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19-0756; Rev 0; 1/07
KIT ATION EVALU ILABLE AVA
SMBus Level 2 Battery Charger with Remote Sense
General Description
The MAX8731A is an SMBusTM programmable multichemistry battery charger. The MAX8731A uses a minimal command set to easily program the charge voltage, charge current, and adapter current limit. The MAX8731A charges one to four Li+ series cells and delivers up to 8A charge current. The MAX8731A drives n-channel MOSFETs for improved efficiency and reduced cost. Low-offset current-sense amplifiers provide high accuracy with 10m sense resistors. The MAX8731A current-sense amplifiers provide high accuracy (3% at 3.5A) and also provide fast cycle-bycycle current-mode control to protect against battery short circuit and system load transients. The charger employs dual remote sense, which reduces charge time by measuring the feedback voltage directly at the battery, improving accuracy of initial transition into constant-voltage mode. The MAX8731A provides 0.5% battery voltage accuracy directly at the battery terminal. The MAX8731A provides a digital output that indicates the presence of the AC adapter, as well as an analog output that indicates the adapter current within 4% accuracy. The MAX8731A is available in a small 5mm x 5mm, 28-pin, thin (0.8mm) QFN package. An evaluation kit is available to reduce design time. The MAX8731A is available in a lead-free package.
SMBus is a trademark of Intel Corp.
Features
0.5% Battery Voltage Accuracy 3% Input Current-Limit Accuracy 3% Charge-Current Accuracy SMBus 2-Wire Serial Interface Cycle-by-Cycle Current Limit Battery Short-Circuit Protection Fast Response for Pulse Charging Fast System-Load-Transient Response Dual-Remote-Sense Inputs Monitor Outputs for Adapter Current (4% Accuracy) AC Adapter Detection 11-Bit Battery Voltage Setting 6-Bit Charge-Current/Input-Current Setting 8A (max) Battery Charger Current 11A (max) Input Current +8V to +26V Input Voltage Range Charges Li+, NiMH, and NiCd Battery Chemistries
MAX8731A

Ordering Information
PART MAX8731AETI+ TEMP RANGE -40C to +85C PINPACKAGE 28 Thin QFN (5mm x 5mm) PKG CODE T2855-6
Applications
Notebook Computers Tablet PCs Medical Devices Portable Equipment with Rechargeable Batteries
+Indicates a lead-free package.
Typical Operating Circuit
EXTERNAL LOAD OPTIONAL CSSP ACIN DCIN ACOK CSSN DHI LX
Pin Configuration
PGND FBSB FBSA CSIN CSIP LDO DLO
N
TOP VIEW
21 DCIN 22 LX 23 DHI 24 BST 25 VCC 26 CSSN 27 CSSP 28 1 GND
20
19
18
17
16
15 14 13 12
REF
MAX8731A DLO
N
BATSEL ACOK
GND
PGND BST CSIP CSIN SELECTOR BATSEL BATTERY A BATTERY B
GND VDD SCL SDA IINP
HOST SCL SDA VDD BATSEL SCL SDA VDD GND IINP DAC CCS LDO VCC CCV CCI FBSB FBSA
MAX8731A
11 10
*EXPOSED PADDLE
9 8
2 ACIN
3 REF
4 CCS
5 CCI
6 CCV
7 DAC
THIN QFN 5mm x 5mm
________________________________________________________________ Maxim Integrated Products
1
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
ABSOLUTE MAXIMUM RATINGS
DCIN, CSSN, CSIN, FBSA, FBSB to GND..............-0.3V to +28V CSSP to CSSN, CSIP to CSIN, PGND to GND ......-0.3V to +0.3V BST to GND ............................................................-0.3V to +32V BST to LX..................................................................-0.3V to +6V DHI to LX.................................................-0.3V to +(VBST + 0.3)V DLO to PGND..........................................-0.3V to +(LDO + 0.3)V LX to GND .................................................................-6V to +28V CCI, CCS, CCV, DAC, REF, IINP to GND...........................................-0.3V to (VVCC + 0.3)V VDD, SCL, SDA, BATSEL, ACIN, ACOK, VCC to GND, LDO to PGND ......................................................-0.3V to +6V Continuous Power Dissipation (TA = +70C) 28-Pin Thin QFN (derate 20.8mW/C above +70C) ........................1666.7 mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-60C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER CHARGE-VOLTAGE REGULATION ChargingVoltage() = 0x41A0 ChargingVoltage() = 0x3130 Battery Full-Charge Voltage and Accuracy ChargingVoltage() = 0x20D0 ChargingVoltage() = 0x1060 Battery Undervoltage-Lockout Trip Point for Trickle Charge CHARGE-CURRENT REGULATION CSIP-to-CSIN Full-Scale CurrentSense Voltage RS2, Figure 1 = 10m; ChargingCurrent() = 0x1f80 Charge Current and Accuracy RS2, Figure 1 = 10m; ChargingCurrent() = 0x0f80 RS2, Figure 1 = 10m; ChargingCurrent() = 0x0080 (128mA) Charge-Current Gain Error FBSA/FBSB/CSIP/CSIN Input Voltage Range Based on ChargeCurrent() = 128mA and 8.064A 78.22 7.822 -3 3.809 -4 64 -2 0 3.968 80.64 8.064 83.06 8.306 +3 4.126 +4 400 +2 19 mV A % A % mA % V 16.716 -0.5 12.491 -0.8 8.333 -0.8 4.150 -1.0 2.5 4.192 8.4 12.592 16.8 16.884 +0.5 12.693 +0.8 8.467 +0.8 4.234 +1.0 V % V % V % V % V CONDITIONS MIN TYP MAX UNITS
2
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SMBus Level 2 Battery Charger with Remote Sense
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER CONDITIONS Adapter present, not charging, ICSIP + ICSIN + ILX + IFBS, VFBS_ = VLX = VCSIN = VCSIP = 19V Adapter absent, ICSIP + ICSIN + ILX + IFBSA + IFBSB + ICSSP + ICSSN, VFBS_ = VLX = VCSIN = VCSIP = 19V, VDCIN = 0V VAdapter = 26V, VBattery = 16.8V, not charging Adapter Quiescent Current IDCIN + ICSSP + ICSSN VAdapter = 19V, VBattery = 16.8V VAdapter = 8V, VBattery = 4V INPUT-CURRENT REGULATION CSSP-to-CSSN Full-Scale Current-Sense Voltage Input Current Accuracy POR Input Current Input Current-Limit Gain Error Input Current-Limit Offset CSSP/CSSN Input Voltage Range IINP Transconductance IINP Offset IINP Accuracy IINP Output Voltage Range SUPPLY AND LINEAR REGULATOR DCIN, Input Voltage Range DCIN Undervoltage-Lockout Trip Point Power-Fail Threshold DCIN falling DCIN rising VCSSP - VCSIN falling VCSSP - VCSIN rising 9 160 8.0 7 7.4 7.5 15 210 7.85 21 271 26.0 V V mV VCSSP - CSSN = 110mV Based on VCSSP - CSSN = 110mV and 20mV VCSSP - CSSN = 110mV VCSSP - CSSN = 55mV or 35mV VCSSP - CSSN = 20mV VFBS_ = 19V RS1, Figure 1 = 10m, InputCurrent() = 11004mA or 3584mA RS1, Figure 1 = 10m, InputCurrent() = 2048mA RS1, Figure 1 = 10m Based on InputCurrent() = 1024mA and 11004mA -2 -1 8 2.85 -1.5 -5 -4 -10 0 3.0 106.7 -3 -5 256 +2 +1 26 3.15 +1.5 +5 +4 +10 3.5 V % 110 113.3 +3 +5 mA % mV V mA/V mV mV Charging Not charging Charging Not charging 200 0.4 200 0.4 200 MIN TYP 2 MAX 5 A +1 500 1 500 1 500 A mA A mA A UNITS
MAX8731A
Battery Quiescent Current
%
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER LDO Output Voltage LDO Load Regulation LDO Undervoltage-Lockout Threshold VDD Range VDD UVLO Rising VDD UVLO Hysteresis VDD Quiescent Current REFERENCE REF Output Voltage REF Undervoltage-Lockout Trip Point ACOK ACOK Sink Current ACOK Leakage Current ACIN ACIN Threshold ACIN Threshold Hysteresis ACIN Input Bias Current REMOTE-SENSE INPUTS FBS_ Range FBS_ Gain CSIN-FBS_ Clamp Voltage FBS_ Bias Current FBS_ Bias Current SWITCHING REGULATOR Off-Time BST Supply Current LX Input Bias Current Maximum Discontinuous-Mode Peak Current (IMIN) DHI On-Resistance Low DHI On-Resistance High DLO On-Resistance High DLO On-Resistance Low IDHI = -10mA IDHI = 10mA IDLO = 10mA IDLO = -10mA VCSIN = 16.0V, VCSSP = 19V VCSIN = 16.0V, VCSSP = 17V DHI high VDCIN = 28V, VCSIN = VLX = 20V, DHI low 0.5 1 3 3 1 3 5 5 3 360 260 400 300 500 440 360 800 2 ns A A A Charger switching, FBS_ selected Charger not switching or FBS_ not selected -2 VCSIN - VFBS VCSIN / (VCSIN - VFBS_) 0 0.95 225 1.00 250 200 1.05 275 14 +2 mV V/V mV A A 2.007 10 -1 2.048 20 2.089 30 +1 V mV A VACOK = 0.4V, ACIN = 1.5V VACOK = 5.5V, ACIN = 2.5V 1 1 mA A 0 < IREF < 500A REF falling 4.071 4.096 3.1 4.120 3.9 V V DCIN < 6V, VDD = 5.5V, SCL = SDA = 5.5V 0 < ILDO < 30mA VDCIN = 8.0V, VLDO falling 3.20 2.7 2.5 100 16 27 CONDITIONS 8.0V < VDCIN < 28V, no load MIN 5.25 TYP 5.4 34 4.00 MAX 5.55 100 5.15 5.5 2.7 UNITS V mV V V V mV A
4
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SMBus Level 2 Battery Charger with Remote Sense
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER ERROR AMPLIFIERS GMV Amplifier Transconductance ChargingVoltage() = 16.8V, VFBS_ = 16.8V GMI Amplifier Transconductance CCI/CCS/CCV Clamp Voltage LOGIC LEVELS SDA/SCL Input-Low_Voltage SDA/SCL Input-High_Voltage SDA/SCL Input Bias Current BATSEL Input-Low_Voltage BATSEL Input-High_Voltage BATSEL Input Bias Current SDA, Output Sink Current PARAMETERS SMBus Frequency Bus Free Time Start Condition Hold Time from SCL Start Condition Setup Time from SCL Stop Condition Setup Time from SCL SDA Hold Time from SCL SDA Setup Time from SCL SCL Low Timeout SCL Low Period SCL High Period Maximum Charging Period Without a ChargeVoltage() or ChargeCurrent() Command V(SDA) = 0.4V SYMBOL fSMB tBUF tHD:STA tSU:STA tSU:STO tHD:DAT tSU:DAT tTIMEOUT TLOW THIGH (Note 1) CONDITIONS SMBus TIMING SPECIFICATIONS (VDD = 2.7V to 5.5V) (see Figures 4 and 5) MIN 10 4.7 4 4.7 4 300 250 25 4.7 4 140 175 210 35 TYP MAX 100 UNITS kHz s s s s ns ns ms s s s 2.1 -1 6 +1 VDD = 2.7V to 5.5V VDD = 2.7V to 5.5V VDD = 2.7V to 5.5V 2.1 -1 +1 0.8 0.8 V V A V V A mA ChargingCurrent() = 3968mA, VCSIP - VCSIN = 39.68mV 0.25V < VCCI/S/V < 2.0V GMS Amplifier Transconductance InputCurrent() = 3968mA, VCSSP - VCSSN = 79.36mV 0.0625 0.5 0.5 120 0.125 1.0 1.0 250 0.2500 2.0 2.0 600 mA/V mA/V mA/V mV CONDITIONS MIN TYP MAX UNITS
MAX8731A
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
ELECTRICAL CHARACTERISTICS
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER CHARGE-VOLTAGE REGULATION ChargingVoltage() = 0x41A0 ChargingVoltage() = 0x3130 Battery Full-Charge Voltage and Accuracy ChargingVoltage() = 0x20D0 ChargingVoltage() = 0x1060 CHARGE-CURRENT REGULATION CSIP-to-CSIN Full-Scale CurrentSense Voltage RS2, Figure 1 = 10m; ChargingCurrent()= 0x1f80 Charge Current and Accuracy RS2, Figure 1 = 10m; ChargingCurrent() = 0x0f80 RS2, Figure 1 =10m; ChargingCurrent() = 0x0080 Charge-Current Gain Error FBSA/FBSB/CSIP/CSIN InputVoltage Range Adapter present, not charging, ICSIP + ICSIN + ILX + IFBS, VFBS_ = VLX = VCSIN = VCSIP = 19V Battery Quiescent Current Adapter absent, ICSIP + ICSIN + ILX + IFBSA + IFBSB + ICSSP + ICSSN, VFBS_= VLX = VCSIN = VCSIP = 19V, VDCIN = 0V VAdapter = 26V, VBattery = 16.8V, not charging Adapter Quiescent Current IDCIN + ICSSP + ICSSN VAdapter = 19V, VBattery = 16.8V VAdapter = 8V, VBattery = 4V Charging Not charging Charging Not charging Based on ChargeCurrent() = 128mA and 8.064A 78.22 7.822 -3 3.809 -4 30 -2 0 83.05 8.305 +3 4.126 +4 400 +2 19 5 A 1 500 1 500 1 500 A mA A mA A mV A % A % mA % V 16.632 -1 12.466 -1 8.316 -1 4.129 -1.5 16.968 +1 12.717 +1 8.484 +1 4.255 +1.5 V % V % V % V % CONDITIONS MIN TYP MAX UNITS
6
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER INPUT-CURRENT REGULATION CSSP-to-CSSN Full-Scale Current-Sense Voltage VFBS_ = 19V RS1, Figure 1 = 10m; InputCurrent() = 11004mA or 3584mA Input Current Accuracy RS1, Figure 1 = 10m; InputCurrent() = 2048mA Input Current-Limit Gain Error Input Current-Limit Offset CSSP/CSSN Input Voltage Range IINP Transconductance IINP Offset IINP Accuracy IINP Output Voltage Range SUPPLY AND LINEAR REGULATOR DCIN, Input Voltage Range DCIN Undervoltage-Lockout Trip Point POWER_FAIL Threshold LDO Output Voltage LDO Load Regulation LDO Undervoltage-Lockout Threshold VDD Range VDD UVLO Rising VDD Quiescent Current REFERENCE REF Output Voltage REF Undervoltage-Lockout Trip Point ACOK ACOK Sink Current VACOK = 0.4V, ACIN = 1.5V 1 mA 0 < IREF < 500A REF falling 4.053 4.139 3.9 V V DCIN < 6V, VDD = 5.5V, SCL = SDA = 5.5V DCIN falling DCIN rising VCSSP - VCSIN falling VCSSP - VCSIN rising 8.0V < VDCIN < 28V, no load 0 < ILDO < 30mA VDCIN = 8.0V, VLDO falling 3.20 2.7 9 160 5.25 8.0 7 7.85 21 271 5.55 100 5.15 5.5 2.7 27 26.0 V -- mV V mV V V V A VCSSP - CSSN = 110mV Based on VCSSP - CSSN = 110mV and 20mV VCSSP - CSSN = 110mV VCSSP - CSSN = 55mV or 35mV VCSSP - CSSN = 20mV Based on InputCurrent() = 1024mA and 11004mA Based on InputCurrent() = 1024mA and 11004mA -5 -5 -1 8 2.7 -1.5 -5 -4 -10 0 +5 +5 +1 26 3.3 +1.5 +5 +4 +10 3.5 V % % mV V mA/V mV 103.3 -6 116.6 +6 % mV CONDITIONS MIN TYP MAX UNITS
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER ACIN ACIN Threshold ACIN Threshold Hysteresis REMOTE-SENSE INPUTS FBS_ Range FBS_ Gain CSIN-FBS_ Clamp Voltage FBS_ Bias Current SWITCHING REGULATOR Off-Time BST Supply Current DHI On-Resistance Low DHI On-Resistance High DLO On-Resistance High DLO On-Resistance Low ERROR AMPLIFIERS GMV Amplifier Transconductance ChargingVoltage() = 16.8V, VFBS_ = 16.8V GMI Amplifier Transconductance CCI/CCS/CCV Clamp Voltage LOGIC LEVELS SDA/SCL Input Low Voltage SDA/SCL Input High Voltage BATSEL Input Low Voltage BATSEL Input High Voltage SDA, Output Sink Current V(SDA) = 0.4V 2.3 6 VDD = 2.7V to 5.5V VDD = 2.7V to 5.5V 2.3 0.8 0.8 V V V V mA ChargingCurrent() = 3968mA, VCSIP - VCSIN = 39.68mV 0.25V < VCCI/S/V < 2.0V GMS Amplifier Transconductance InputCurrent() = 3968mA, VCSSP - VCSSN = 79.36mV 0.0625 0.5 0.5 150 0.2500 2.0 2.0 600 mA/V mA/V mA/V mV VCSIN = 16.0V, VCSSP = 19V VCSIN = 16.0V, VCSSP = 17V DHI high IDHI = -10mA IDHI = 10mA IDLO = 10mA IDLO = -10mA 360 260 440 350 800 3 5 5 3 ns A Charger switching, FBS_ selected VCSIN - VFBS VCSIN / (VCSIN - VFBS_) 0 0.9 220 200 1.1 280 14 mV V/V mV A 2.007 10 2.089 30 V mV CONDITIONS MIN TYP MAX UNITS
8
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SMBus Level 2 Battery Charger with Remote Sense
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0, CLDO = 1F, VCC = LDO, CREF = 1F, CDAC = 0.1F , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per Figure 1; TA = -40C to +85C, unless otherwise noted.) (Note 2)
SMB TIMING SPECIFICATION (VDD = 2.7V to 5.5V) (see Figures 4 and 5) PARAMETERS SMBus Frequency Bus Free Time START Condition Hold Time from SCL START Condition Setup Time from SCL STOP Condition Setup Time from SCL SDA Hold Time from SCL SDA Setup Time from SCL SCL Low Timeout SCL Low Period SCL High Period Maximum Charging Period Without a ChargeVoltage() or ChargeCurrent() Command SYMBOL fSMB tBUF tHD:STA tSU:STA tSU:STO tHD:DAT tSU:DAT tTIMEOUT TLOW THIGH (Note 1) CONDITIONS MIN 10 4.7 4 4.7 4 300 250 25 4.7 4 140 210 35 TYP MAX 100 UNITS kHz s s s s ns ns ms s s s
MAX8731A
Note 1: Devices participating in a transfer time out when any clock low exceeds the 25ms minimum timeout period. Devices that have detected a timeout condition must reset the communication no later than the 35ms maximum timeout period. Both a master and a slave must adhere to the maximum value specified as it incorporates the cumulative stretch limit for both a master (10ms) and a slave (25ms). Note 2: Specifications to -40C are guaranteed by design, not production tested.
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
Typical Operating Characteristics
(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25C, unless otherwise noted.)
INPUT CURRENT-LIMIT ERROR vs. INPUT CURRENT-LIMIT SETTING
MAX8731A toc01
INPUT CURRENT-LIMIT ERROR vs. SYSTEM CURRENT
MAX8731A toc02
INPUT CURRENT-LIMIT ERROR vs. SYSTEM CURRENT
0.8 INPUT CURRENT-LIMIT ERROR (%) 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 4 0 1 2 3 SYSTEM CURRENT (A) 4 VBATT = 8.4V VBATT = 12.6V VBATT = 16.8V INPUT CURRENT LIMIT = 3.584A
MAX8731A toc03
6 INPUT CURRENT-LIMIT ERROR (%) 4 MAXIMUM 2 TYPICAL 0 -2 -4 -6 0 2 4 6 8 10 INPUT CURRENT-LIMIT SETTING (A) MINIMUM
0.4 INPUT CURRENT-LIMIT ERROR (%) INPUT CURRENT LIMIT = 3.584A 0.2
1.0
0 INPUT CURRENT LIMIT = 4.096A -0.2 INPUT CURRENT LIMIT = 2.048A -0.4 0 1 2 3 SYSTEM CURRENT (A)
IINP ERROR vs. SYSTEM CURRENT
MAX8731A toc04
IINP ERROR vs. SYSTEM CURRENT
MAX8731A toc05
IINP ERROR vs. INPUT CURRENT
8 6 4 IINP ERROR (%) NOT SWITCHING MAXIMUM
MAX8731A toc06
1.0 0.8 0.6 0.4 IINP ERROR (%) 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 0 0.5 1.0 1.5 2.0 2.5 3.0 SYSTEM CURRENT (A) 3.5 INPUT CURRENT LIMIT = 4.096A OPERATING AT INPUT CURRENT LIMIT INPUT CURRENT LIMIT = 3.584A INPUT CURRENT LIMIT = 2.048A
2.5 VBATT = 12.6V 2.0 IINP ERROR (%)
10
1.5 VBATT = 16.8V 1.0 VBATT = 8.4V
2 0 -2 -4 TYPICAL MINIMUM
0.5 INPUT CURRENT LIMIT = 3.584A 0 4.0 0 1 2 3 SYSTEM CURRENT (A) 4
-6 -8 -10 0 1
2 3 4 INPUT CURRENT (A)
5
6
CHARGE-CURRENT ERROR vs. CHARGE CURRENT-LIMIT SETTING
8 6 4 2 0 -2 -4 -6 -8 -10 0 2 4 6 CHARGE-CURRENT SETTING (A) 8 -4 3 TYPICAL MINIMUM MAXIMUM
MAX8731A toc07
CHARGE-CURRENT ERROR vs. BATTERY VOLTAGE
MAX8731A toc08
TRICKLE-CHARGE CURRENT ERROR vs. BATTERY VOLTAGE
TRICKLE-CHARGE CURRENT ERROR (%)
MAX8731A toc09
10 CHARGE-CURRENT LIMIT ERROR (%)
4
5 0 -5 -10 -15 -20 -25 ChargeCurrent( ) = 128mA -30 0 3 6 9 12 BATTERY VOLTAGE (V) 15
CHARGE-CURRENT ERROR (%)
2
3.072A
3.968A
0 8.064A -2
6
9 12 15 BATTERY VOLTAGE (V)
18
18
10
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
Typical Operating Characteristics (continued)
(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25C, unless otherwise noted.)
CHARGE-VOLTAGE ERROR vs. CHARGE-VOLTAGE SETTING
MAX8731A toc10
BATTERY-VOLTAGE ERROR vs. CHARGE CURRENT
MAX8731A toc11
0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 4 8 12 16 CHARGE-VOLTAGE SETTING (V)
0.3 0.2 0.1 0 -0.1 -0.2 3 CELLS -0.3 4 CELLS
BATTERY-VOLTAGE ERROR (%)
CHARGE-VOLTAGE ERROR (%)
2 CELLS
20
0
1
2 3 4 CHARGE CURRENT (A)
5
6
BATTERY REMOVAL
MAX8731A toc12
SYSTEM LOAD TRANSIENT
MAX8731A toc13
VOUT OUTPUT CAPACITOR = 10F
13.5V 13.0V 12.5V
LOAD CURRENT ADAPTER CURRENT INDUCTOR CURRENT CCS VOLTAGE 500mV/div CCI VOLTAGE 500mV/div CCS CCI CCS 200s/div CCI
5A 0A 5A 0A 5A 0A 500mV/div 500mV/div
VOUT OUTPUT CAPACITOR = 22F ChargeVoltage( ) = 12.6V
20s/div
EFFICIENCY vs. CHARGE CURRENT
MAX8731A toc14
LDO LOAD REGULATION
-5 CHARGER OFF -10 LDO ERROR (mV) -15 -20 -25 -30 -35 -40
MAX8731A toc15
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 0 2 4 6 CHARGE CURRENT (A) 8 3 CELLS 2 CELLS 4 CELLS
0
0
20
40 60 ILDO (mA)
80
100
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11
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
Typical Operating Characteristics (continued)
(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25C, unless otherwise noted.)
LDO LINE REGULATION
MAX8731A toc16
REF LOAD REGULATION
MAX8731A toc17
REF ERROR vs. TEMPERATURE
MAX8731A toc18
0 -1 LDO ERROR (mV) -2 -3 -4 -5 -6 8 13 18 VDCIN (V) 23 NOT SWITCHING
0.20 NOT SWITCHING 0.15 0.10 REF ERROR (%) 0.05 0 -0.05 -0.10 -0.15 -0.20 0 0.2 0.4 0.6 IREF (mA) 0.8
0.3 0.2 0.1 0 -0.1 -0.2 -0.3
1.0
REF ERROR (%)
-40
-20
0 20 40 TEMPERATURE (C)
60
80
SWITCHING FREQUENCY
MAX8731A toc19
BATTERY-CHARGE CURVE
5 2.8Ah x 3S3P BATTERY 4 CHARGE CURRENT (A) BATTERY VOLTAGE 12.5 12.0 3 11.5 2 11.0 1 CHARGE CURRENT 0 10.5 10.0 0 1 2 3 TIME (h) 4 5 6 BATTERY VOLTAGE (V)
MAX8731A toc20
450 400 FREQUENCY (kHz) 350 300 250 200 150 0 5 10 15 VADAPTER - VBATTERY (V)
13.0
20
ADAPTER CURRENT vs. ADAPTER VOLTAGE
MAX8731A toc21
BATTERY-LEAKAGE CURRENT vs. BATTERY VOLTAGE
ADAPTER PRESENT OR ABSENT 2.0 BATTERY CURRENT (A)
MAX8731A toc22
3.0 SWITCHING, NO LOAD 2.5 ADAPTER CURRENT (mA) 2.0 ChargeVoltage( ) = 4.192V 1.5 1.0 0.5 0 0 5 10 15 20 ADAPTER VOLTAGE (V) 25 NOT SWITCHING
2.5
1.5
1.0
0.5
0 30 0 5 10 15 BATTERY VOLTAGE (V) 20
12
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SMBus Level 2 Battery Charger with Remote Sense
Pin Description
PIN 1, 12 2 3 4 5 6 7 8 9 10 11 13 NAME GND ACIN REF CCS CCI CCV DAC IINP SDA SCL VDD ACOK FUNCTION Analog Ground. Connect directly to the paddle. AC Adapter Detect Input. ACIN is the input to an uncommitted comparator. 4.096V Voltage Reference. Bypass REF with a 1F capacitor to GND. Input Current Regulation Loop-Compensation Point. Connect 0.01F from CCS to GND. Output Current Regulation Loop-Compensation Point. Connect 0.01F from CCI to GND. Voltage Regulation Loop-Compensation Point. Connect 10k in series with 0.01F to GND. DAC Voltage Output. Bypass with 0.1F from DAC to GND. Input Current Monitor Output. IINP sources the current proportional to the current sensed across CSSP and CSSN. The transconductance from (CSSP - CSSN) to IINP is 3mA/V. SMBus Data I/O. Open-drain output. Connect an external pullup resistor according to SMBus specifications. SMBus Clock Input. Connect an external pullup resistor according to SMBus specifications. Logic Circuitry Supply-Voltage Input. Bypass with a 0.1F capacitor to GND. AC Detect Output. This open-drain output is high impedance when ACIN is greater than REF/2. The ACOK output remains low when the MAX8731A is powered down. Connect a 10k pullup resistor from VCC to ACOK. Battery Voltage Select Input. Drive BATSEL high to select battery B, or drive BATSEL low to select battery A. Any change of BATSEL immediately stops charging. Charging begins again in approximately 10ms. Remote-Sense Input for the Output Voltage of Battery A. Connect a 100 resistor from FBSA to the battery connector, and a 10nF capacitor from FBSA to PGND. Remote-Sense Input for the Output Voltage of Battery B. Connect a 100 resistor from FBSB to the battery connector, and a 10nF capacitor from FBSB to PGND. Charge Current-Sense Negative Input Charge Current-Sense Positive Input. Connect a 10m current-sense resistor between CSIP and CSIN. Power Ground Low-Side Power MOSFET Driver Output. Connect to low-side n-channel MOSFET. DLO drives between LDO and PGND. Linear-Regulator Output. LDO is the output of the 5.4V linear regulator supplied from DCIN. LDO also directly supplies the DLO driver and the BST charge pump. Bypass with a 1F ceramic capacitor from LDO to PGND. Charger Bias Supply Input. Bypass DCIN with a 0.1F capacitor to PGND. High-Side Power MOSFET Driver Source Connection. Connect to the source of the high-side n-channel MOSFET. High-Side Power MOSFET Driver Output. Connect to the high-side n-channel MOSFET gate. High-Side Power MOSFET Driver Power-Supply Connection. Connect a 0.1F capacitor from BST to LX. Device Power-Supply Input. Connect to LDO through an RC filter as shown in Figure 1. Input Current-Sense Negative Input Input Current-Sense Positive Input. Connect a 10m current-sense resistor between CSSP and CSSN. Backside Paddle. Connect the backside paddle to analog ground.
MAX8731A
14 15 16 17 18 19 20
BATSEL FBSA FBSB CSIN CSIP PGND DLO
21 22 23 24 25 26 27 28 --
LDO DCIN LX DHI BST VCC CSSN CSSP BP
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13
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
ADAPTER INPUT R2 100k R13 100k DCIN C1 1F R3 49.9k N MAX8731A LDO R4 10k ACOK BST VDD R5 10k C2 0.1F SCL LX DLO SDA IINP R7 10k C3 0.1F CSIP CSIN VOUT CCV R8 10k C4 0.01F C6 0.01F C5 0.01F CCI BATSEL R10 100 CCS REF DAC C7 1F FBSB FBSA GND R9 100 SELECTOR COUT1 10F COUT2 10F PGND C9 220pF N3 L1 4.3H L1: SUMIDA CEP125-4R3MC-U RS2 10m DHI C10 0.1F N1 R11 1 DHI N2 LDO D2 ACIN CSSN VCC C12 1F R12 33 LDO C11 1F R1 150k D1
CSSP RS1 10m SYSTEM LOAD CIN2 10F
CIN1 10F
INPUT VDD
N1, N2,: SI4800BDY N3: SI4810BDY
KBC SCL
R6 10k SDA
BP C8 0.1F
BATTERY A
BATTERY B
Figure 1. Typical Dual-Battery Application Circuit
Detailed Description
The typical application circuit is shown in Figure 1. The MAX8731A includes all the functions necessary to charge Li+, NiMH, and NiCd smart batteries. A highefficiency, synchronous-rectified, step-down DC-DC converter is used to implement a precision constantcurrent, constant-voltage charger. The DC-DC converter drives a high-side n-channel MOSFET and provides synchronous rectification with a low-side n-channel MOSFET. The charge current and input current-sense
14
amplifiers have low input-offset error (64V typ), allowing the use of small-valued sense resistors. The MAX8731A features a voltage-regulation loop (CCV) and two current-regulation loops (CCI and CCS). The loops operate independently of each other. The CCV voltage-regulation loop monitors either FBSA or FBSB to ensure that its voltage never exceeds the voltage set by the ChargeVoltage() command. The CCI battery current-regulation loop monitors the current delivered to the selected battery to ensure that it never exceeds the current limit set by the ChargeCurrent()
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SMBus Level 2 Battery Charger with Remote Sense
command. The charge current-regulation loop is in control as long as the selected battery voltage is below the charge voltage set point. When the selected battery voltage reaches its set point, the voltage-regulation loop takes control and maintains the battery voltage at the set point. A third loop (CCS) takes control and reduces the charge current when the adapter current exceeds the input current limit set by the InputCurrent() command. A functional diagram is shown in Figure 2.
MAX8731A
150mV (750mA FOR RS2 = 10m)
2V (10A FOR RS2 = 10m)
MAX8731A
CSSP POWER-FAIL ZCMP CSIN ACIN VCC ACOK LVC REF/2 GND OVP LOWEST VOLTAGE CLAMP CCV PGND CCMP LOWSIDE DRIVER DLO 100mV ENABLE IMIN DC-DC CONVERTER LEVEL SHIFT IMAX HIGHSIDE DRIVER DHI BST
LX LDO
CCI GMV
5.4V LINEAR REGULATOR
DCIN VCC
GMI
+ 200mV -
CCS CSI CSA: CURRENT-SENSE AMPLIFIER GMS 11-BIT DAC A = 1V/V CSS CSA 6-BIT DAC A = 20V/V GM CSA A = 20V/V 6-BIT DAC
4.096V REFERENCE REF
SMBus LOGIC CHARGE VOLTAGE ( ) SCL
CHARGE CURRENT ( )
SDA
INPUT CURRENT ( )
VDD
BATSEL
CSSN
CSSP
CSIN
IINP
CSIP
FBSB
FBSA
Figure 2. Functional Diagram ______________________________________________________________________________________ 15
DAC
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
Setting Charge Voltage
To set the output voltage, use the SMBus to write a 16-bit ChargeVoltage() command using the data format listed in Table 1. The ChargeVoltage() command uses the WriteWord protocol (see Figure 3). The command code for ChargeVoltage() is 0x15 (0b00010101). The MAX8731A provides a 1.024V to 19.200V charge voltage range, with 16mV resolution. Set ChargeVoltage() below 1.024V to terminate charging. Upon reset, the ChargeVoltage() and ChargeCurrent() values are cleared and the charger remains off until both the ChargeVoltage() and the ChargeCurrent() command are sent. Both DHI and DLO remain low until the charger is restarted.
Table 1. ChargeVoltage () (0x15)
BIT 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 BIT NAME -- -- -- -- Charge voltage, DACV 0 Charge voltage, DACV 1 Charge voltage, DACV 2 Charge voltage, DACV 3 Charge voltage, DACV 4 Charge voltage, DACV 5 Charge voltage, DACV 6 Charge voltage, DACV 7 Charge voltage, DACV 8 Charge voltage, DACV 9 Charge voltage, DACV 10 -- DESCRIPTION Not used. Normally a 1mV weight. Not used. Normally a 2mV weight. Not used. Normally a 4mV weight. Not used. Normally a 8mV weight. 0 = Adds 0mV of charger voltage compliance, 1024mV min. 1 = Adds 16mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance, 1024mV min. 1 = Adds 32mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance, 1024mV min. 1 = Adds 64mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance, 1024mV min. 1 = Adds 128mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance, 1024mV min. 1 = Adds 256mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance, 1024mV min. 1 = Adds 512mV of charger voltage compliance. 0 = Adds 0mA of charger voltage compliance. 1 = Adds 1024mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance. 1 = Adds 2048mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance. 1 = Adds 4096mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance. 1 = Adds 8192mV of charger voltage compliance. 0 = Adds 0mV of charger voltage compliance. 1 = Adds 16,384mV of charger voltage compliance, 19,200mV max. Not used. Normally a 32,768mV weight.
16
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SMBus Level 2 Battery Charger with Remote Sense
Setting Charge Current
To set the charge current, use the SMBus to write a 16bit ChargeCurrent() command using the data format listed in Table 2. The ChargeCurrent() command uses the Write-Word protocol (see Figure 3). The command code for ChargeCurrent() is 0x14 (0b00010100). When RS2 =10m, the MAX8731A provides a charge current range of 128mA to 8.064A, with 128mA resolution. Set ChargeCurrent() to 0 to terminate charging. Upon reset, the ChargeVoltage() and ChargeCurrent() values are cleared and the charger remains off until both the ChargeVoltage() and the ChargeCurrent() commands are sent. Both DHI and DLO remain low until the charger is restarted. The MAX8731A includes a foldback current limit when the battery voltage is low. If the battery voltage is less than 2.5V, the charge current is temporarily set to 128mA. The ChargeCurrent() register is preserved and becomes active again when the battery voltage is higher than 2.5V. This function effectively provides a foldback current limit, which protects the charger during short circuit and overload.
Setting Input-Current Limit
System current normally fluctuates as portions of the system are powered up or put to sleep. By using the input-current-limit circuit, the output-current requirement of the AC wall adapter can be lowered, reducing system cost. The total input current, from a wall cube or other DC source, is the sum of the system supply current and the current required by the charger. When the input current exceeds the set input current limit, the MAX8731A decreases the charge current to provide priority to system load current. As the system supply rises, the available charge current drops linearly to zero. Thereafter, the total input current can increase without limit. The internal amplifier compares the differential voltage between CSSP and CSSN to a scaled voltage set by the InputCurrent() command (see Table 3). The total input current is the sum of the device supply current, the charger input current, and the system load current. The total input current can be estimated as follows:
MAX8731A
Table 2. ChargeCurrent() (0x14) (10m Sense Resistor, RS2)
BIT 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 BIT NAME -- -- -- -- -- -- -- Charge Current, DACI 0 Charge Current, DACI 1 Charge Current, DACI 2 Charge Current, DACI 3 Charge Current, DACI 4 Charge Current, DACI 5 -- -- -- DESCRIPTION Not used. Normally a 1mA weight. Not used. Normally a 2mA weight. Not used. Normally a 4mA weight. Not used. Normally an 8mA weight. Not used. Normally a 16mA weight. Not used. Normally a 32mA weight. Not used. Normally a 64mA weight. 0 = Adds 0mA of charger current compliance. 1 = Adds 128mA of charger current compliance. 0 = Adds 0mA of charger current compliance. 1 = Adds 256mA of charger current compliance. 0 = Adds 0mA of charger current compliance. 1 = Adds 512mA of charger current compliance. 0 = Adds 0mA of charger current compliance. 1 = Adds 1024mA of charger current compliance. 0 = Adds 0mA of charger current compliance. 1 = Adds 2048mA of charger current compliance. 0 = Adds 0mA of charger current compliance. 1 = Adds 4096mA of charger current compliance, 8064mA max. Not used. Normally a 8192mA weight. Not used. Normally a 16,386mA weight. Not used. Normally a 32,772mA weight.
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17
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
(ICHARGE x VBATTERY ) IINPUT = ILOAD + + IBIAS (VIN x ) where is the efficiency of the DC-DC converter (typically 85% to 95%). To set the input current limit, use the SMBus to write a 16-bit InputCurrent() command using the data format listed in Table 3. The InputCurrent() command uses the Write-Word protocol (see Figure 3). The command code for InputCurrent() is 0x3F (0b00111111). When RS1 = 10m, the MAX8731A provides an input-currentlimit range of 256mA to 11.004A, with 256mA resolution. InputCurrent() settings from 1mA to 256mA result in a current limit of 256mA. Upon reset the input current limit is 256mA.
Remote Sense
The MAX8731A features dual remote sense, which allows the rejection of board resistance and selector resistance when used in either single- or dual-battery systems. To fully utilize remote sensing, connect FBS_ directly to the battery interface through an unshared battery sense trace in series with a 100 resistor, and 10nF capacitor (see Figure 1). In single-battery systems, connect BATSEL directly to GND and use only FBSA. Remote sensing cancels the effect of impedance in series with the battery. This impedance normally causes the battery charger to prematurely enter constantvoltage mode with reducing charge current. The result is that the last 20% of charging takes longer than necessary. When in constant-voltage mode, the remaining charge time is proportional to the total resistance in series with the battery. Remote sensing reduces charge time according to the following equation: t CVRS = t CV 0 x RPack RPack + RBoard
Charger Timeout
The MAX8731A includes a timer to terminate charging if the charger does not receive a ChargeVoltage() or ChargeCurrent() command within 175s. If a timeout occurs, both ChargeVoltage() and ChargeCurrent() commands must be resent to reenable charging.
Table 3. InputCurrent() (0x3F) (10m Sense Resistor, RS1)
BIT 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 BIT NAME -- -- -- -- -- -- -- Input Current, DACS 0 Input Current, DACS 1 Input Current, DACS 2 Input Current, DACS 3 Input Current, DACS 4 Input Current, DACS 5 -- -- -- DESCRIPTION Not used. Normally a 2mA weight. Not used. Normally a 4mA weight. Not used. Normally an 8mA weight. Not used. Normally a 16mA weight. Not used. Normally a 32mA weight. Not used. Normally a 64mA weight. Not used. Normally a 128mA weight. 0 = Adds 0mA of input current compliance. 1 = Adds 256mA of input current compliance. 0 = Adds 0mA of input current compliance. 1 = Adds 512mA of input current compliance. 0 = Adds 0mA of input current compliance. 1 = Adds 1024mA of input current compliance. 0 = Adds 0mA of input current compliance. 1 = Adds 2048mA of input current compliance. 0 = Adds 0mA of input current compliance. 1 = Adds 4096mA of input current compliance. 0 = Adds 0mA of input current compliance. 1 = Adds 8192mA of input current compliance, 11,004mA max. Not used. Normally a 16,384mA weight. Not used. Normally a 32,768mA weight. Not used. Normally a 65,536mA weight.
18
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SMBus Level 2 Battery Charger with Remote Sense
where RPack is the total resistance in the battery pack, RBoard is the board resistance in series with the battery charge path, tCV0 is the constant-voltage charge time without remote sense, and tCVRS is the constant-voltage charge time with remote sense. The MAX8731A includes a safety feature, which limits the charge voltage when FBS_ or the selector is disconnected. The MAX8731A guarantees that CSIN does not regulate more than 250mV above the selected charging voltage. This also limits the extent to which remote sense can cancel charge-path impedance.
VDD Supply The VDD input provides power to the SMBus interface. Connect VDD to LDO, or apply an external supply to VDD to keep the SMBus interface active while the supply to DCIN is removed. When VDD is biased the internal registers are maintained. Bypass VDD to GND with a 0.1F or greater ceramic capacitor. Operating Conditions
The MAX8731A has the following operating states: * Adapter Present: When DCIN is greater than 7.5V, the adapter is considered to be present. In this condition, both the LDO and REF function properly and battery charging is allowed: a) Charging: The total MAX8731A quiescent current when charging is 1mA (max) plus the current required to drive the MOSFETs. b) Not Charging: To disable charging, set either ChargeCurrent() or ChargeVoltage() to zero. When the adapter is present and charging is disabled, the total adapter quiescent current is less than 1mA and the total battery quiescent current is less than 5A. * Adapter Absent (Power Fail): When VCSSP is less than VCSIN + 10mV, the MAX8731A is in the powerfail state, since the DC-DC converter is in dropout. The charger does not attempt to charge in the power-fail state. Typically, this occurs when the adapter is absent. When the adapter is absent, the total MAX8731A quiescent battery current is less than 1A (max). * VDD Undervoltage (POR): When VDD is less than 2.5V, the VDD supply is in an undervoltage state and the internal registers are in their POR state. The SMBus interface does not respond to commands. When VDD rises above 2.5V, the MAX8731A is in a power-on reset state. Charging does not occur until the ChargeVoltage() and ChargeCurrent() commands are sent. When V DD is greater than 2.5V, SMBus registers are preserved. The MAX8731A allows charging under the following conditions: 1) DCIN > 7.5V, LDO > 4V, REF > 3.1V 2) VCSSP > VCSIN + 210mV (15mV falling threshold) 3) VDD > 2.5V
MAX8731A
Input Current Measurement
Use IINP to monitor the system-input current sensed across CSSP and CSSN. The voltage at IINP is proportional to the input current by the equation: VIINP = IINPUT x RS1 x GIINP x R8 where I INPUT is the DC current supplied by the AC adapter, GIINP is the transconductance of IINP (3mA/V typ), and R8 is the resistor connected between IINP and ground. Typically, IINP has a 0 to 3.5V output voltage range. Leave IINP open if not used.
LDO Regulator
An integrated low-dropout (LDO) linear regulator provides a 5.4V supply derived from DCIN, and delivers over 30mA of load current. The LDO powers the gate drivers of the n-channel MOSFETs. See the MOSFET Drivers section. LDO has a minimum current limit of 35mA. This allows the MAX8731A to work with 87nC of total gate charge (both high-side and low-side MOSFETs). Bypass LDO to PGND with a 1F or greater ceramic capacitor.
AC Adapter Detection
The MAX8731A includes a hysteretic comparator that detects the presence of an AC power adapter. When ACIN is greater than 2.048V, the open-drain ACOK output becomes high impedance. Connect 10k pullup resistance between LDO and ACOK. Use a resistive voltage-divider from the adapter's output to the ACIN pin to set the appropriate detection threshold. Select the resistive voltage-divider not to exceed the 6V absolute maximum rating of ACIN.
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19
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
SMBus Interface
The MAX8731A receives control inputs from the SMBus interface. The MAX8731A uses a simplified subset of the commands documented in System Management Bus Specification V1.1, which can be downloaded from www.smbus.org. The MAX8731A uses the SMBus Read-Word and Write-Word protocols (Figure 3) to communicate with the smart battery. The MAX8731A performs only as an SMBus slave device with address 0b0001001_ (0x12) and does not initiate communication on the bus. In addition, the MAX8731A has two identification (ID) registers (0xFE): a 16-bit device ID register and a 16-bit manufacturer ID register (0xFF). The data (SDA) and clock (SCL) pins have Schmitt-trigger inputs that can accommodate slow edges. Choose pullup resistors (10k) for SDA and SCL to achieve rise times according to the SMBus specifications. Communication starts when the master signals a START condition, which is a high-to-low transition on SDA, while SCL is high. When the master has finished communicating, the master issues a STOP condition, which is a low-to-high transition on SDA, while SCL is high. The bus is then free for another transmission. Figures 4 and 5 show the timing diagram for signals on the SMBus interface. The address byte, command byte, and data bytes are transmitted between the START and STOP conditions. The SDA state changes only while SCL is low, except for the START and STOP conditions. Data is transmitted in 8-bit bytes and is sampled on the rising edge of SCL. Nine clock cycles are required to transfer each byte in or out of the MAX8731A because either the master or the slave acknowledges the receipt of the correct byte during the ninth clock cycle. The MAX8731A supports the charger commands as described in Table 4.
a) WRITE-WORD FORMAT S SLAVE W ADDRESS 7 BITS 1b MSB LSB 0 PRESET TO 0b0001001 ACK 1b 0 COMMAND BYTE 8 BITS MSB LSB ChargerMode() = 0x12 ChargeCurrent() = 0x14 ChargeVoltage() = 0x15 AlarmWarning() = 0x16 InputCurrent() = 0x3F ACK 1b 0 LOW DATA ACK BYTE 8 BITS 1b MSB LSB 0 D7 D0 HIGH DATA ACK P BYTE 8 BITS 1b MSB LSB 0 D15 D8
b) READ-WORD FORMAT S SLAVE W ADDRESS 7 BITS 1b MSB LSB 0 Preset to 0b0001001 ACK 1b 0
COMMAND ACK BYTE 8 BITS 1b MSB LSB 0 ChargerSpecInfo() = 0x11 ChargerStatus() = 0x13
S
SLAVE ADDRESS 7 BITS MSB LSB PRESET TO 0b0001001
R 1b 1
ACK 1b 0
LOW DATA ACK BYTE 8 BITS 1b MSB LSB 0 D7 D0
HIGH DATA NACK P BYTE 8 BITS 1b MSB LSB 1 D15 D8
LEGEND: S = START CONDITION OR REPEATED START CONDITION ACK = ACKNOWLEDGE (LOGIC-LOW) W = WRITE BIT (LOGIC-LOW) MASTER TO SLAVE SLAVE TO MASTER
P = STOP CONDITION NACK = NOT ACKNOWLEDGE (LOGIC-HIGH) R = READ BIT (LOGIC-HIGH)
Figure 3. SMBus Write-Word and Read-Word Protocols
20
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
A tLOW B tHIGH C D E F G H I J K L M
SMBCLK
SMBDATA
tSU:STA
tHD:STA
tSU:DAT
tHD:DAT
tHD:DAT
tSU:STO tBUF J = ACKNOWLEDGE CLOCKED INTO MASTER K = ACKNOWLEDGE CLOCK PULSE L = STOP CONDITION, DATA EXECUTED BY SLAVE M = NEW START CONDITION
A = START CONDITION B = MSB OF ADDRESS CLOCKED INTO SLAVE C = LSB OF ADDRESS CLOCKED INTO SLAVE D = R/W BIT CLOCKED INTO SLAVE E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER G = MSB OF DATA CLOCKED INTO SLAVE H = LSB OF DATA CLOCKED INTO SLAVE I = SLAVE PULLS SMBDATA LINE LOW
Figure 4. SMBus Write Timing
A tLOW
B tHIGH
C
D
E
F
G
H
I
J
K
SMBCLK
SMBDATA
tSU:STA tHD:STA A = START CONDITION B = MSB OF ADDRESS CLOCKED INTO SLAVE C = LSB OF ADDRESS CLOCKED INTO SLAVE D = R/W BIT CLOCKED INTO SLAVE
tSU:DAT
tHD:DAT E = SLAVE PULLS SMBDATA LINE LOW F = ACKNOWLEDGE BIT CLOCKED INTO MASTER G = MSB OF DATA CLOCKED INTO MASTER H = LSB OF DATA CLOCKED INTO MASTER
tSU:DAT
tSU:STO
tBUF
I = ACKNOWLEDGE CLOCK PULSE J = STOP CONDITION K = NEW START CONDITION
Figure 5. SMBus Read Timing
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21
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
Table 4. Battery-Charger Command Summary
COMMAND 0x14 0x15 0x3F 0xFE 0xFF COMMAND NAME ChargeCurrent() ChargeVoltage() InputCurrent() ManufacturerID() DeviceID() READ/WRITE Write Only Write Only Write Only Read Only Read Only DESCRIPTION 6-Bit Charge-Current Setting 11-Bit Charge-Voltage Setting 6-Bit Charge-Current Setting Manufacturer ID Device ID POR STATE 0x0000 0x0000 0x0080 0x004D 0x0008
Battery-Charger Commands
The MAX8731A supports four battery-charger commands that use either Write-Word or Read-Word protocols, as summarized in Table 4. ManufacturerID() and DeviceID() can be used to identify the MAX8731A. On the MAX8731A, the ManufacturerID() command always returns 0x004D and the DeviceID() command always returns 0x0008.
DC-DC Converter
The MAX8731A employs a synchronous step-down DCDC converter with an n-channel high-side MOSFET switch and an n-channel low-side synchronous rectifier. The MAX8731A features a pseudo-fixed-frequency, current-mode control scheme with cycle-by-cycle current limit. The controller's constant off-time (tOFF) is calculated based on VCSSP, VCSIN, and a time constant with a minimum value of 300ns. The MAX8731A can also operate in discontinuous-conduction mode for improved light-load efficiency. The operation of the DC-DC controller is determined by the following four comparators as shown in the functional diagrams in Figures 2 and 6:
FBS_ ChargeVoltage ( ) +200mV CSI IMAX 2V R Q DH DRIVER OVP
The IMIN comparator triggers a pulse in discontinuous mode when the accumulated error is too high. IMIN compares the control signal (LVC) against 100mV (typ). When LVC is less than 100mV, DHI and DLO are both forced low. Indirectly, IMIN sets the peak inductor current in discontinuous mode. The CCMP comparator is used for current-mode regulation in continuous-conduction mode. CCMP compares LVC against the inductor current. The high-side MOSFET on-time is terminated when the CSI voltage is higher than LVC. The IMAX comparator provides a secondary cycle-bycycle current limit. IMAX compares CSI to 2V (corresponding to 10A when RS2 = 10m). The high-side MOSFET on-time is terminated when the current-sense signal exceeds 10A. A new cycle cannot start until the IMAX comparator's output goes low. The ZCMP comparator provides zero-crossing detection during discontinuous conduction. ZCMP compares the current-sense feedback signal to 750mA (RS2 = 10m). When the inductor current is lower than the 750mA threshold, the comparator output is high and DLO is turned off. The OVP comparator is used to prevent overvoltage at the output due to battery removal. OVP compares FBS_ against the set voltage (ChargeVoltage()). When FBS_ is 200mV above the set value, the OVP comparator output goes high and the high-side MOSFET on-time is terminated. DHI and DLO remain off until the OVP condition is removed.
CCMP LVC
CCV, CCI, CCS, and LVC Control Blocks
DL DRIVER
IMIN 100mV
S
Q
ZCMP 150mV DCIN CSIN OFF-TIME COMPUTE OFF-TIME ONE-SHOT
Figure 6. DC-DC Converter Functional Diagram 22
The MAX8731A controls input current (CCS control loop), charge current (CCI control loop), or charge voltage (CCV control loop), depending on the operating condition. The three control loops--CCV, CCI, and CCS--are brought together internally at the lowest voltage-clamp (LVC) amplifier. The output of the LVC amplifier is the feedback control signal for the DC-DC controller. The minimum voltage at the CCV, CCI, or CCS appears at the output of the LVC amplifier and clamps the other control loops to within 0.3V above the
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SMBus Level 2 Battery Charger with Remote Sense
control point. Clamping the other two control loops close to the lowest control loop ensures fast transition with minimal overshoot when switching between different control loops (see the Compensation section). IMIN must be high, IMAX must be low, and OVP must be low for the controller to initiate a new cycle. If the peak inductor current exceeds the IMAX comparator threshold or the output voltage exceeds the OVP threshold, then the on-time is terminated. The cycle-bycycle current limit effectively protects against overcurrent and short-circuit faults. If during the off-time the inductor current goes to zero, the ZCMP comparator output pulls high, turning off the low-side MOSFET. Both the high- and low-side MOSFETs are turned off until another cycle is ready to begin. ZCOMP causes the MAX8731A to enter into discontinuous-conduction mode (see the Discontinuous Conduction section). There is a 0.3s minimum off-time when the (VCSSP VCSIN) differential becomes too small. If VCSIN 0.88 x VCSSP, the threshold for the 0.3s minimum off-time is reached. The switching frequency in this mode varies according to the equation: f= 1 L x IRIPPLE + 0.3s VCSSN - VBATT
MAX8731A
Continuous-Conduction Mode
With sufficient charge current, the MAX8731A's inductor current never crosses zero, which is defined as continuous-conduction mode. The regulator switches at 400kHz (nominal) if VCSIN < 0.88 x VCSSP. The controller starts a new cycle by turning on the high-side MOSFET and turning off the low-side MOSFET. When the charge-current feedback signal (CSI) is greater than the control point (LVC), the CCMP comparator output goes high and the controller initiates the off-time by turning off the high-side MOSFET and turning on the low-side MOSFET. The operating frequency is governed by the off-time and is dependent upon VCSIN and VCSSP. The off-time is set by the following equation:
-V V t OFF = 2.5s x CSSP CSIN VCSSP
The on-time can be determined using the following equation: t ON = where: V xt IRIPPLE = BATT OFF L The switching frequency can then be calculated: fSW = 1 t ON + t OFF L x IRIPPLE VCSSN - VBATT
Discontinuous Conduction
The MAX8731A can also operate in discontinuous-conduction mode to ensure that the inductor current is always positive. The MAX8731A enters discontinuousconduction mode when the output of the LVC control point falls below 100mV. This corresponds to peak inductor current = 500mA: 100mV ICHG = 1 x = 250mA 2 20 x RS2 charge current for RS2 = 10m. In discontinuous mode, a new cycle is not started until the LVC voltage rises above 100mV. Discontinuousmode operation can occur during conditioning charge of overdischarged battery packs, when the charge current has been reduced sufficiently by the CCS control loop, or when the charger is in constant-voltage mode with a nearly full battery pack.
These equations describe the controller's pseudofixed-frequency performance over the most common operating conditions. At the end of the fixed off-time, the controller initiates a new cycle if the control point (LVC) is greater than 100mV and the peak charge current is less than the cycle-by-cycle current limit. Restated another way,
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
Compensation
The charge-voltage and charge-current regulation loops are independent and compensated separately at the CCV, CCI, and CCS. CCV Loop Compensation The simplified schematic in Figure 7 is sufficient to describe the operation of the MAX8731A when the voltage loop (CCV) is in control. The required compensation network is a pole-zero pair formed with CCV and RCV. The zero is necessary to compensate the pole formed by the output capacitor and the load. RESR is the equivalent series resistance (ESR) of the charger output capacitor (COUT). RL is the equivalent charger output load, where RL = VBATT / ICHG. The equivalent output impedance of the GMV amplifier, ROGMV, is greater than 10M. The voltage amplifier transconductance, GMV = 0.125A/mV. The DC-DC converter transconductance is dependent upon the charge-current sense resistor RS2: GMOUT = 1 A CSI x RS2
where ACSI = 20V/V, and RS2 = 10m in the typical application circuits, so GMOUT = 5A/V. The loop-transfer function is given by: LTF = GMOUT x RL x GMV x ROGMV (1 + sCOUT x RESR )(1 + sCCV x RCV ) x (1 + sCCV x ROGMV )(1 + sCOUT x RL ) The poles and zeros of the voltage loop-transfer function are listed from lowest frequency to highest frequency in Table 5.
FBS_ GMOUT RESR COUT GMV RCV CCV ROGMV ChargeVoltage( ) RL
CCV
Near crossover CCV is much lower impedance than ROGMV. Since CCV is in parallel with ROGMV, CCV dominates the parallel impedance near crossover. Additionally, RCV is much higher impedance than CCV and dominates the series combination of RCV and CCV, so near crossover: ROGMV x (1+ sCCV x RCV ) RCV (1+ sCCV x ROGMV )
Figure 7. CCV Loop Diagram
Table 5. CCV Loop Poles and Zeros
NAME CCV Pole EQUATION DESCRIPTION Lowest frequency pole created by CCV and GMV's finite output resistance.
f P _ CV =
1 2ROGMV x CCV
CCV Zero
f Z _ CV =
1 2RCV x CCV
Voltage-loop compensation zero. If this zero is at the same frequency or lower than the output pole fP_OUT, then the loop-transfer function approximates a single-pole response near the crossover frequency. Choose CCV to place this zero at least 1 decade below crossover to ensure adequate phase margin. Output pole formed with the effective load resistance RL and the output capacitance COUT. RL influences the DC gain but does not affect the stability of the system or the crossover frequency. Output ESR Zero. This zero can keep the loop from crossing unity gain if fZ_OUT is less than the desired crossover frequency; therefore, choose a capacitor with an ESR zero greater than the crossover frequency.
Output Pole
f P _ OUT =
1 2RL x COUT 1 2RL x COUT
Output Zero
f P _ OUT =
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SMBus Level 2 Battery Charger with Remote Sense
C OUT is also much lower impedance than RL near crossover so the parallel impedance is mostly capacitive and: 1 RL (1+ sCOUT x RL ) sCOUT If RESR is small enough, its associated output zero has a negligible effect near crossover and the loop-transfer function can be simplified as follows: RCV LTF = GMOUT x GMV sCOUT Setting LTF = 1 to solve for the unity-gain frequency yields: fCO _ CV = GMOUT x GMV x RCV 2 x COUT GMOUT = 5A/V COUT = 2 x 10F FOSC = 400kHz RL = 0.2 FCO_CV = 50kHz To ensure that the compensation zero adequately cancels the output pole, select fZ_CV fP_OUT: CCV (RL / RCV) COUT C CV 400pF (assuming 2 cells and 2A maximum charge current.) Figure 8 shows the Bode plot of the voltage-loop frequency response using the values calculated above. CCI Loop Compensation The simplified schematic in Figure 9 is sufficient to describe the operation of the MAX8731A when the battery current loop (CCI) is in control. Since the output capacitor's impedance has little effect on the response of the current loop, only a simple single pole is required to compensate this loop. ACSI is the internal gain of the current-sense amplifier. RS2 is the charge currentsense resistor (10m). ROGMI is the equivalent output impedance of the GMI amplifier, which is greater than 10M. GMI is the charge-current amplifier transconductance = 1A/mV. GMOUT is the DC-DC converter transconductance = 5A/V.
MAX8731A
For stability, choose a crossover frequency lower than 1/10 the switching frequency. For example, choose a crossover frequency of 50kHz and solve for RVC using the component values listed in Figure 1 to yield RCV = 10k: RCV = 2 x COUT x fCO _ CV GMV x GMOUT 10k
GMV = 0.125A/mV
80 60 40 20 0 -20 -40 0.1 1 10 100 1k 10k 100k FREQUENCY (Hz) MAG PHASE
0
CSIP GMOUT RS2 CSIN
-45
PHASE (DEGREES)
MAGNITUDE (dB)
CSI
-90
CCI GMI
-135 1M
CCI
ROGMI ChargeCurrent( )
Figure 8. CCV Loop Response
Figure 9. CCI Loop Diagram
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SMBus Level 2 Battery Charger with Remote Sense MAX8731A
The loop-transfer function is given by: LTF = GMOUT x A CSI x RS2 x GMI ROGMI 1+ sROGMI x CCI CCS Loop Compensation The simplified schematic in Figure 11 is sufficient to describe the operation of the MAX8731A when the input current-limit loop (CCS) is in control. Since the output capacitor's impedance has little effect on the response of the input current-limit loop, only a single pole is required to compensate this loop. ACSS is the internal gain of the current-sense resistor; RS1 = 10m in the typical application circuits. ROGMS is the equivalent output impedance of the GMS amplifier, which is greater than 10M. GMS is the charge-current amplifier transconductance = 1A/mV. GMIN is the DC-DC converter's input-referred transconductance = (1/D) x GMOUT = (1 / D) x 5A/V. The loop-transfer function is given by: LTF = GMIN x ACSS x RSI x GMS Since: ROGMS 1 + SROGMS x CCS
This describes a single-pole system. Since: 1 GMOUT = A CSI x RS2 the loop-transfer function simplifies to: LTF = GMI ROGMI 1+ sROGMI x CCI
The crossover frequency is given by: GMI fCO _ CI = 2CCI For stability, choose a crossover frequency lower than 1/10 the switching frequency: CCI > 10 x GMI / (2 fOSC) = 4nF, for a 400kHz switching frequency. Values for CCI greater than 10 times the minimum value can slow down the current-loop response. Choosing CCI = 10nF yields a crossover frequency of 15.9kHz. Figure 10 shows the Bode plot of the current-loop frequency response using the values calculated above.
GMIN =
1 A CSS x RS2
the loop-transfer function simplifies to: LTF = GMS ROGMS 1 + SROGMS x CCS
100 80 MAGNITUDE (dB) 60 40 MAG PHASE
0
ADAPTER INPUT CSSP InputCurrent( ) CSS RS1 CSSI GMS
-45 20 0 -20 -40 0.1 10 1k FREQUENCY (Hz) 100k -90 CCS
GMIN CCS ROGMS
SYSTEM LOAD
Figure 10. CCI Loop Response
Figure 11. CCS Loop Diagram
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SMBus Level 2 Battery Charger with Remote Sense
The crossover frequency is given by: fCO _ CS = GMS 2CCS the device). Unlike the DLO output, the DHI output uses a 50ns (typ) delay time to prevent the low-side MOSFET from turning on until DHI is fully off. The same considerations should be used for routing the DHI signal to the high-side MOSFET. The high-side driver (DHI) swings from LX to 5V above LX (BST) and has a typical impedance of 3 sourcing and 1 sinking. The low-side driver (DLO) swings from DLOV to ground and has a typical impedance of 1 sinking and 3 sourcing. This helps prevent DLO from being pulled up when the high-side switch turns on, due to capacitive coupling from the drain to the gate of the low-side MOSFET. This places some restrictions on the MOSFETs that can be used. Using a low-side MOSFET with smaller gate-to-drain capacitance can prevent these problems.
MAX8731A
For stability, choose a crossover frequency lower than 1/10 the switching frequency: CCS = 5 x GMS /(2fOSC ) Choosing a crossover frequency of 30kHz and using the component values listed in Figure 1 yields CCS > 5.4nF. Values for CCS greater than 10 times the minimum value may slow down the current-loop response excessively. Figure 12 shows the Bode plot of the input current-limit-loop frequency response using the values calculated above.
Design Procedure
MOSFET Selection
Choose the n-channel MOSFETs according to the maximum required charge current. The MOSFETs must be able to dissipate the resistive losses plus the switching losses at both VDCIN(MIN) and VDCIN(MAX). For the high-side MOSFET, the worst-case resistive power losses occur at the maximum battery voltage and minimum supply voltage: PDCONDUCTION(HighSide) = VFBS _ VCSSP x ICHG2 x RDS(ON)
MOSFET Drivers
The DHI and DLO outputs are optimized for driving moderate-sized power MOSFETs. The MOSFET drive capability is the same for both the low-side and highsides switches. This is consistent with the variable duty factor that occurs in the notebook computer environment where the battery voltage changes over a wide range. There must be a low-resistance, low-inductance path from the DLO driver to the MOSFET gate to prevent shoot-through. Otherwise, the sense circuitry in the MAX8731A interprets the MOSFET gate as "off" while there is still charge left on the gate. Use very short, wide traces measuring 10 to 20 squares or less (1.25mm to 2.5mm wide if the MOSFET is 25mm from
100 80 MAGNITUDE (dB) 60 40 -45 20 0 -20 -40 0.1 10 1k FREQUENCY (Hz) 100k -90 10M MAG PHASE PHASE (DEGREES) 0
Generally a low-gate charge high-side MOSFET is preferred to minimize switching losses. However, the RDS(ON) required to stay within package power-dissipation limits often limits how small the MOSFET can be. The optimum occurs when the switching (AC) losses equal the conduction (RDS(ON)) losses. Calculating the power dissipation in N1 due to switching losses is difficult since it must allow for difficult quantifying factors that influence the turn-on and turn-off times. These factors include the internal gate resistance, gate charge, threshold voltage, source inductance, and PCB layout characteristics. The following switching-loss calculation provides a rough estimate and is no substitute for breadboard evaluation, preferably including a verification using a thermocouple mounted on N1:
PDSWITCHING (High Side) = 1 x t Trans x VDCIN x ICHG x fSW 2
Figure 12. CCS Loop Response
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27
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
where tTRANS is the driver's transition time and can be calculated as follows:
1 1 2QG t TRANS = + , and fSW 400kHz x IGsrc IGsnk IGATE
The total power low-side MOSFET dissipation is: PDTOTAL (Low Side) PDCONDUCTION(Low Side) + PDBDY (Low Side) These calculations provide an estimate and are not a substitute for breadboard evaluation, preferably including a verification using a thermocouple mounted on the MOSFET.
IGATE is the peak gate-drive current. The following is the power dissipated due to the highside n-channel MOSFET's output capacitance (CRSS): V2 x CRSS x fSW PDCOSS (HighSide) DCIN 2 The following high-side MOSFET's loss is due to the reverse-recovery charge of the low-side MOSFET's body diode: PDQRR(HighSide) = QRR2 x VDCIN x fSW x 0.5 Ignore PDQRR (HighSide) if a Schottky diode is used parallel to the low-side MOSFET. The total high-side MOSFET power dissipation is: PDTOTAL (HighSide) PDCONDUCTION(HighSide) + PDSWITCHING (HighSide) + PDCOSS (HighSide) +PDQRR (HighSide) Switching losses in the high-side MOSFET can become an insidious heat problem when maximum AC adapter voltages are applied. If the high-side MOSFET chosen for adequate RDS(ON) at low-battery voltages becomes hot when biased from V IN(MAX), consider choosing another MOSFET with lower parasitic capacitance. For the low-side MOSFET (N2), the worst-case power dissipation always occurs at maximum input voltage: VFBS _ PDCONDUCTION(Low Side) = 1- VCSSP x ICHG2 x RDS(ON) The following additional loss occurs in the low-side MOSFET due to the body diode conduction losses:
PDBDY (Low Side) = 0.05 x IPEAK x 0.4V
Inductor Selection
The charge current, ripple, and operating frequency (off-time) determine the inductor characteristics. For optimum efficiency, choose the inductance according to the following equation: V xt L = BATT OFF 0.3 x ICHG This sets the ripple current to 1/3 the charge current and results in a good balance between inductor size and efficiency. Higher inductor values decrease the ripple current. Smaller inductor values save cost but require higher saturation current capabilities and degrade efficiency. Inductor L1 must have a saturation current rating of at least the maximum charge current plus 1/2 the ripple current (IL): ISAT = ICHG + (1/2) IL The ripple current is determined by: IL = VBATT x tOFF / L where: tOFF = 2.5s (VDCIN - VBATT) / VDCIN for VBATT < 0.88 VDCIN or during dropout: tOFF = 0.3s for VBATT > 0.88 VDCIN
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SMBus Level 2 Battery Charger with Remote Sense
Input-Capacitor Selection
The input capacitor must meet the ripple current requirement (IRMS) imposed by the switching currents. Nontantalum chemistries (ceramic, aluminum, or OS-CON) are preferred due to their resilience to powerup surge currents: V BATT ( VDCIN - VBATT ) IRMS = ICHG VDCIN The input capacitors should be sized so that the temperature rise due to ripple current in continuous conduction does not exceed approximately +10C. The maximum ripple current occurs at 50% duty factor or VDCIN = 2 x VBATT, which equates to 0.5 x ICHG. If the application of interest does not achieve the maximum value, size the input capacitors according to the worst-case conditions.
AC-TO-DC CONVERTER (ADAPTER) SYSTEM POWER SUPPLIES
MAX8731A
MAX8731A SMART-BATTERY CHARGER/ POWER-SOURCE SELECTOR SMBus CONTROL SIGNALS FOR BATTERY
BATT+ SMART BATTERY
BATT-
SMBus CONTROL SIGNALS FOR BATTERY
SYSTEM HOST (KEYBOARD CONTROLLER)
Output-Capacitor Selection
The output capacitor absorbs the inductor ripple current and must tolerate the surge current delivered from the battery when it is initially plugged into the charger. As such, both capacitance and ESR are important parameters in specifying the output capacitor as a filter and to ensure stability of the DC-DC converter (see the Compensation section). Beyond the stability requirements, it is often sufficient to make sure that the output capacitor's ESR is much lower than the battery's ESR. Either tantalum or ceramic capacitors can be used on the output. Ceramic devices are preferable because of their good voltage ratings and resilience to surge currents.
Figure 13. Typical Smart-Battery System
configurations and chemistries. Microcontroller programs can perform frequent tests on the battery's state of charge and dynamically change the voltage and current applied to enhance safety. Multiple batteries can also be utilized with a selector that is programmable over the SMBus.
Setting Input-Current Limit
The input-current limit should be set based on the current capability of the AC adapter and the tolerance of the input-current limit. The upper limit of the input current threshold should never exceed the adapter's minimum available output current. For example, if the adapter's output current rating is 5A 10%, the input current limit should be selected so that its upper limit is less than 5A x 0.9 = 4.5A. Since the input current-limit accuracy of the MAX8731A is 3%, the typical value of the input-current limit should be set at 4.5A / 1.03 4.36A. The lower limit for input current must also be considered. For chargers at the low end of the spec, the input-current limit for this example could be 4.36A x 0.95, or approximately 4.14A.
Applications Information
Smart-Battery System Background Information
Smart-battery systems have evolved since the conception of the smart-battery system (SBS) specifications. Originally, such systems consisted of a smart battery and smart-battery charger that were compatible with the SBS specifications and communicated directly with one another using SMBus protocols. Modern systems still employ the original commands and protocols, but often use a keyboard controller or similar digital intelligence to mediate the communication between the battery and the charger (Figure 13). This arrangement permits considerable freedom in the implementation of charging algorithms at the expense of standardization. Algorithms can vary from the simple detection of the battery with a fixed set of instructions for charging the battery to highly complex programs that can accommodate multiple battery
Layout and Bypassing
Bypass DCIN with a 1F ceramic to ground (Figure 1). D1 protects the MAX8731A when the DC power source input is reversed. Bypass VDD, DCIN, LDO, VCC, DAC, and REF as shown in Figure 1.
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SMBus Level 2 Battery Charger with Remote Sense
Good PCB layout is required to achieve specified noise immunity, efficiency, and stable performance. The PCB layout artist must be given explicit instructions--preferably, a sketch showing the placement of the powerswitching components and high-current routing. Refer to the PCB layout in the MAX8731A evaluation kit for examples. A ground plane is essential for optimum performance. In most applications, the circuit will be located on a multilayer board, and full use of the four or more copper layers is recommended. Use the top layer for high-current connections, the bottom layer for quiet connections, and the inner layers for uninterrupted ground planes. Use the following step-by-step guide: 1) Place the high-power connections first, with their grounds adjacent: a) Minimize the current-sense resistor trace lengths, and ensure accurate current sensing with Kelvin connections. b) Minimize ground trace lengths in the high-current paths. c) Minimize other trace lengths in the high-current paths. Use > 5mm wide traces in the high-current paths. d) Connect C1 and C2 to high-side MOSFET (10mm max length). Place the input capacitor between the input current-sense resistor and drain of the high-side MOSFET. e) Minimize the LX node (MOSFETs, rectifier cathode, inductor (15mm max length)). Keep LX on one side of the PCB to reduce EMI radiation. f) Since the return path of DHI is LX, route DHI near LX. Optimally, LX and DHI should overlap. The same principle is applied to DLO and PGND.
MAX8731A
not go through vias. The resulting top-layer subground plane is connected to the normal innerlayer ground plane at the paddle. Other high-current paths should also be minimized, but focusing primarily on short ground and currentsense connections eliminates approximately 90% of all PCB layout problems. 2) Place the IC and signal components. Keep the main switching node (LX node) away from sensitive analog components (current-sense traces and REF capacitor). Important: The IC must be no further than 10mm from the current-sense resistors. Quiet connections to REF, CCS, DAC, CCV, CCI, ACIN, and VCC should be returned to a separate ground (GND) island. The analog ground is separately worked from power ground in Figure 1. There is very little current flowing in these traces, so the ground island need not be very large. When placed on an inner layer, a sizable ground island can help simplify the layout because the low-current connections can be made through vias. The ground pad on the backside of the package should also be connected to this quiet ground island. 3) Keep the gate-drive traces (DHI and DLO) as short as possible (L < 20mm), and route them away from the current-sense lines and REF. These traces should also be relatively wide (W > 1.25mm). 4) Place ceramic bypass capacitors close to the IC. The bulk capacitors can be placed further away. Place the current-sense input filter capacitors under the part, connected directly to the GND pin. 5) Use a single-point star ground placed directly below the part at the PGND pin. Connect the power ground (ground plane) and the quiet ground island at this location.
g) Ideally, surface-mount power components are flush against one another with their ground terminals almost touching. These high-current grounds are then connected to each other with a wide, filled zone of top-layer copper, so they do
Chip Information
TRANSISTOR COUNT: 10,234 PROCESS: BiCMOS
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SMBus Level 2 Battery Charger with Remote Sense
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
MAX8731A
PACKAGE OUTLINE, 16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm
21-0140
K
1
2
______________________________________________________________________________________
QFN THIN.EPS
31
SMBus Level 2 Battery Charger with Remote Sense MAX8731A
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
PACKAGE OUTLINE, 16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm
21-0140
K
2
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
32 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products. Inc.

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