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19-1182; Rev 2; 12/98
UAL IT MAN TION K SHEET VALUA E TA WS DA FOLLO
Switch-Mode Lithium-Ion Battery-Charger
____________________________Features
o Charges 1 to 4 Lithium-Ion Battery Cells o 0.75% Voltage-Regulation Accuracy Using 1% Resistors o Provides up to 4A without Excessive Heating o 90% Efficient o Uses Low-Cost Set Resistors and N-Channel Switch o Up to 24V Input o Up to 18V Maximum Battery Voltage o 300kHz PWM Operation: Low-Noise, Small Components o Stand-Alone Operation; No Microcontroller Needed
General Description
The MAX745 provides all functions necessary for charging lithium-ion battery packs. It provides a regulated charging current of up to 4A without getting hot, and a regulated voltage with only 0.75% total error at the battery terminals. It uses low-cost, 1% resistors to set the output voltage, and a low-cost N-channel MOSFET as the power switch. The MAX745 regulates the voltage set point and charging current using two loops that work together to transition smoothly between voltage and current regulation. The per-cell battery voltage regulation limit is set between 4.0V and 4.4V using standard 1% resistors, and then the number of cells is set from 1 to 4 by pinstrapping. Total output voltage error is less than 0.75%. For a similar device with an SMBusTM microcontroller interface and the ability to charge NiCd and NiMH cells, refer to the MAX1647 and MAX1648. For a low-cost lithium-ion charger using a linear-regulator control scheme, refer to the MAX846A.
MAX745
________________________Applications
Lithium-Ion Battery Packs Desktop Cradle Chargers Cellular Phones Notebook Computers Hand-Held Instruments
Pin Configuration appears on last page.
Ordering Information
PART MAX745C/D MAX745EAP TEMP. RANGE 0C to +70C -40C to +85C PIN-PACKAGE Dice* 20 SSOP
*Dice are tested at TA = +25C.
___________________________________________________Typical Operating Circuit
VIN (UP TO 24V) DCIN VL BST DHI N
CELL COUNT SELECT ON OFF
CELL0 CELL1 THM/SHDN REF SETI VADJ
MAX745
LX
DLO
N ICHARGE
CS RSENSE
SET PER CELL VOLTAGE WITH 1% RESISTORS
STATUS BATT CCV CCI GND IBAT PGND VOUT 1-4 Li+ CELLS (UP TO 18V)
SMBus is a trademark of Intel Corp.
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769.
Switch-Mode Lithium-Ion Battery Charger MAX745
ABSOLUTE MAXIMUM RATINGS
DCIN to GND ............................................................-0.3V to 26V BST, DHI to GND ......................................................-0.3V to 30V BST to LX ....................................................................-0.3V to 6V DHI to LX............................................(LX - 0.3V) to (BST + 0.3V) LX to GND ................................................-0.3V to (DCIN + 0.3V) VL to GND...................................................................-0.3V to 6V CELL0, CELL1, IBAT, STATUS, CCI, CCV, REF, SETI, VADJ, DLO, THM/SHDN to GND ..-0.3V to (VL + 0.3V) BATT, CS to GND .....................................................-0.3V to 20V PGND to GND..........................................................-0.3V to 0.3V VL Current ...........................................................................50mA Continuous Power Dissipation (TA = +70C) SSOP (derate 8.00mW/C above +70C) ......................640mW Operating Temperature Range ...........................-40C to +85C Storage Temperature.........................................-60C to +150C Lead Temperature (soldering, 10sec) .............................+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 = 18V, VBATT = 8.4V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.) PARAMETER SUPPLY AND REFERENCE DCIN Input Voltage Range DCIN Quiescent Supply Current VL Output Voltage REF Output Voltage REF Output Load Regulation SWITCHING REGULATOR Oscillator Frequency DHI Maximum Duty Cycle DHI On-Resistance DLO On-Resistance BATT Input Current Output high or low Output high or low VL < 3.2V, VBATT = 12V VL > 5.15V, VBATT = 12V VL < 3.2V, VCS = 12V VL > 5.15V, VCS = 12V 4V < VBATT < 16V SETI = VREF (full scale) SETI = 400mV Not including VADJ resistor tolerance With 1% tolerance VADJ resistors 0 1.5 170 14 -0.65 -0.75 185 18 205 22 0.65 0.75 6.0V < VDCIN < 24V, logic inputs = VL 6.0V < VDCIN < 24V, no load TA = +25C 6.0V < VDCIN < 24V 0 < IREF < 1mA 270 89 5.15 4.17 4.16 CONDITIONS MIN 6 4 5.40 4.2 4.2 10 300 93 4 6 7 14 5 500 5 400 19 TYP MAX 24 6 5.65 4.23 4.24 20 330 UNITS V mA V V mV/mA kHz % A A V mV mV %
CS Input Current BATT, CS Input Voltage Range CS to BATT Offset Voltage (Note 1) CS to BATT Current-Sense Voltage Absolute Voltage Accuracy
2
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Switch-Mode Lithium-Ion Battery Charger
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = 18V, VBATT = 8.4V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.) PARAMETER ERROR AMPLIFIERS GMV Amplifier Transconductance GMI Amplifier Transconductance GMV Amplifier Output Current GMI Amplifier Output Current CCI Clamp Voltage with Respect to CCV CCV Clamp Voltage with Respect to CCI CONTROL INPUTS/OUTPUTS CELL0, CELL1 Input Bias Current SETI Input Voltage Range (Note 1) VADJ Adjustment Range SETI, VADJ Input Bias Current VADJ Input Voltage Range THM/SHDN Rising Threshold THM/SHDN Falling Threshold STATUS Output Low Voltage STATUS Output Leakage Current IBAT Output Current vs. Current-Sense Voltage IBAT Compliance Voltage Range Charger in current-regulation mode, STATUS sinking 1mA Charger in voltage-regulation mode, VSTATUS = 5V VIBAT = 2V 0 0.9 2 1.1V < VCCV < 3.5V 1.1V < VCCI < 3.5V 25 25 -1 0 10 -10 0 2.20 2.01 2.3 2.1 10 VREF 2.34 2.19 0.2 1 CONDITIONS MIN TYP 800 200 130 320 80 80 200 200 1 VREF MAX UNITS A/V A/V A A mV mV A V % nA V V V V A A/mV V
MAX745
ELECTRICAL CHARACTERISTICS
(VDCIN = 18V, VBATT = 8.4V, TA = -40C to +85C, unless otherwise noted. Limits over temperature are guaranteed by design.) PARAMETER SUPPLY AND REFERENCE VL Output Voltage REF Output Voltage SWITCHING REGULATOR (Note 1) Oscillator Frequency DHI On-Resistance DLO On-Resistance CS to BATT Full-Scale Current-Sense Voltage Absolute Voltage Accuracy Not including VADJ resistors Note 1: When VSETI = 0V, the battery charger turns off. Output high or low Output high or low 165 -1.0 260 340 7 14 205 1.0 kHz mV % CONDITIONS 6.0V < VDCIN < 24V, no load 6.0V < VDCIN < 24V MIN 5.10 4.14 TYP MAX 5.70 4.26 UNITS V V
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3
Switch-Mode Lithium-Ion Battery Charger MAX745
__________________________________________Typical Operating Characteristics
(TA = +25C, VDCIN = 18V, VBATT = 4.2V, CELL0 = CELL1 = GND, CVL = 4.7F, CREF = 0.1F. Circuit of Figure 1, unless otherwise noted.)
BATTERY VOLTAGE vs. CHARGING CURRENT
MAX745/TOC-01
CURRENT-SENSE VOLTAGE vs. SETI VOLTAGE
180 CURRENT-SENSE VOLTAGE (mV) 160 140 120 100 80 60 40 20 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 SETI VOLTAGE (V) R1 = 0.2
MAX745/TOC-02
4.5 4.0 3.5 BATTERY VOLTAGE (V) 3.0 2.5 2.0 1.5 1.0 0.5 0 0 R1 = 0.2 R16 = SHORT R12 = OPEN CIRCUIT
200
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 CHARGING CURRENT (A)
REFERENCE VOLTAGE vs. TEMPERATURE
MAX745/TOC-06
VOLTAGE LIMIT vs. VADJ VOLTAGE
4.40 PER-CELL VOLTAGE LIMIT (V) 4.35 4.30 4.25 4.20 4.15 4.10 4.05 4.00 3.95
MAX745/TOC-03
4.205 4.204 REFERENCE VOLTAGE (V) 4.203 4.202 4.201 4.200 4.199 4.198 4.197 4.196 4.195 0 25 50 TEMPERATURE (C) 75
4.45
100
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 VADJ VOLTAGE (V)
VL LOAD REGULATION
MAX745/TOC-04
REFERENCE LOAD REGULATION
4.24 REFERENCE VOLTAGE (V) 4.23 4.22 4.21 4.20 4.19 4.18 4.17 4.16 4.15
MAX745/TOC-05
5.50 5.45 VL OUTPUT VOLTAGE (V) 5.40 5.35 5.30 5.25 5.20 5.15 5.10 5.05 0 0 5 10 15 20
4.25
25
0
500
1000
1500
2000
2500
3000
VL OUTPUT CURRENT (mA)
REFERENCE CURRENT (A)
4
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Switch-Mode Lithium-Ion Battery Charger
______________________________________________________________Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11, 12 NAME IBAT DCIN VL CCV CCI THM/ SHDN REF VADJ SETI GND CELL1, CELL0 STATUS BATT CS PGND DLO DHI LX BST FUNCTION Current-Sense Amplifier's Analog Current-Source Output. See Monitoring Charge Current section for detailed description. Charger Input Voltage. Bypass DCIN with a 0.1F capacitor. Chip Power Supply. Output of the 5.4V linear regulator from DCIN. Bypass VL with a 4.7F capacitor. Voltage-Regulation-Loop Compensation Point Current-Regulation-Loop Compensation Point Thermistor Sense-Voltage Input. THM/SHDN also performs the shutdown function. If pulled low, the charger turns off. 4.2V Reference Voltage Output. Bypass REF with a 0.1F or greater capacitor. Voltage-Adjustment Pin. VADJ is tied to a 1% tolerance external resistor-divider to adjust the voltage set point by 10%, eliminating the need for precision 0.1% resistors. The input voltage range is 0V to VREF. SETI is externally tied to the resistor-divider between REF and GND to set the charging current. Analog Ground Logic Inputs to Select Cell Count. See Table 1 for cell-count programming. An open-drain MOSFET sinks current when in current-regulation mode, and is high impedance when in voltage-regulation mode. Connect STATUS to VL through a 1k to 100k pull-up resistor. STATUS may also drive an LED for visual indication of regulation mode (see MAX745 evaluation kit). Leave STATUS floating if not used. Battery-Voltage-Sense Input and Current-Sense Negative Input Current-Sense Positive Input Power Ground Low-Side Power MOSFET Driver Output High-Side Power MOSFET Driver Output Power Connection for the High-Side Power MOSFET Source Power Input for the High-Side Power MOSFET Driver
MAX745
13 14 15 16 17 18 19 20
_______________Detailed Description
The MAX745 is a switch-mode, lithium-ion battery charger that can achieve 90% efficiency. The charge voltage and current are set independently by external resistor-dividers at SETI and VADJ, and at pin connections at CELL0 and CELL1. VADJ is connected to a resistor-divider to set the charging voltage. The output voltage-adjustment range is 5%, eliminating the need for 0.1% resistors while still achieving 0.75% set accuracy using 1% resistors. The MAX745 consists of a current-mode, pulse-widthmodulated (PWM) controller and two transconductance error amplifiers: one for regulating current (GMI) and the other for regulating voltage (GMV) (Figure 2). The error amplifiers are controlled via the SETI and VADJ pins. Whether the MAX745 is controlling voltage or current at any time depends on the battery state. If the battery is discharged, the MAX745 output reaches the
current-regulation limit before the voltage limit, causing the system to regulate current. As the battery charges, the voltage rises to the point where the voltage limit is reached and the charger switches to regulating voltage. The STATUS pin indicates whether the charger is regulating current or voltage.
Voltage Control
To set the voltage limit on the battery, tie a resistordivider to VADJ from REF. A 0V to V REF change at VADJ sets a 5% change in the battery limit voltage around 4.2V. Since the 0 to 4.2V range on VADJ results in only a 10% change on the voltage limit, the resistordivider's accuracy does not need to be as high as the output voltage accuracy. Using 1% resistors for the voltage dividers typically results in no more than 0.1% degradation in output voltage accuracy. VADJ is internally buffered so that high-value resistors can be used to set the output voltage. When the voltage at VADJ is
5
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Switch-Mode Lithium-Ion Battery Charger MAX745
VREF / 2, the voltage limit is 4.2V. Table 1 defines the battery cell count. The battery limit voltage is set by the following: 1 VREF VADJ - 2 VBATT = cell count x VREF + 9.523 where V REF = 4.2V and cell count is 1, 2, 3, or 4 (Table 1). The voltage-regulation loop is compensated at the CCV pin. Typically, a series-resistor-capacitor combination can be used to form a pole-zero doublet. The pole introduced rolls off the gain starting at low frequencies. The zero of the doublet provides sufficient AC gain at mid-frequencies. The output capacitor (C1) rolls off the mid-frequency gain to below unity. This guarantees stability before encountering the zero introduced by the C1's equivalent series resistance (ESR). The GMV amplifier's output is internally clamped to between onefourth and three-fourths of the voltage at REF.
(
)
Solving for VADJ, we get: 9.523 VBATT VADJ = - 9.023VREF (cell count) Set VADJ by choosing a value for R11 (typically 100k), and determine R3 by: R3 = [1 - (VADJ / VREF)] x R11 (Figure 1)
Current Control
The charging current is set by a combination of the current-sense resistor value and the SETI pin voltage. The current-sense amplifier measures the voltage across the current-sense resistor, between CS and BATT. The current-sense amplifier's gain is 6. The voltage on SETI is buffered and then divided by 4. This voltage is compared to the current-sense amplifier's output. Therefore, full-scale current is accomplished by connecting SETI to REF. The full-scale charging current (IFS) is set by the following: IFS = 185mV / R1 (Figure 1)
Table 1. Cell-Count Programming Table
CELL0 GND VL GND VL CELL1 GND GND VL VL CELL COUNT 1 2 3 4
VIN C5 4.7F
(UP TO 24V) D2 IN4148 VL REF DCIN BST C7 0.1F DHI LX M1A 1/2 IRF7303 L1 22H C6 0.1F
R16 C4 0.1F R3 100k 1%
R15 10k THM/SHDN
MAX745
THM 1 SETI R12 DLO
1/2 IRF7303 M1B
D1 MBRS 340T3
D6 MBRS 340T3 0.2 R1
PGND VADJ C2, 0.1F R2 10k CCV CCI C3 47nF STATUS GND IBAT CS BATT
R11 100k 1%
BATTERY
C1 68F
Figure 1. Standard Application Circuit
6 _______________________________________________________________________________________
Switch-Mode Lithium-Ion Battery Charger
To set currents below full scale without changing R1, adjust the voltage at SETI according to the following formula: ICHG = IFS (VSETI / VREF) A capacitor at CCI sets the current-feedback loop's dominant pole. While the current is in regulation, CCV voltage is clamped to within 80mV of the CCI voltage. This prevents the battery voltage from overshooting when the voltage setting is changed. The converse is true when the voltage is in regulation and the current setting is changed. Since the linear range of CCI or CCV is about 2V (1.5V to 3.5V), the 80mV clamp results in negligible overshoot when the loop switches from voltage regulation to current regulation, or vice versa. RIBAT must be chosen to limit VIBAT to voltages below 2V for the maximum charging current. Connect IBAT to GND if unused.
MAX745
PWM Controller
The battery voltage or current is controlled by a current-mode, PWM DC/DC converter controller. This controller drives two external N-channel MOSFETs, which control power from the input source. The controller sets the switched voltage's pulse width so that it supplies the desired voltage or current to the battery. Total component cost is reduced by using a dual, N-channel MOSFET. The heart of the PWM controller is a multi-input comparator. This comparator sums three input signals to determine the switched signal's pulse width, setting the battery voltage or current. The three signals are the current-sense amplifier's output, the GMV or GMI error amplifier's output, and a slope-compensation signal that ensures that the current-control loop is stable. The PWM comparator compares the current-sense amplifier's output to the lower output voltage of either the GMV or GMI amplifiers (the error voltage). This current-mode feedback reduces the effect of the inductor on the output filter LC formed by the output inductor (L1) and C1 (Figure 1). This makes stabilizing the circuit much easier, since the output filter changes to a first-order RC from a complex, second-order RLC.
Monitoring Charge Current
The battery-charging current can be externally monitored by placing a scaling resistor (R IBAT) between IBAT and GND. IBAT is the output of a voltage-controlled current source, with output current given by: IBAT = 0.9A/VSENSE where VSENSE is the voltage across the current-sense resistor (in millivolts) given by: VSENSE = VCS - VBATT = ICHG x R1 The voltage across RIBAT is then given by: R 0.9A VIBAT = x IBAT ICHG R1
IBAT DCIN
BATT
CS
CURRENT SENSE AV = 6 ON
5.4V REG
4.2 REF
REF
VL
STATUS GMI SETI CCI 1/4 BST DHI PWM LOGIC LX VL DLO PGND
CLAMP
GMV VADJ CCV CELL0 CELL1 CELL LOGIC REF 2
THM/SHDN
GND
Figure 2. Functional Diagram
_______________________________________________________________________________________ 7
Switch-Mode Lithium-Ion Battery Charger MAX745
MOSFET Drivers
The MAX745 drives external N-channel MOSFETs to switch the input source generating the battery voltage or current. Since the high-side N-channel MOSFET's gate must be driven to a voltage higher than the input source voltage, a charge pump is used to generate such a voltage. The capacitor (C7) charges through D2 to approximately 5V when the synchronous rectifier (M1B) turns on (Figure 1). Since one side of C7 is connected to LX (the source of M1A), the high-side driver (DHI) drives the gate up to the voltage at BST, which is greater than the input voltage while the high-side MOSFET is on. The synchronous rectifier (M1B) behaves like a diode but has a smaller voltage drop, improving efficiency. A small dead time is added between the time when the high-side MOSFET is turned off and when the synchronous rectifier is turned on, and vice versa. This prevents crowbar currents during switching transitions. Place a Schottky rectifier from LX to ground (D1, across M1B's drain and source) to prevent the synchronous rectifier's body diode from conducting during the dead time. The body diode typically has slower switchingrecovery times, so allowing it to conduct degrades efficiency. D1 can be omitted if efficiency is not a concern, but the resulting increased power dissipation in the synchronous rectifier must be considered. Since the BST capacitor is charged while the synchronous rectifier is on, the synchronous rectifier may not be replaced by a rectifier. The BST capacitor will not fully charge without the synchronous rectifier, leaving the highside MOSFET with insufficient gate drive to turn on. However, the synchronous rectifier can be replaced with a small MOSFET (such as a 2N7002) to guarantee that the BST capacitor is allowed to charge. In this case, the majority of the high charging currents are carried by D1, and not by the synchronous rectifier.
Minimum Input Voltage
The input voltage to the charger circuit must be greater than the maximum battery voltage by approximately 2V so the charger can regulate the voltage properly. The input voltage can have a large AC-ripple component when operating from a wall cube. The voltage at the low point of the ripple waveform must still be approximately 2V greater than the maximum battery voltage. Using components as indicated in Figure 1, the minimum input voltage can be determined by the following formula: [VBATT + VD6 + ICHG (RDS(ON) + RL + R1)] VIN x 0.89 where: VIN is the input voltage; VD6 is the voltage drop across D6 (typically 0.4V to 0.5V); ICHG is the charging current; RDS(ON) is the high-side MOSFET M1A's on-resistance; RL is the the inductor's series resistance; R1 is the current-sense resistor R1's value.
__________________Pin Configuration
TOP VIEW
IBAT 1 DCIN 2 VL 3 CCV 4 CCI 5 THM/SHDN 6 REF 7 VADJ 8 20 BST 19 LX 18 DHI 17 DLO
MAX745
16 PGND 15 CS 14 BATT 13 STATUS 12 CELL0 11 CELL1
Internal Regulator and Reference
The MAX745 uses an internal low-dropout linear regulator to create a 5.4V power supply (VL), which powers its internal circuitry. The VL regulator can supply up to 25mA. Since 4mA of this current powers the internal circuitry, the remaining 21mA can be used for external circuitry. MOSFET gate-drive current comes from VL, which must be considered when drawing current for other functions. To estimate the current required to drive the MOSFETs, multiply the sum of the MOSFET gate charges by the switching frequency (typically 300kHz). Bypass VL with a 4.7F capacitor to ensure stability. The MAX745 internal 4.2V reference voltage must be bypassed with a 0.1F or greater capacitor.
SETI 9 GND 10
SSOP
___________________Chip Information
TRANSISTOR COUNT: 1695 SUBSTRATE CONNECTED TO GND
8
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