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19-1095; Rev 2; 12/97
1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters
General Description
The MAX848/MAX849 boost converters set a new standard of high efficiency and high integration for noisesensitive power-supply applications, such as portable phones and small systems with RF data links. The heart of the these devices is a synchronous boost-topology regulator that generates a fixed 3.3V output (or 2.7V to 5.5V adjustable output) from one to three NiCd/NiMH cells or one Li-Ion cell. Synchronous rectification provides a 5% efficiency improvement over similar nonsynchronous boost regulators. In standby mode, pulse-skipping PFM operation keeps the output voltage alive with only 150W quiescent power consumption. Fixed-frequency PWM operation ensures that the switching noise spectrum is limited to the 300kHz fundamental and its harmonics, allowing easy post-filtering noise reduction. For even tighter noise spectrum control, synchronize to a 200kHz to 400kHz external clock. Battery monitoring is provided by a two-channel, voltage-to-frequency analog-to-digital converter (ADC). One channel is intended for a single-cell battery input (0.625V to 1.875V range), while the other channel is for monitoring higher voltages (0V to 2.5V range). Two control inputs are provided for push-on, push-off control via a momentary pushbutton switch. Upon power-up, an internal comparator monitors the output voltage to generate a power-good output (POK). The devices differ only in the current limit of the N-channel MOSFET power switch: 0.8A for the MAX848, and 1.4A for the MAX849.
INPUT 0.8V TO 5.5V
Features
o Up to 95% Efficiency (see Typical Output Selector Guide below) o 3.3V Dual ModeTM or 2.7V to 5.5V Adj. Output o 0.7V to 5.5V Input Range o 0.15mW Standby Mode o 300kHz PWM Mode or Synchronizable o Two-Channel ADC with Serial Output o Power-Good Function
MAX848/MAX849
Applications
Digital Cordless Phones Cellular Phones Palmtop Computers Local 3.3V to 5V Supplies PCS Phones Hand-Held Instruments Personal Communicators
Ordering Information
PART MAX848ESE MAX849ESE TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 16 Narrow SO 16 Narrow SO
Typical Operating Circuit
Typical Output Selector Guide
VIN (V) 0.9 1.2 2.4 2.7 3.6 VOUT (V) 3.3 5 3.3 5 3.3 5 3.3 5 5 MAX849 IOUT (mA) 100 70 300 200 750 500 800 600 1000 MAX848 IOUT (mA) 70 40 110 70 200 130 250 150 300
A/D CHANNEL 1 IN A/D CHANNEL 2 IN A/D CHANNEL SELECT A/D OUTPUT ON/OFF CONTROL SYNC INPUT
AIN1 AIN2 AINSEL DATA ON1 ON2 CLK/SEL POKIN REF
MAX848 MAX849
LX OUT POUT POK FB
OUTPUT
VOLTAGE MONITOR OUTPUT
GND
PGND
Pin Configuration appears at end of data sheet. Dual Mode is a trademark of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products 1
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
ABSOLUTE MAXIMUM RATINGS
ON1, ON2, OUT, POUT to GND..................................-0.3V, +6V PGND to GND ..........................................................-0.3V, +0.3V LX to PGND ...............................................-0.3V, (VPOUT + 0.3V) CLK/SEL, DATA, POKIN, REF, AINSEL, AIN1, AIN2, FB, POK to GND .....-0.3V, (VOUT + 0.3V) Continuous Power Dissipation (TA = +70C) Narrow SO (derate 8.7mW/C above +70C) ................696mW Operating Temperature Range MAX848ESE/MAX849ESE .................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature.........................................-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
(VOUT = 3.6V, GND = PGND = CLK/SEL = ON1 = ON2 = AINSEL = AIN1 = AIN2 = FB = POKIN, POUT = OUT, TA = 0C to +85C, unless otherwise noted.) PARAMETER Minimum Operating Voltage (Note 1) REFERENCE Reference Output Voltage REF Load Regulation REF Supply Rejection DC-DC CONVERTER Output Voltage (Note 2) IREF = 0mA -1A < IREF < 50A 2.5V < VOUT < 5V VFB < 0.1V, CLK/SEL = OUT VOUT = 3.3V VIN = 1.2V VOUT = 5V VOUT = 3.3V VIN = 2.4V Output Current VOUT = 5V VOUT = 3.3V VIN = 2.7V VOUT = 5V VOUT = 5V FB Regulation Voltage FB Input Current Output Voltage Adjust Range Output Voltage Lockout Range Load Regulation (Note 4) Minimum Start-Up Voltage (Note 5) Frequency in Start-Up Mode Operating Current in Shutdown (Note 3) CLK/SEL = OUT ILOAD < 1mA, TA > +25C VOUT = 1.5V Current into OUT pin, V ON2 = 3.6V 40 4 MAX848 MAX849 MAX848 MAX849 MAX848 MAX849 MAX848 MAX849 MAX848 MAX849 MAX848 MAX849 3.17 1.23 CONDITIONS MIN TYP 0.7 1.25 5 0.2 3.34 110 300 70 200 200 750 130 500 250 600 150 800 300 1000 1.215 2.7 2.1 -1.6 0.9 1.1 300 20 1.240 1.265 200 5.5 2.4 V nA V V % V kHz A mA mA 1.27 15 5 3.40 MAX UNITS V V mV mV V
MAX848, VIN = 3.3V MAX849, VIN = 3.6V
Adjustable output, CLK/SEL = OUT VFB = 1.25V
2
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(VOUT = 3.6V, GND = PGND = CLK/SEL = ON1 = ON2 = AINSEL = AIN1 = AIN2 = FB = POKIN, POUT = OUT, TA = 0C to +85C, unless otherwise noted.) PARAMETER Operating Current in Low-Power Mode (Note 6) Operating Current in Low-Noise Mode (Note 6) DC-DC SWITCHES POUT Leakage Current LX Leakage Current Switch On-Resistance VLX = 0V, V ON2 = VOUT = 5.5V VLX = V ON2 = VOUT = 5.5V N-channel P-channel CLK/SEL = OUT N-Channel Current Limit VCLK/SEL = 0V (Note 7) ADC Data Output Voltage Low Data Output Voltage High AIN1 Input Voltage Range AIN2 Input Voltage Range AIN1, AIN2 Input Current Accuracy POWER-GOOD Internal Trip Level External Trip Level POK Low Voltage POK High Leakage Current POKIN Leakage Current LOGIC AND CONTROL INPUTS Input Low Voltage Input High Voltage Logic Input Current Internal Oscillator Frequency Oscillator Maximum Duty Cycle External Clock Frequency Range CLK/SEL Pulse Width CLK/SEL Rise/Fall Time Not tested Not tested 1.2V < VOUT < 5.5V, ON1 and ON2 (Note 8) VOUT = 2.7V, AINSEL and CLK/SEL 1.2V < VOUT < 5.5V, ON1 and ON2 (Note 8) VOUT = 5.5V, AINSEL and CLK/SEL ON1, ON2, AINSEL and CLK/SEL CLK/SEL = OUT 260 80 200 200 100 300 85 0.8VOUT 0.8VOUT 1 340 90 400 0.2VOUT 0.2VOUT V V A kHz % kHz ns ns 3 Rising VOUT, VPOKIN < 0.1V Rising VPOKIN ISINK = 1mA, VOUT = 3.6V or ISINK = 20A, VOUT = 1V VOUT = VPOK = 5.5V VPOKIN = 1.5V 0.01 2.95 1.225 3.10 1.275 0.4 1 50 V V V A nA ISINK = 1mA ISOURCE = 1mA AINSEL = GND AINSEL = OUT fCLK = 400kHz, VAIN1 = VAIN2 = 2.5V fCLK = 400kHz, 5ms conversion, monotonic to 8 bits VOUT - 0.4 0.625 0 1 1.875 2.5 2 4 0.4 V V V V A % FSR CLK/SEL = GND CLK/SEL = OUT CLK/SEL = OUT MAX848 MAX849 MAX848 MAX849 600 1100 120 250 0.1 0.1 0.3 0.13 0.25 800 1350 200 400 20 20 0.6 0.25 0.5 1000 1600 300 550 mA A A CONDITIONS Current into OUT pin, CLK/SEL = GND Current into OUT pin, CLK/SEL = OUT, does not include switching losses MIN TYP 35 150 MAX 90 300 UNITS A A
MAX848/MAX849
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
ELECTRICAL CHARACTERISTICS
(VOUT = 3.6V, GND = PGND = CLK/SEL = ON1 = ON2 = AINSEL = AIN1 = AIN2 = FB = POKIN, POUT = OUT, TA = -40C to +85C, unless otherwise noted.) (Note 9) PARAMETER REFERENCE Reference Output Voltage DC-DC CONVERTER Output Voltage (Note 3) FB Regulation Voltage Output Voltage Lockout Range OUT Supply Current in Shutdown OUT Supply Current in Low-Power Mode (Note 6) OUT Supply Current in Low-Noise Mode (Note 6) DC-DC SWITCHES Switch On-Resistance N-channel P-channel CLK/SEL = OUT N-Channel Current Limit CLK/SEL = GND (Note 7) ADC Accuracy POWER-GOOD Internal Trip Level External Trip Level LOGIC CONTROL INPUTS Internal Oscillator Frequency Oscillator Maximum Duty Cycle CLK/SEL = OUT 260 80 340 90 kHz % Rising VOUT, VPOKIN < 0.1V Rising VPOKIN 2.95 1.225 3.10 1.275 V V fCLK = 400kHz, 5ms conversion 4 % FSR CLK/SEL = GND CLK/SEL = OUT CLK/SEL = OUT MAX848 MAX849 MAX848 MAX849 600 1100 120 250 0.6 0.25 0.5 1100 1800 300 550 mA VFB < 0.1V, CLK/SEL = OUT, includes load-regulation error Adjustable output, CLK/SEL = OUT (Note 3) V ON2 = 3.6V CLK/SEL = GND CLK/SEL = OUT, does not include switching losses 3.13 1.21 2.05 3.47 1.27 2.45 20 90 300 V V V A A A IREF = 0mA 1.225 1.275 V CONDITIONS MIN TYP MAX UNITS
Note 1: Minimum operating voltage. Because the MAX848/MAX849 are bootstrapped to the output, it will operate down to a 0.7V input. Note 2: In low-power mode (CLK/SEL = GND), the output voltage regulates 1% higher than in low-noise mode (CLK/SEL = OUT or synchronized). Note 3: The part is in start-up mode until it reaches this voltage level. Do not apply full-load current. Note 4: Load regulation is measured from no load to full load, where full load is determined by the N-channel switch current limit. Note 5: Start-up is tested with Figure 2's circuit. Output current is measured when the input and output voltages are applied. Note 6: Supply current from the 3.34V output is measured between the 3.34V output and the OUT pin. This current correlates directly with actual battery supply current, but is reduced in value according to the step-up ratio and efficiency. VOUT = 3.6V to keep the internal switch open when measuring the current into the device. Note 7: When VCLK/SEL = 0V, the inductor is forced into constant-peak-current, discontinuous operation. This is guaranteed by testing in Figure 2's circuit. Note 8: ON1 and ON2 inputs have approximately 0.15VOUT hysteresis. Note 9: Specifications to -40C are guaranteed by design, not production tested. 4 _______________________________________________________________________________________
1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
MAX849 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX848/9 TOC-01
MAX848/MAX849
MAX849 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V)
MAX848/9 TOC-02
MAX848 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX848/9 TOC-03
100 90 EFFICIENCY (%) 80 70 60 50 40 0.1 1 10 100 VIN = 0.9V VIN = 2.4V VIN = 1.2V
100 VIN = 3.6V 90 80 70 60 50 VIN = 1.2V VIN = 2.4V
100 VIN = 2.4V 90 EFFICIENCY (%)
EFFICIENCY (%)
80 VIN = 1.2V
70 PFM PWM 60 0.1 1 10 100 1000 0.1
VIN = 0.9V PFM PWM 1 10 100 1000
PFM PWM 1000
40 30
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE
MAX848/9 TOC-04
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX848/9 TOC-05
START-UP VOLTAGE vs. LOAD CURRENT (VOUT = 3.3V, PWM MODE)
1.8 START-UP VOLTAGE (V) 1.6 1.4 1.2 1.0 0.8 TA = +85C TA = +25C
6
MAX848/9 TOC-06
14 12 INPUT CURRENT (mA) 10 8 TA = +25C 6 4 2 0 0 1 2 3 4 5 TA = -40C TA = +85C
18 16 SHUTDOWN CURRENT (A) 14 12 10 8 6 4 2 0 TA = -40C 0 1 2 3 4 5 INCLUDES ALL EXTERNAL COMPONENT LEAKAGES. CAPACITOR LEAKAGE DOMINATES AT TA = +85C TA = +85C TA = +25C
2.0
TA = -40C
6
0.6 0.01 0.1 1 10 100 1000 LOAD CURRENT (mA)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
REFERENCE VOLTAGE vs. TEMPERATURE
MAX848/9 TOC-07
REFERENCE VOLTAGE vs. REFERENCE CURRENT
MAX848/9 TOC-08
ADC LINEARITY ERROR vs. FULL-SCALE INPUT VOLTAGE
MAX848/9 TOC-09
1.252
1.252 1.250 REFERENCE VOLTAGE (V) 1.248 1.246 1.244 1.242 1.240
0.25
0.15 LINEARITY ERROR (%FS)
REFERENCE VOLTAGE (V)
1.251
0.05
AIN2
1.250
-0.05 AIN1 -0.15
1.249
1.248 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.238 0 10 20 30 40 50 60 70 80 REFERENCE CURRENT (A)
-0.25 0.1875 0.4375 0.6875 0.9375 FULL-SCALE INPUT VOLTAGE (V)
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
HEAVY-LOAD SWITCHING WAVEFORMS (VOUT = 3.3V)
MAX848/9 TOC-10
LINE-TRANSIENT RESPONSE
MAX848/9 TOC-11
VOUT A 0V B 0V 0A C B A
1s/div VIN = 1.1V, IOUT = 200mA, VOUT = 3.3V A = LX VOLTAGE, 2V/div B = INDUCTOR CURRENT, 0.5A/div C = VOUT RIPPLE, 50mV/div, AC COUPLED IOUT = 0mA, VOUT = 3.3V
5ms/div A = VIN, 1.1V TO 2.1V, 1V/div B = VOUT RIPPLE, 50mV/div, AC COUPLED
LOAD-TRANSIENT RESPONSE
MAX848/9 TOC-12
POWER-ON DELAY (PFM MODE)
MAX848/9 TOC-13
3.3V A 200mA 0A A B
B
C
0mA
2ms/div VIN = 1.1V, VOUT = 3.3V A = LOAD CURRENT, 0mA TO 200mA, 0.2A/div B = VOUT RIPPLE, 50mV/div, AC COUPLED
5ms/div A = VON1, 2V/div B = VOUT, 1V/div C = INPUT CURRENT, 0.2A/div
6
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
MAX849 GSM LOAD-TRANSIENT RESPONSE
MAX848/9 TOC-14
MAX848/MAX849
MAX849 DECT LOAD-TRANSIENT RESPONSE
MAX848/9 TOC-15
5V A A
3.3V
B 0A
B 0A
1ms/div VIN = 3.6V, VOUT = 5V, COUT = 440F A = VOUT RIPPLE, 200mV/div, AC COUPLED B = LOAD CURRENT, 100mA TO 1A, 0.5A/div, PULSE WIDTH = 577s
2ms/div VIN = 1.2V, VOUT = 3.3V, COUT = 440F A = VOUT RIPPLE, 200mV/div, AC COUPLED B = LOAD CURRENT, 50mA TO 400mA, 0.2A/div, PULSE WIDTH = 416s
MAX849 NOISE SPECTRUM (VOUT = 3.3V, VIN = 1.2V, RLOAD = 50)
MAX848/9 TOC-16
2.7
NOISE (mVRMS) 0 0.1k 1k 10k FREQUENCY (Hz) 100k 1M
MAX849 INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE
MAX848/9 TOC-17
MAX849 PEAK INDUCTOR CURRENT vs. OUTPUT VOLTAGE
1.8 PEAK INDUCTOR CURRENT (A) 1.7 1.6 1.5 1.4 1.3
MAX848/9 TOC-18
380 INTERNAL OSC. FREQUENCY (kHz)
2.0
360 VOUT = 5V
340
320 VOUT = 3.3V 300
280 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.2 2.5 3.0 3.5 4.0 4.5 5.0 5.5 OUTPUT VOLTAGE (V)
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
Pin Description
PIN NAME AIN1 AIN2 REF GND OUT FUNCTION ADC's Channel 1 Input. Analog input voltage range is 0.625V to 1.875V. ADC's Channel 2 Input. Analog input voltage range is 0V to 2.5V. Reference Output. Bypass with a 0.22F capacitor to GND. Ground. Use for low-current ground paths. Connect to PGND with a short trace. Output Sense Input. The IC is powered from OUT. Bypass to GND with a 0.1F ceramic capacitor. Connect OUT to POUT through a 10 series resistor. Power-Good Comparator Input. Connect to GND for fixed threshold (VOUT x 0.9). To adjust the threshold, connect to a resistor divider from OUT to GND. Dual Mode DC-DC Converter Feedback Input. Connect to GND for fixed 3.3V output voltage. Connect to a resistor divider from OUT to GND to adjust the output voltage. Minimize noise coupling from switching signals to FB. Power-Good Output. This open-drain output is pulled low when the output voltage (VOUT) drops below the internally set threshold (fixed threshold), or when the voltage at POKIN drops below VREF (adjustable threshold). ADC's Input Channel Selector. Pull low to select AIN1 and drive high to select AIN2. ADC's Serial Output. Pulsed output, RZ format. Full scale is fOSC/2 (fCLK/2 in external sync mode). The DATA output is low when VCLK/SEL = 0V (PFM mode). External Clock Input/Regulator's Switching Mode Selector. CLK/SEL = low: low-power, low-quiescent PFM mode. Delivers 100mW of output power. CLK/SEL = high: low-noise, high-power PWM mode, switching at a constant frequency (300kHz). CLK/SEL = driven with external clock: low-noise, high-power, synchronized PWM mode. The internal oscillator is synchronized to the external clock (200kHz ~ 400kHz). Turning the DC-DC converter on with VCLK/SEL = 0V also serves as a soft-start function, since the peak inductor current is limited to 30% of the nominal value. Source of the Internal N-Channel Power MOSFET. Connect to high-current ground path. Drain of the Internal N-Channel Power MOSFET and P-Channel Synchronous Rectifier Source of the Internal P-Channel Synchronous Rectifier MOSFET. Connect an external Schottky diode from LX to POUT. Bypass to PGND with a 0.1F ceramic capacitor as close to the IC as possible. OFF Control Input. When ON1 = 0 and ON2 = 1, the IC is off. ON Control Input. When ON1 = 1 or ON2 = 0, the IC is on.
1
2 3 4 5
6
POKIN
7
FB
8
POK
9 10
AINSEL DATA
11
CLK/SEL
12 13 14 15 16
PGND LX POUT ON2 ON1
8
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
OUT
MAX848/MAX849
START-UP OSCILLATOR EN Q D Q PCH 0.25 POUT
2.25V
ON1 ON2 REF GND CLK/SEL FB POKIN AINSEL AIN1 AIN2 FEEDBACK AND POWER-GOOD SELECT FEEDBACK 1.25V ON REF RDY EN 300kHz OSCILLATOR PFM/PWM EN OSC Q NCH 0.13 PGND POK N EN ADC DATA LX
MODE PFM/PWM CONTROLLER
Figure 1. Functional Diagram
_______________Detailed Description
The MAX848/MAX849 combine a switching regulator, N-channel power MOSFET, P-channel synchronous rectifier, precision reference voltage, power-good indicator, and battery voltage monitor, all in a single monolithic device. The MAX848/MAX849 are powered directly from the output. The output voltage is factory preset to 3.3V or adjustable from 2.7V to 5V with external resistors (Dual ModeTM operation). These devices start from a low 1V input voltage and remain operational down to 0.7V. The MAX848/MAX849 operate with either one to three NiCd/NiMH cells or one Li-Ion cell. At power-up, an internal low-voltage oscillator drives the N-channel power switch, and the output voltage slowly builds up. The oscillator has a 25% nominal duty cycle to prevent current build-up in the inductor. An output voltage in excess of the nominal 2.25V lockout voltage activates the error comparator and internal timing circuitry. The device resumes operation in either pulse-frequency-modulation (PFM) low-power mode or pulse-width-modulation (PWM) low-noise mode, selected by the logic control, CLK/SEL. Figure 2 shows the
standard application circuit for the MAX849 configured in the high-power PWM mode.
On/Off Control
The MAX848/MAX849 are turned on or off by logic input pins ON1 and ON2 (Table 1). When ON1 = 1 or ON2 = 0, the part is on. When ON1 = 0 and ON2 = 1, the part is off. Both inputs have logic trip points near 0.5 x VOUT with 0.15 x VOUT hysteresis.
Table 1. On/Off Logic Control
ON1 0 0 1 1 ON2 0 1 0 1 MAX848/MAX849 On Off On On
Operating Modes
The MAX848/MAX849 operate in either PFM, PWM, or PWM synchronized to an externally applied clock signal. Table 2 lists each operating mode.
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9
1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
VIN = 1.1V
Q
D
LOGIC HIGH POUT
10 OUT R3 100k C2 0.1F POUT
*
D1 MBR0520L C5 0.1F L1 10H C1 22F
3.3V @ 200mA C4 2 x 100F
R
MAX849
GND POK ON1 ON2
LX FEEDBACK REF R PFM-MODE CURRENTLIMIT LEVEL CURRENT SENSE PGND S Q N
LX CLK/SEL PGND C3 FB 0.22F POKIN REF
* HEAVY LINES INDICATE
HIGH-CURRENT PATH.
Figure 2. 3.3V Preset Output
Figure 3. Controller Block Diagram in PFM Mode
Table 2. Selecting Operating Mode
CLK/SEL 0 1 External clock (200kHz ~ 400kHz) MODE PFM PWM Synchronized PWM
Low-Power PFM Mode When CLK/SEL is pulled low, the MAX848/MAX849 operate in low-power, low-supply-current PFM mode. Pulsefrequency modulation provides the highest efficiency at light loads. The P-channel rectifier is turned off to reduce gate-charge losses, and the regulator operates in discontinuous mode. The N-channel power MOSFET is kept on until the inductor current ramps to 30% of the current limit. The inductor energy is delivered to the output capacitor when the switch turns off. A new cycle is inhibited until the inductor current crosses zero. Zero current detection is accomplished by sensing the LX voltage crossing the output voltage. Figure 3 shows the block diagram for the PFM controller. Low-Noise PWM Mode When CLK/SEL is pulled high, the MAX848/MAX849 operate in high-power, low-noise, current-mode PWM, switching at the 300kHz nominal internal oscillator frequency. The internal rectifier is active in this mode, and the regulator operates in continuous mode. The N-channel power MOSFET turns on until either the output voltage is in regulation or the inductor current limit is reached (0.8A for the MAX848 and 1.4A for the MAX849). The switch turns off for the remainder of the cycle and the inductor energy is delivered to the output
10
capacitor. A new cycle is initiated on the next oscillator cycle. In low-noise applications, the fundamental and the harmonics generated by the fixed switching frequency can easily be filtered. Figure 4 shows the block diagram for the PWM controller. The MAX848/MAX849 enter synchronized current-mode PWM when a clock signal (200kHz < fCLK < 400kHz) is applied to CLK/SEL. The internal synchronous rectifier is active and the switching frequency is synchronized to the externally applied clock signal. For wireless applications, this ensures that the harmonics of the switching frequencies are predictable and can be kept outside the IF band(s). High-frequency operation permits low-magnitude output ripple voltage. The MAX848/MAX849 are capable of providing a stable output even with a rapidly pulsing load (GSM, DECT), such as from a transmitter power amplifier in digital cordless phones (see Typical Operating Characteristics). In PWM mode, the use of the synchronous rectifier ensures constant-frequency operation, regardless of the load current.
Setting the Output Voltage Externally
The MAX848/MAX849 feature Dual Mode operation. The output voltage is preset to 3.3V (FB = 0V), or it can be adjusted from 2.7V to 5.5V with external resistors R1, R2, and R3, as shown in Figure 5. To set the output voltage externally, select resistor R3 in the 10k to 100k range. The values for R1 and R2 are given by: R2 = R3(VOUT / VTRIP - 1) R1 = (R3 + R2)(VTRIP / VREF - 1)
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
POUT P FEEDBACK REF RQ N S LX
OUT
OUTPUT R1
MAX848 MAX849
POKIN
R2 FB
PGND
GND
POK
R3
PWM-MODE CURRENTLIMIT LEVEL OSC
Figure 4. Controller Block Diagram in PWM Mode
Figure 5. Adjustable Output Voltage and Power-Good Trip Level
where VREF = 1.25V, VOUT is the desired output voltage, and VTRIP is the desired trip level for the powergood comparator.
AIN1 C C
Power-OK
The MAX848/MAX849 feature a power-good comparator. This comparator's open-drain output, POK, is pulled low when the output voltage falls below the nominal internal threshold level of 3V with POKIN = 0V. To set the power-good trip level externally, refer to the Setting the Output Voltage Externally section.
AINSEL
AIN2 C/2 2 x REF REF D Q
Analog-to-Digital Converter (ADC)
The MAX848/MAX849 have an internal, two-channel, serial ADC. The ADC converts an analog input voltage into a digital stream available at the DATA pin. The converter skips clock pulses in proportion to the input voltage. Output format is a return-to-zero bit stream with a bit duration of 1/fCLK. At zero-scale input voltage, all pulses are skipped and DATA remains low; with a positive fullscale input voltage, no pulses are skipped; and at midscale, every other pulse is skipped. The ADC's clock is one-half of the externally applied clock signal or one-half of the internal 300kHz clock available at LX. In PFM mode, the converter is not active and DATA is driven low. Channel 1, AIN1, has an input voltage range of 0.625V to 1.875V and is selected when AINSEL is low. Channel 2, AIN2, accepts inputs in the 0V to 2.5V range and is selected when AINSEL is pulled high (Figure 6). The ADC is a switched-capacitor type; therefore, an anti-aliasing filter might be required at the inputs. Insert a 1k series resistor and a 0.01F filter capacitor in noisy environments.
C/2
OSC
/2
DATA
Figure 6. A/D Converter Block Diagram
Timer Function Implementation Implement the necessary counter functions either with discrete hardware or with microcontroller (C) implementations. The output resolution depends on how many of the ADC clock pulses are counted, as shown in Figure 7. Hardware Implementation A complete hardware solution can be implemented using either two counters or an ASIC. Resolution depends on how many pulses are counted. The main advantage of the discrete hardware implementation is that accuracy is not affected by interrupt latency associated with the C solution.
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
COUNTING FOUR PULSES fOSC/2
DATA GIVES YOU 2-BIT RESOLUTION
Figure 7. Bit Stream at 1/2 Full Scale
When using two counters of the same length, as shown in Figure 8, one counter (A) just counts the A/D clock pulses (fOSC/2), and the other counter (B) counts DATA output pulses. When counter A overflows (for example, after 256 clock cycles for an 8-bit counter), counter B is disabled. The controller reads the counter B output data and calculates the analog voltage present at the ADC's input.
VCC CLOCK/SEL OR LX CLEAR DATA OUTPUT CLR CLK /2 EN CLK CLR
A CARRY OUTPUT 8-BIT COUNTER B 8-BIT COUNTER EN RC
All C Implementation This implementation uses a C timer and a counter. The timer and the counter are reset at the same time. The counter counts data-output pulses applied at its input. When the timer times out, an interrupt is asserted. The C then reads the state of the counter register. The interrupt-handling overhead can cause the counter to count more pulses than desired. Accuracy depends on how long the C needs to read the counter. No errors will occur if the counter is disabled within one clock period. Interrupt latency reduces accuracy. The main advantage of this implementation is that no external hardware is required.
LATCH
Figure 8. Discrete Hardware Solution for Counting A/D Output Data Pulses
Diode Selection
The MAX848/MAX849's high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817 or MBR0520L, are recommended. Make sure the diode's current rating exceeds the maximum load current and that its breakdown voltage exceeds VOUT. The Schottky rectifier diode carries load currents only in the PFM operating mode, since the P-channel synchronous rectifier is disabled. Therefore, the current rating need not be high (0.5A is sufficient). In PFM mode, the voltage drop across the rectifier diode causes efficiency loss. However, when operating in PWM mode, the internal P-channel synchronous rectifier is active and efficiency loss due to the rectifier diode is minimized. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; use high-speed silicon diodes such as the MUR105 or EC11FS1. At heavy loads and high temperatures, the benefits of a Schottky diode's low forward voltage may outweigh the disadvantage of high leakage current. See Table 4 for a list of suggested diode suppliers.
__________________Design Procedure
Inductor Selection
The MAX848/MAX849's high switching frequency allows the use of a small inductor. Use a 10H inductor for the MAX849 and a 22H inductor for the MAX848. Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use with high switching frequencies. Make sure the inductor's saturation rating (the current at which the core begins to saturate and inductance starts to fall) exceeds the internal current limit: 0.8A for the MAX848 and 1.4A for the MAX849. However, it is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). For highest efficiency, use a coil with low DC resistance, preferably under 100m. To minimize radiated noise, use a toroid, pot core, or shielded inductor. See Table 5 for a list of suggested inductor suppliers.
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
1M
MAX848 MAX849
ON2 OUT VDD I/O ON1 I/O
C
MAX848 MAX849
MAX8865/MAX8866 DUALS MAX8863/MAX8864 SINGLES
PA 1M
C
RADIO
Figure 9. Momentary Pushbutton On/Off Switch
Figure 10. Typical Phone Application
Capacitor Selection
Input Bypass Capacitors A 22F, low-ESR input capacitor will reduce peak currents and reflected noise due to inductor current ripple. Smaller ceramic capacitors may also be used for light loads or in applications that can tolerate higher input ripple. Output Filter Capacitors Two 100F (single 100F for the MAX848), 10V, lowESR, output filter capacitors typically exhibit 30mV ripple when stepping up from 1.2V to 3.3V at 200mA (100mA for the MAX848). Bypass the MAX848/MAX849 supply input, OUT, with a 0.1F ceramic capacitor to GND. Also bypass POUT to PGND with a 0.1F ceramic capacitor. The filter capacitors' equivalent series resistance (ESR) affects efficiency and output ripple. The output voltage ripple is the product of the peak inductor current and the output capacitor's ESR. Low-ESR, surface-mount tantalum capacitors are currently available from Sprague (595D series) and AVX (TPS series). Sanyo OS-CON organic-semiconductor, through-hole capacitors also exhibit very low ESR, and are especially useful for operation at cold temperatures. See Table 5 for a list of suggested capacitor suppliers.
Applications Information
Using a Momentary On/Off Switch
A momentary pushbutton switch can be used to turn the MAX848/MAX849 on and off. As shown in Figure 9, ON1 is pulled low and ON2 is pulled high when the part is off. When the momentary switch is pressed, ON2 is pulled low and the regulator turns on. The switch should be on long enough for the C to exit reset. The controller issues a logic high to ON1, which guarantees that the part will stay on, regardless of the switch state. To turn off the regulator, the switch is pressed and held. The controller reads the switch status and pulls ON1 low. The switch is released and ON2 is pulled high.
Power Amplifier (PA) and Radio Supply in a Typical Phone Application
The MAX849 is an ideal power supply for the power amplifier (PA) and the radio used in digital cordless and PCS phones (Figure 10). The PA is directly powered by the MAX849 for maximum output swing. Postlinear regulators power the controller and the radio. In addition, they reduce switching noise and ripple. Table 3 lists the output power available when operating with one or more NiCd/NiMH cells or one Li-Ion cell.
Table 3. Available Output Power
NUMBER OF CELLS 1 NiCd/NiMH 2 NiCd/NiMH 2 NiCd/NiMH 3 NiCd/NiMH or 1 Li-Ion INPUT VOLTAGE (V) 1.2 2.4 2.4 3.6 OUTPUT VOLTAGE: PA POWER SUPPLY (V) 3.3 3.3 5.0 5.0 OUTPUT POWER (W) 0.9 2.4 2.6 4.3 13
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
C
MAX848 MAX849
the timing resistor should not exceed the difference between the output voltage and the C reset threshold voltage. This resistor should be large enough to minimize the shutdown current.
OUT R POK C
VCC
C-Controlled Shutdown
The MAX848/MAX849 turn on when ON1 = 1 or ON2 = 0. The C monitors the battery voltage and turns off the device (forces ON1 low and ON2 high) when the battery is weak.
RESET
Layout Considerations
Figure 11. Power-On Reset Delay
Power-On Reset Delay
Adding a timing capacitor from POK to GND generates a power-on reset delay. The reset time constant is determined by the pull-up resistor and timing capacitor (Figure 11). When power is turned on, POK is low and the capacitor is shorted. When the output voltage reaches regulation, POK goes high and the capacitor slowly charges to the output voltage. The timing resistor value depends on the controller's RESET input leakage current. The voltage drop across
Due to high inductor current levels and fast switching waveforms, which radiate noise, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting PGND, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). Also, minimize lead lengths to reduce stray capacitance and trace resistance. If an external resistor divider is used to set the output voltage (Figure 5), the trace from FB to the resistors must be extremely short and must be shielded from switching signals, such as CLK, DATA, or LX.
Table 4. Component Selection Guide
PRODUCTION Surface Mount INDUCTORS Sumida CDR63B, CD73, CDR73B, CD74B series Coilcraft DO1608, DO3308, DT3316 series Sumida RCH654 series CAPACITORS Matsuo 267 series Sprague 595D series AVX TPS series Sanyo OS-CON series Nichicon PL series DIODES Motorola MBR0520L
Through Hole
Motorola 1N5817
Table 5. 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
Chip Information
TRANSISTOR COUNT: 2059
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters
Pin Configuration
TOP VIEW
AIN1 1 AIN2 2 REF 3 GND 4 OUT 5 POKIN 6 FB 7 POK 8 16 ON1 15 ON2 14 POUT
MAX848/MAX849
MAX848 MAX849
13 LX 12 PGND 11 CLK/SEL 10 DATA 9 AINSEL
Narrow SO
Package Information
SOICN.EPS
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1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters MAX848/MAX849
NOTES
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.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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