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19-1336; Rev 2; 11/98
NUAL KIT MA ATION ET EVALU TA SHE WS DA FOLLO
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
____________________________Features
o Up to 95% Efficiency o Up to 1.5A Output o Fixed 5V or Adjustable Step-Up Output (2.5V to 5.5V) o 0.7V to 5.5V Input Range o Low-Power Mode (300W) o Low-Noise, Constant-Frequency Mode (300kHz) o Synchronizable Switching Frequency (200kHz to 400kHz) o 1A Logic-Controlled Shutdown o Power-Good Comparator o Uncommitted Gain Block
________________General Description
The MAX1703 is a high-efficiency, low-noise, step-up DC-DC converter intended for use in battery-powered wireless applications. It uses a synchronous-rectified pulse-width-modulation (PWM) boost topology to generate a 2.5V to 5.5V output from battery inputs, such as one to three NiCd/NiMH cells or one Li-Ion cell. The device includes a 2A, 75m, N-channel MOSFET switch and a 140m, P-channel synchronous rectifier. With its internal synchronous rectifier, the MAX1703 delivers up to 5% better efficiency than similar nonsynchronous converters. It also features a pulse-frequencymodulation (PFM) low-power mode to improve efficiency at light loads, and a 1A shutdown mode. The MAX1703 comes in a 16-pin narrow SO package and includes an uncommitted comparator that generates a power-good or low-battery-warning output. It also contains a linear gain block that can be used to build a linear regulator. For lower-power outputs and a smaller package, refer to the MAX1700/MAX1701. For dual outputs (step-up plus linear regulator), refer to the MAX1705/MAX1706. For an on-board analog-to-digital converter, refer to the MAX848/MAX849. The MAX1703 evaluation kit is available to speed designs.
MAX1703
Ordering Information
PART MAX1703ESE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 Narrow SO
________________________Applications
Digital Cordless Phones PCS Phones Wireless Handsets Two-Way Pagers Personal Communicators Palmtop Computers Hand-Held Instruments
Typical Operating Circuit
INPUT 0.7V TO 5.5V
Pin Configuration
TOP VIEW
MAX1703
OFF ON PWM OR PFM POWER-GOOD INPUT GAIN-BLOCK INPUT POKIN AIN REF FB POK AO GND PGND SYNC ON CLK/ SEL OUT LXP, LXN POUT
OUTPUT 5V OR ADJ UP TO 1.5A
REF 1 FB 2 POKIN 3 OUT 4 GND 5
16 ON 15 POUT 14 LXP
MAX1703
13 POUT 12 PGND 11 LXN 10 PGND 9 CLK/SEL
POWER-GOOD OUTPUT GAIN-BLOCK OUTPUT
AIN 6 AO 7 POK 8
Narrow SO
________________________________________________________________ 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.
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
ABSOLUTE MAXIMUM RATINGS
OUT, ON, AO, POK to GND .....................................-0.3V to +6V PGND to GND.....................................................................0.3V LXP, LXN to PGND .................................-0.3V to (VPOUT + 0.3V) POUT, CLK/SEL, AIN, REF, FB, POKIN to GND.......................................-0.3V to (VOUT + 0.3V) Continuous Power Dissipation (TA = +70C) Narrow SO (derate 8.70mW/C above +70C) .............696mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +160C 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
(CLK/SEL = AIN = ON = POKIN = FB = PGND = GND, OUT = POUT, LXP = LXN, VOUT = 5.3V (Note 1), TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER DC-DC CONVERTER Input Supply Range Minimum Start-Up Voltage Frequency in Start-Up Mode Output Voltage (Note 4) FB Regulation Voltage FB Input Current Output Voltage Adjust Range Output Voltage Lockout Threshold Load Regulation (Note 6) Supply Current in Shutdown Supply Current in Low-Power Mode Supply Current in Low-Noise Mode DC-DC SWITCHES POUT, LXP Leakage Current LXN Leakage Current Switch On-Resistance Switch On-Resistance VLXP = 0V, VOUT = V ON = 5.5V V ON = VLXN = VOUT = 5.5V N-channel P-channel N-Channel Current Limit P-Channel Turn-Off Current POWER-GOOD COMPARATOR POKIN Trip Level POKIN Input Current POK Low Voltage POK High Leakage Current 2 Rising VPOKIN VPOKIN = 0.7V ISINK(POK) = 1mA, VOUT = 3.6V or ISINK(POK) = 20A, VOUT = 1V VOUT = VPOK = 5.5V 1.225 -20 0.03 0.01 1.250 1.275 20 0.4 1 V nA V A CLK/SEL = OUT CLK/SEL = GND CLK/SEL = GND 2200 500 20 CLK/SEL = GND CLK/SEL = OUT 0.1 0.1 0.14 0.075 0.13 2700 800 160 20 20 0.25 0.13 0.25 3200 1100 260 mA mA mA A A (Note 5) CLK/SEL = OUT, no load to full load ON = OUT CLK/SEL = GND (Note 1) CLK/SEL = OUT (Note 1) (Note 2) ILOAD < 1mA, TA = +25C (Note 3) VOUT = 1.5V VFB < 0.1V, CLK/SEL = OUT, 0 ILX 1.1A, VBATT = 3.7V Adjustable output, CLK/SEL = OUT, 0 ILX 1.1A, VBATT = 2.2V, VOUT = 3.3V VFB = 1.25V 2.5 2.0 2.15 -1.6 0.1 65 150 20 120 300 40 4.87 1.21 0.7 0.9 140 5.05 1.24 0.1 5.5 1.1 300 5.20 1.255 20 5.5 2.3 V V kHz V V nA V V % A A A CONDITIONS MIN TYP MAX UNITS
_______________________________________________________________________________________
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(CLK/SEL = AIN = ON = POKIN = FB = PGND = GND, OUT = POUT, LXP = LXN, VOUT = 5.3V (Note 1), TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER GAIN BLOCK AIN Reference Voltage AIN Input Current Transconductance AO Output Low Voltage AO Output High Leakage REFERENCE Reference Output Voltage REF Load Regulation REF Supply Rejection LOGIC INPUTS Input Low Voltage Input High Voltage Logic Input Current Internal Oscillator Frequency Oscillator Maximum Duty Cycle External Clock Frequency Range Minimum CLK/SEL Pulse Width Maximum CLK/SEL Rise/Fall Time ON, 1.2V < VOUT < 5.5V (Note 7) CLK/SEL, VOUT = 2.5V ON, 1.2V < VOUT < 5.5V CLK/SEL, VOUT = 5.5V ON, CLK/SEL CLK/SEL = OUT, VFB = 0.5V CLK/SEL = OUT, VFB = 0.5V 0.8VOUT 0.8VOUT -1 260 80 200 200 100 0.01 300 86 1 340 90 400 0.2VOUT 0.2VOUT V V V A kHz % kHz ns ns IREF = 0A -1A < IREF < 50A 2.5V < VOUT < 5.5V 1.237 1.250 5 0.2 1.263 15 5 V mV mV IAO = 20A VAIN = 1.5V 10A < IAO < 100A VAIN = 0.5V, IAO = 100A VAIN = 1.5V, VAO = 5.5V 1.237 -30 5 10 0.1 0.01 1.25 1.263 30 16 0.4 1 V nA mmho V A CONDITIONS MIN TYP MAX UNITS
MAX1703
ELECTRICAL CHARACTERISTICS
(CLK/SEL = AIN = ON = POKIN = FB = PGND = GND, OUT = POUT, LXP = LXN, VOUT = 5.3V (Note 1), TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 8) PARAMETER DC-DC CONVERTER Output Voltage (Note 4) FB Regulation Voltage Output Voltage Lockout Threshold Supply Current in Shutdown Supply Current in Low-Power Mode Supply Current in Low-Noise Mode VFB < 0.1V, CLK/SEL = OUT, 0 ILX 1.1A, VBATT = 3.7V Adjustable output, CLK/SEL = OUT, 0 ILX 1.1A, VOUT = 3.3V, VBATT = 2.2V (Note 5) ON = OUT CLK/SEL = GND (Note 1) CLK/SEL = OUT (Note 1) 4.87 1.20 2.0 5.20 1.27 2.3 20 120 300 V V V A A A CONDITIONS MIN TYP MAX UNITS
_______________________________________________________________________________________
3
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
ELECTRICAL CHARACTERISTICS (continued)
(CLK/SEL = AIN = ON = POKIN = FB = PGND = GND, OUT = POUT, LXP = LXN, VOUT = 5.3V (Note 1), TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 8) PARAMETER DC-DC SWITCHES Switch On-Resistance N-channel P-channel N-Channel Current Limit POWER-GOOD COMPARATOR POKIN Trip Level GAIN BLOCK AIN Reference Voltage Transconductance REFERENCE Reference Output Voltage LOGIC INPUTS Internal Oscillator Frequency Oscillator Maximum Duty Cycle CLK/SEL = OUT, VFB = 0.5V CLK/SEL = OUT, VFB = 0.5V 260 80 340 92 kHz % IREF = 0A 1.23 1.27 V IAO = 20A 10A < IAO < 100A 1.23 5 1.27 16 V mmho Rising VPOKIN 1.225 1.275 V CLK/SEL = OUT CLK/SEL = GND 2200 500 CLK/SEL = GND CLK/SEL = OUT 0.25 0.13 0.25 3600 1100 mA mA CONDITIONS MIN TYP MAX UNITS
Note 1: Supply current from the 5.05V output is measured between the 5.05V output and the OUT pin. This current correlates directly to the actual battery supply current, but is reduced in value according to the step-up ratio and efficiency. Set VOUT = 5.3V to keep the internal switch open when measuring the device operating current. Note 2: Minimum operating voltage. Since the regulator is bootstrapped to the output, once started it will operate down to a 0.7V input. Note 3: Start-up is tested with the circuit of Figure 2. Note 4: In low-power mode (CLK/SEL = GND) the output voltage regulates 1% higher than low-noise mode (CLK/SEL = OUT or synchronized). Note 5: The regulator is in start-up mode until this voltage is reached. Do not apply full-load current below this voltage. Note 6: Load regulation is measured from no-load to full load, where full load is determined by the N-channel switch current limit. Note 7: The ON input has a total hysteresis of approximately 0.15 x VOUT. Note 8: Specifications to -40C are guaranteed by design and not production tested.
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_______________________________________________________________________________________
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
__________________________________________Typical Operating Characteristics
(VIN = +3.6V, VOUT = 5V, TA = +25C, unless otherwise noted.)
MAX1703
EFFICIENCY vs. LOAD CURRENT (VOUT = 5V)
MAX1703-01
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX1703-02
NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE
LOW-POWER MODE BATTERY CURRENT (mA) 3
MAX1703-03
100 90 EFFICIENCY (%) 80 70 60 50 40 0.1 1 10 100
VIN = 3.6V
100 VIN = 2.4V 90 EFFICIENCY (%) 80 70 60 50 40 PFM PWM VIN = 1.2V VIN = 0.9V
4
VIN = 2.4V
VIN = 1.2V
2
TA = +85C
1
TA = +25C TA = -40C 0 1 2 3 4 5 6 INPUT VOLTAGE (V)
PFM PWM 1000 10,000
0 0.1 1 10 100 1000 10,000 LOAD CURRENT (mA)
LOAD CURRENT (mA)
SHUTDOWN SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1703-04
START-UP VOLTAGE vs. LOAD CURRENT
MAX1703-05
REFERENCE VOLTAGE vs. TEMPERATURE
MAX1703-06
2.5
SHUTDOWN CURRENT (A)
2.0
1.6 1.4 1.2 1.0 0.8 0.6
1.5
TA = +85C
REFERENCE VOLTAGE (V) 0.01 0.1 1 10 100 1000
START-UP VOLTAGE (V)
INCLUDES ALL EXTERNAL COMPONENT LEAKAGES. CAPACITOR LEAKAGE DOMINATES AT TA = +85C
2.0 1.8 VOUT = 5V PWM MODE
1.2550
1.2530
1.2510
1.0 TA = +25C, TA = -40C
1.2490
0.5
1.2470
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 INPUT VOLTAGE (V)
1.2450 -40 -20 0 20 40 60 80 100 LOAD CURRENT (mA) TEMPERATURE (C)
REFERENCE VOLTAGE vs. REFERENCE CURRENT
MAX1703-07
NOISE SPECTRUM
8 7 NOISE (mVRMS) 6 5 4 3 2 1 0 VOUT = 5V VIN = 3.6V ILOAD = 500mA
MAX1703-14
1.2520 1.2510 REFERENCE VOLTAGE (V) 1.2500 1.2490 1.2480 1.2470 1.2460 0 10 20 30 40 50 60 70 80 VIN = 3.6V VOUT = 5V LOW-POWER MODE
9
-1 90 100 1k 10k 100k 1M 10M REFERENCE CURRENT (A) FREQUENCY (Hz)
_______________________________________________________________________________________
5
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
_________________________________Typical Operating Characteristics (continued)
(VIN = +3.6V, VOUT = 5V, TA = +25C, unless otherwise noted.)
PEAK INDUCTOR CURRENT LIMIT vs. OUTPUT VOLTAGE
MAX1703-15 MAX1703-16
FREQUENCY vs. TEMPERATURE
340 330 320 CURRENT LIMIT(A) FREQUENCY (kHz) 310 300 290 280 VOUT = 3.3V 270 260 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) 1.0 0.5 2.5 2.5 2.0 1.5 VOUT = 5V 3.5 3.0
PWM
LOW POWER (PFM)
3.0
3.5
4.0
4.5
5.0
5.5
OUTPUT VOLTAGE (V)
HEAVY LOAD SWITCHING
MAX1703-08
LINE-TRANSIENT RESPONSE
MAX1703-09
VOUT (50mV/div)
4V VIN 2V 0V
2s/div ILOAD = 1.5A, C7 = 0.47F
2ms/div VIN = 2.6V TO 3.6V, VOUT IS AC COUPLED
LOAD-TRANSIENT RESPONSE
MAX1703-10
POWER-ON DELAY (PFM MODE)
MAX1703-11
VON VOUT (50mV/div) VOUT (2V/div) 1.0A 0.5A 0A 2ms/div VOUT IS AC COUPLED LOAD CURRENT = 0A TO 1.5A 1ms/div VIN = 2.6V TO 3.6V, VOUT IS AC COUPLED
IIN (0.2A/div)
6
_______________________________________________________________________________________
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
_________________________________Typical Operating Characteristics (continued)
(VIN = +3.6V, VOUT = 5V, TA = +25C, unless otherwise noted.)
GSM LOAD-TRANSIENT RESPONSE
MAX1703-12
MAX1703
DECT LOAD-TRANSIENT RESPONSE
MAX1703-13
VOUT (100mV/div)
VOUT (100mV/div)
ILOAD (0.5A/div)
ILOAD (0.2A/div)
1ms/div VIN = 3.6V, VOUT = 5V, COUT = 470F, PULSE WIDTH = 577s, LOAD CURRENT = 100mA TO 1A
2ms/div VIN = 1.2V, VOUT = 3.3V, COUT = 470F, PULSE WIDTH = 416s, LOAD CURRENT = 50mA TO 400mA
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME REF FB POKIN OUT GND AIN AO POK FUNCTION Reference Output. Bypass with a 0.22F bypass capacitor to GND. Dual-ModeTM Feedback Input. Connect FB to ground to set a fixed output voltage of +5V. Connect a divider between the output voltage and GND to set the output voltage from 2.5V to 5.5V. Power-Good Comparator Input. Threshold is 1.250V, with 1% hysteresis on the threshold's rising edge. DC-DC Converter Output. Power source for the IC. Ground Gain-Block Input. When AIN is low, AO sinks current. The nominal transconductance from AIN to AO is 10mmhos. Gain-Block Output. This open-drain output sinks current when VAIN < VREF. Power-Good Comparator Output. This open-drain N-channel output is low when VPOKIN < 1.250V. Switch-Mode Selection and External-Clock Synchronization Input: * CLK/SEL = Low: Low-power, low-quiescent-current PFM mode. Delivers up to 10% of full load current. * CLK/SEL = High: High-power PWM mode. Full output power available. Operates in low-noise, constantfrequency mode. * CLK/SEL = External Clock: High-power PWM mode with the internal oscillator synchronized to the external CLK Turning on with CLK/SEL = 0V also serves as a soft-start function, since peak inductor current is limited to 25% of that allowed in PWM mode. Source of N-Channel Power MOSFET Switch Drain of N-Channel Power Switch. Connect LXP to LXN. Source of P-Channel Synchronous Rectifier MOSFET Switch. Connect an external Schottky diode from LXN and LXP to POUT. Drain of P-Channel Synchronous Rectifier. Connect LXP to LXN. On/Off Input. When ON is low, the IC turns on.
9
CLK/SEL
10, 12 11 13, 15 14 16
PGND LXN POUT LXP ON
Dual Mode is a trademark of Maxim Integrated Products.
_______________________________________________________________________________________ 7
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
OUT IC POWER 2.15V
UNDERVOLTAGE LOCKOUT
PFM/PWM CONTROLLER EN START-UP OSCILLATOR Q D Q P
POUT
OUT
LXP ON ON REF GND CLK/SEL FB POKIN 1.25V REF OUT DUAL MODE/ FB COMPARATOR N REF AIN GAIN BLOCK REF N RDY REFERENCE EN 300kHz OSCILLATOR PFM/PWM EN OSC Q MODE FB POK N PGND LXN
MAX1703
AO
Figure 1. Functional Block Diagram
_______________Detailed Description
The MAX1703 is a highly efficient, low-noise power supply for portable RF and data-acquisition instruments. It combines a boost switching regulator, Nchannel power MOSFET, P-channel synchronous rectifier, precision reference, shutdown control, versatile gain block, and power-good (POK) comparator (Figure 1) in a 16-pin narrow SO package. The switching DC-DC converter boosts a 1- to 3-cell input to a fixed 5V or an adjustable output between 2.5V and 5.5V. Typically the MAX1703 starts from a low, 0.9V input and remains operational down to 0.7V.
The MAX1703 is optimized for use in cellular phones and other applications requiring low noise during fullpower operation, as well as low quiescent current for maximum battery life in low-power mode and shutdown. It features constant-frequency (300kHz), lownoise PWM operation with up to 1.5A output capability. See Table 1 for typical outputs. A low-quiescent-current, low-power mode offers an output up to 150mA and reduces quiescent power consumption to 300W. In shutdown mode, the quiescent current is further reduced to just 1A. Figure 2 shows the standard application circuit for the MAX1703.
8
_______________________________________________________________________________________
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
Table 1. Typical Available Output Current
NO. OF CELLS 1 NiCd/NiMH 2 NiCd/NiMH 2.4 3 NiCd/NiMH 3.6 5.0 5.0 950 1600 INPUT VOLTAGE (V) 1.2 2.4 OUTPUT VOLTAGE (V) 3.3 3.3 OUTPUT CURRENT (mA) 600 1400
Table 2. Selecting the Operating Mode
CLK/SEL 0 1 External Clock (200kHz to 400kHz) MODE Low power PWM Synchronized PWM FEATURES Low supply current Low noise, high output current Low noise, high output current
MAX1703
Step-Up Converter
VBATT
L1 4.7H
C1 100F
CLK/SEL LXP, LXN
MBR0520L D1 POUT R5 10 OUT C2 0.22F C5 0.22F C4 2 x 220F
MAX1703
ON
The step-up switching DC-DC converter generates an adjustable output from 2.5V to 5.5V. The internal Nchannel MOSFET switch is turned on during the first part of each cycle, allowing current to ramp up in the inductor and store energy in a magnetic field. During the second part of each cycle, when the MOSFET is turned off, the voltage across the inductor reverses and forces current through the diode and synchronous rectifier to the output filter capacitor and load. As the energy stored in the inductor is depleted, the current ramps down and the output diode and synchronous rectifier turn off. Depending on the CLK/SEL pin setting, voltage across the load is regulated using either low-noise PWM or low-power operation (Table 2).
R3
AIN AO POKIN REF POK FB GND
R4
C3 0.22F
PGND
SIGNAL GROUND POWER GROUND
NOTE: HEAVY LINES INDICATE HIGH-CURRENT PATHS.
Figure 2. MAX1703 in High-Power PWM Mode
Additional features include synchronous rectification for high efficiency and improved battery life, and an uncommitted comparator (POK) for monitoring the regulator's output or battery voltage. The MAX1703 also includes a gain block that can be used to build a linear regulator using an external P-channel MOSFET pass device; this gain block can also function as a second comparator. A CLK input allows frequency synchronization to reduce interference.
Low-Noise PWM Operation When CLK/SEL is pulled high, the MAX1703 operates in a high-power, low-noise PWM mode. During PWM operation, the MAX1703 switches at a constant frequency (300kHz), and modulates the MOSFET-switch pulse width to control the power transferred per cycle and regulate the voltage across the load. In PWM mode the device can output up to 1.5A. Switching harmonics generated by fixed-frequency operation are consistent and easily filtered. See the Noise Spectrum plot in the Typical Operating Characteristics. During PWM operation, each of the internal clock's rising edges sets a flip-flop, which turns on the N-channel MOSFET switch (Figure 3). The switch turns off when the sum of the voltage-error, slope-compensation, and current-feedback signals trips a multi-input comparator and resets the flip-flop; the switch remains off for the rest of the cycle. When a change occurs in the output voltage error signal, the comparator shifts the level to which the inductor current ramps during each cycle. A second comparator enforces an inductor current limit of 2.7A (typical).
_______________________________________________________________________________________
9
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
POUT FB REF R Q N LXP LXN ERROR COMPARATOR FB REF 2.7A TYP CURRENT LIMIT OSC PGND R PGND 800mA TYP CURRENT LIMIT S Q N LXP LXN S Q P R D LOGIC HIGH POUT
P
S
Figure 3. Simplified PWM Controller Block Diagram
Figure 4. Controller Block Diagram in Low-Power PFM Mode
Synchronized PWM Operation The MAX1703 can be synchronized in PWM mode to a 200kHz to 400kHz frequency by applying an external clock to CLK/SEL. This allows the user to set the harmonics to avoid IF bands in wireless applications. The synchronous rectifier is also active during synchronized PWM operation. Low-Power PFM Operation Pulling CLK/SEL low places the MAX1703 in a lowpower mode. During low-power mode, PFM operation regulates the output voltage by transferring a fixed amount of energy during each cycle, and then modulating the pulse frequency to control the power delivered to the output. The devices switch only as needed to service the load, resulting in the highest possible efficiency at light loads. Output current capability in PFM mode is 150mA (max). The output voltage is typically 1% higher than in PWM mode.
During PFM operation, the error comparator detects the output voltage falling out of regulation and sets a flipflop, which turns on the N-channel MOSFET switch (Figure 4). When the inductor current ramps to the PFM mode current limit (800mA typical) and stores a fixed amount of energy, the current-sense comparator resets a flip-flop. The flip-flop turns off the N-channel switch and turns on the P-channel synchronous rectifier. A second flip-flop, previously reset by the switch's "on" sig10
nal, inhibits the error comparator from initiating another cycle until the energy stored in the inductor is transferred to the output filter capacitor and the synchronous rectifier current has ramped down to 80mA. This forces operation with a discontinuous inductor current.
Synchronous Rectifier The MAX1703 features an internal 140m, P-channel synchronous rectifier to enhance efficiency. Synchronous rectification provides a 5% efficiency improvement over similar nonsynchronous boost regulators. In PWM mode, the synchronous rectifier is turned on during the second half of each switching cycle. In lowpower mode, an internal comparator turns on the synchronous rectifier when the voltage at LX exceeds the boost regulator output, and then turns it off when the inductor current drops below 80mA.
Low-Voltage Start-Up Oscillator
The MAX1703 uses a CMOS, low-voltage start-up oscillator for a 1.1V guaranteed minimum start-up input voltage at +25C. On start-up, the low-voltage oscillator switches the N-channel MOSFET until the output voltage reaches 2.15V. Above this level, the normal boostconverter feedback and control circuitry take over. Once the device is in regulation, it can operate down to a 0.7V input, since internal power for the IC is bootstrapped from the output via the OUT pin. Do not apply full load until the output exceeds 2.3V (max).
______________________________________________________________________________________
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
Shutdown
The MAX1703 shuts down to reduce quiescent current to 1A. During shutdown (ON = VOUT), the reference, low-battery comparator, gain block, and all feedback and control circuitry are off. The boost converter's output drops to one Schottky diode drop below the input.
Reference
The MAX1703 has an internal 1.250V, 1% bandgap reference. Connect a 0.22F bypass capacitor to GND within 0.2in. (5mm) of the REF pin. REF can source up to 50A of external load current.
MAX1703
Gain Block
The MAX1703 gain block can function as a second comparator, or can be used to build a linear regulator using an external P-channel MOSFET pass device. The gain-block output is a single-stage transconductance amplifier that drives an open-drain N-channel MOSFET. The g m of the entire gain-block stage is 10mmho. Figure 6 shows the gain block used in a linear-regulator application. The output of an external P-channel pass element is compared to the internal reference. The difference is amplified and used to drive the gate of the pass element. Use a logic-level PFET, such as an NDS336P (RDS(ON) = 270m) from Fairchild. This configuration allows ripple reduction at the output. If a lower RDS(ON) PFET is used, then the linear regulator output filter capacitance may need to be increased. To use the gain block as a comparator, refer to the Power-Good (POK) Comparator section.
Power-Good (POK) Comparator
The MAX1703 features an uncommitted POK comparator. The internal POK comparator has an open-drain output (POK) capable of sinking 1mA. When the input (POKIN) rises above the 1.25V reference, the POK open-drain output turns off. The POKIN input has 10mV of hysteresis. To provide a power-good signal, connect the POKIN input to an external resistor-divider between OUT and GND (Figure 5). Calculate the resistor values as follows: R3 = R4(VTH / VREF - 1) where VTH is the desired input voltage trip threshold. Since the input bias current into POKIN is less than 20nA, R4 can be a large value (such as 270k or less) without sacrificing accuracy. Connect the resistor voltage-divider as close to the IC as possible, within 0.2in. (5mm) of POKIN.
VIN
VIN
L1 4.7H CLK/SEL LXP, LXN D1 POUT R5 10 OUT
C1 100F
L1 4.7H
C1 100F
BOOST OUTPUT LINEAR REGULATED OUTPUT 47F
MBR0520L
OUTPUT C4 2 x 220F
CLK/SEL LXP, LXN POUT ON
MBR0520L C4 330F R5 10 C2 0.22F AO POK R1 C5 0.22F
P
MAX1703
ON
C5 0.22F
R3 AIN
R6 20k
MAX1703
OUT
C2 0.22F
R3
AIN AO POKIN REF POK R1 FB GND R2 SIGNAL GROUND POWER GROUND
R4
POKIN
R4 C3 0.22F PGND
REF PGND
FB GND R2 100k SIGNAL GROUND POWER GROUND
NOTE: HEAVY LINES INDICATE HIGH-CURRENT PATHS.
Figure 5. Adjustable Output (PWM Mode)
Figure 6. Using the Gain Block as a Linear Regulator
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1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
Table 3. Component Selection Guide
PRODUCTION Surface Mount Coilcraft DO3316 Through Hole Sumida RCH654 series INDUCTORS Sumida CDR125 CAPACITORS Matsuo 267 series Sprague 595D series AVX TPS series Sanyo OS-CON series Nichicon PL series 1N5817 Motorola MBR0520L DIODES
__________________Design Procedure
Setting the Output Voltages
Set the output voltage between 2.5V and 5.5V by connecting a resistor voltage-divider to FB from OUT to GND, as shown in Figure 2. The resistor values are then as follows: R1 = R2(VOUT / VFB - 1) where VFB, the boost-regulator feedback setpoint, is 1.24V. Since the input bias current into FB is less than 20nA, R2 can have a large value (such as 270k or less) without sacrificing accuracy. Connect the resistor voltage-divider as close to the IC as possible, within 0.2in. (5mm) of the FB pin.
Table 4. Component Suppliers
SUPPLIER AVX Coilcraft Matsuo Motorola Sanyo Sumida PHONE USA: (803) 946-0690 (800) 282-4975 USA: (847) 639-6400 USA: (714) 969-2491 USA: (602) 303-5454 USA: (619) 661-6835 Japan: 81-7-2070-6306 USA: (847) 956-0666 Japan: 81-3-3607-5111 FAX (803) 626-3123 (847) 639-1469 (714) 960-6492 (602) 994-6430 (619) 661-1055 81-7-2070-1174 (847) 956-0702 81-3-3607-5144
Inductor Selection
The MAX1703's high switching frequency allows the use of a small surface-mount inductor. A 4.7H inductor should have a saturation-current rating that exceeds the N-channel switch current limit. However, it is generally acceptable to bias the inductor current into saturation by as much as 20%, although this will slightly reduce efficiency. For high efficiency, choose an inductor with a high-frequency core material, such as ferrite, to reduce core losses. To minimize radiated noise, use a toroid, pot core, or shielded bobbin inductor. See Table 3 for suggested components and Table 4 for a list of component suppliers. Connect the inductor from the battery to the LX pins as close to the IC as possible.
Input and Output Filter Capacitors
Choose input and output filter capacitors that will service the input and output peak currents with acceptable voltage ripple. Choose input capacitors with working voltage ratings over the maximum input voltage, and output capacitors with working voltage ratings higher than the output. A 330F, 100m, low-ESR tantalum capacitor is recommended for a 5V output. For full output load current, one 470F or two 220F, 100m low-ESR tantalum capacitors are recommended for a 3.3V output. The input filter capacitor (CIN) also reduces peak currents drawn from the input source and reduces input switching noise. The input voltage source impedance determines the required size of the input capacitor. When operating directly from one or two NiCd cells placed close to the MAX1703, use a 100F, low-ESR input filter capacitor. Sanyo OS-CON and Panasonic SP/CB-series ceramic capacitors offer the lowest ESR. Low-ESR tantalum capacitors are a good choice and generally offer a good tradeoff between price and performance. Do not exceed the ripple current ratings of tantalum capacitors. Avoid most aluminum-electrolytic capacitors, because their ESR is often too high.
Output Diode
Use a Schottky diode such as a 1N5817, MBR0520L, or equivalent. The Schottky diode carries current during both start-up and PFM mode after the synchronous rectifier turns off. Thus, its current rating only needs to be 500mA. Connect the diode between LXN/LXP and POUT, as close to the IC as possible. Do not use ordinary rectifier diodes, since slow switching speeds and long reverse recovery times will compromise efficiency and load regulation.
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1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
Bypass Capacitors
A few ceramic bypass capacitors are required for proper operation. Bypass REF with a 0.22F capacitor to GND. Connect a 0.22F ceramic capacitor from OUT to GND. Each of these should be placed as close to their respective pins as possible, within 0.2in. (5mm) of the DC-DC converter IC. See Table 4 for suggested suppliers.
MAX1703
CON 4.7nF
16
__________ Applications Information
Intermittent Supply/Battery Connections
When boosting an input supply connected via a mechanical switch, or a battery connected via spring contacts, input power may sometimes be intermittent as a result of contact bounce. When operating in PFM mode with input voltages greater than 2.5V, restarting after such dropouts may initiate high current pulses that interfere with the MAX1703's internal MOSFET switch control. If contact or switch bounce is anticipated in the design, use one of the following solutions: 1) Connect a capacitor (CON) from ON to VIN and a 1M resistor (RON) from ON to GND, as shown in Figure 7. This resistor-capacitor network differentiates fast input edges at VIN and momentarily holds the IC off until VIN settles. The appropriate value of CON is 10-5 times the total output filter capacitance (COUT), so a COUT of 440F results in CON = 4.7nF. 2) Use the system microcontroller to hold the MAX1703 in shutdown from the time when power is applied (or reapplied) until COUT has charged to at least the input voltage. Standard power-on-reset times accomplish this. 3) Ensure that the IC operates, or at least powers up, in PWM mode (CLK/SEL = high). Activate PFM mode only after the output voltage has settled and all of the system's power-on-reset flags are cleared.
RON 1M
ON
LXP, LXN
11, 14 COUT 2 x 220F
MAX1703
OUT 4
POUT
15, 13
Figure 7. Connecting CON and RON when Switch or BatteryContact Bounce is Anticipated
Designing a PC Board
High switching frequencies and large peak currents make PC board layout an important part of design. Poor design can cause excessive EMI and ground bounce, both of which can cause instability or regulation errors by corrupting the voltage and current feedback signals. Power components--such as the inductor, converter IC, filter capacitors, and output diode--should be placed as close together as possible, and their traces should be kept short, direct, and wide. A separate lownoise ground plane containing the reference and signal grounds should only connect to the power-ground plane at one point. This minimizes the effect of powerground currents on the part. Keep the voltage feedback network very close to the IC, within 0.2in. (5mm) of the FB pins. Keep noisy traces, such as from the LX pin, away from the voltage feedback networks and separated from them using grounded copper. Consult the MAX1703 EV kit for a full PC board example.
Use in a Typical Wireless Phone Application
The MAX1703 is ideal for use in digital cordless and PCS phones. The power amplifier (PA) is connected directly to the boost-converter output for maximum voltage swing (Figure 8). Low-dropout linear regulators are used for post-regulation to generate low-noise power for DSP, control, and RF circuitry. Typically, RF phones spend most of their life in standby mode with only short periods in transmit/receive mode. During standby, maximize battery life by setting CLK/SEL = 0; this places the IC in low-power mode (for the lowest quiescent power consumption). See Gain Block section for information on configuring an external MOSFET as a linear regulator.
Soft-Start
To implement soft-start, set CLK/SEL low on power-up; this forces PFM operation and reduces the peak switching current to 800mA max. Once the circuit is in regulation and start-up transients have settled, CLK/SEL can be set high for full-power operation.
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1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
___________________Chip Information
TRANSISTOR COUNT: 554 SUBSTRATE CONNECTED TO GND
POUT
LX
MAX1703
LDOs MAX8865/MAX8866
PA C RADIO
Figure 8. Typical Phone Application
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1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter
________________________________________________________Package Information
SOICN.EPS
MAX1703
______________________________________________________________________________________
15
1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter MAX1703
NOTES
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