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19-2068; Rev 0; 7/01
High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter
General Description
The MAX1708 sets a new standard of space savings for high-power, step-up DC-DC conversion. It delivers up to 10W at a fixed (3.3V or 5V) or adjustable (2.5V to 5.5V) output, using an on-chip power MOSFET from a +0.7V to +5V supply. Fixed-frequency PWM operation ensures that the switching noise spectrum is constrained to the 600kHz fundamental and its harmonics, allowing easy postfiltering for noise reduction. External clock synchronization capability allows for even tighter noise spectrum control. Quiescent power consumption is less than 1mW to extend operating time in battery-powered systems. Two control inputs (ONA, ONB) allow simple push-on, push-off control through a single momentary push-button switch, as well as conventional on/off logic control. The MAX1708 also features programmable soft-start and current limit for design flexibility and optimum performance with batteries. The maximum RMS switch current rating is 5A. For a device with a higher (10A) switch current rating, refer to the MAX1709 data sheet. o On-Chip 5A Power MOSFET o 5V, 2A Output from a 3.3V Input o Fixed 3.3V or 5V Output Voltage or Adjustable (2.5V to 5.5V) o Input Voltage Range Down to 0.7V o Low Power Consumption 1mW Quiescent Power 1A Current in Shutdown Mode o Low-Noise, Constant Frequency Operation (600kHz) o Synchronizable Switching Frequency (350kHz to 1000kHz) o Small QSOP Package
Features
MAX1708
________________________Applications
Routers, Servers, Workstations, Card Racks Local 2.5V to 3.3V or 5V Conversion Local 3.3V to 5V Conversion 3.6V or 5V RF PAs in Communications Handsets
PART MAX1708EEE
Ordering Information
TEMP. RANGE -40C to +85C PIN-PACKAGE 16 QSOP
Typical Operating Circuit
INPUT 1V TO 5V 2.2H OUTPUT 3.3V, 5V, OR ADJ UP TO 2A
Pin Configuration
TOP VIEW
ONB 1 ONA 2 LX 3 LX 4 LX 5 16 CLK 15 3.3/5 14 PGND
OFF ON SYNC OR INTERNAL
ONA MAX1708 CLK SS/LIM REF
LX
MAX1708
13 PGND 12 PGND 11 FB 10 OUT 9 GND
GND GND 6 OUT SS/LIM 7 REF 8
QSOP
________________________________________________________________ Maxim Integrated Products
1
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
ABSOLUTE MAXIMUM RATINGS
ONA, ONB, OUT, SS/LIM, 3.3/5 to GND ...............-0.3V to +6.0V LX to PGND ...........................................................-0.3V to +6.0V FB, CLK, REF to GND.............................. -0.3V to (VOUT + 0.3V) PGND to GND .......................................................-0.3V to +0.3V Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 8.30mW/C above +70C). .......667mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C 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
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Output Voltage Load Regulation FB Regulation Voltage (VFB) FB Input Current Output Voltage Adjust Range Output Undervoltage Lockout (Note 3) Frequency in Startup Mode Minimum Startup Voltage Minimum Operating Voltage Soft-Start Pin Current OUT Supply Current OUT Leakage Current In Shutdown LX Leakage Current N-Channel Switch On-Resistance N-Channel Current Limit RMS Switch Current Reference Voltage Reference Load Regulation Reference Supply Rejection Input Low Level (Note 7) IREF = 0 -1A IREF 50A +2.5V VOUT +5.5V ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V CLK, 2.7V < VOUT < 5.5V 1.245 1.260 4 0.2 0.2
CONDITIONS VFB < 0.1V (Note 1) 3.3/5 = GND, ISW = 0.5A 3.3/5 = OUT, ISW = 0.5A
MIN 3.26 4.90 1.215 2.5
TYP 3.34 5.05 -0.40 1.240 1
MAX 3.42 5.20 -0.60 1.265 200 5.5 2.3 400
UNITS V %/A V nA V V kHz V V A A A A m A ARMS V mV mV V
Measured between 0.5A < ISW < 1.5A (Note 2) ISW = 0.5A VFB = +1.5V
Rising and falling VOUT =1.5V IOUT < 1mA, TA = +25C (Note 4) (Note 5) VSS/LIM = 1V VFB = 1.5V (Note 6) V ONB = 3.6V VLX = V ONB = VOUT = +5.5V
2.0 40 0.9 0.7 3.2 4 200 0.1 1 30
1.1 5.0 300 2 25 80 7.0 3.85 5 1.275 10 5 VOUT 0.2 VOUT
SS/LIM = open SS/LIM = 150k to GND
4.5 1.80
5.3 3.00
2
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input High Level Logic Input Current Internal Oscillator Frequency Maximum Duty Cycle External Clock Frequency Range CLK Pulse Width CLK Rise/Fall Time CONDITIONS ONA, ONB, 3.3/5, 1.2V < VOUT <5.5V CLK, 2.7 V< VOUT < 5.5V ONA, ONB, CLK, 3.3/5 = 0, 5.5V MIN 0.8 VOUT 0.8 VOUT -1 520 82 350 (Note 8) (Note 8) 100 50 600 88 1 680 94 1000 TYP MAX UNITS V A kHz % kHz ns ns
MAX1708
ELECTRICAL CHARACTERISTICS
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = -40C to +85C, unless otherwise noted.) (Note 9)
PARAMETER Output Voltage FB Regulation Voltage FB Input Current (VFB) Load Regulation Soft-Start Pin Current OUT Leakage Current in Shutdown OUT Supply Current N-Channel Switch On-Resistance N-Channel Current Limit Reference Voltage SS/LIM = open SS/LIM = 150k to GND IREF = 0 4.5 1.8 1.24 CONDITIONS VFB < 0.1V, VIN = +2.4V (Note 1) ISW = 0.5A VFB = +1.5V Measured between 0.5A < ISW < 1.5A (Note 2) SS/LIM = 1V V ONB = 3.6V VFB = 1.5V (Note 6) 3.2 3.3/5 = GND, ISW = 0.5A 3.3/5 = OUT, ISW = 0.5A MIN 3.24 4.90 1.20 MAX 3.45 5.22 1.28 200 -0.60 5.2 2 300 80 7.5 4.0 1.28 UNITS V V nA %/A A A A m A V
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
ELECTRICAL CHARACTERISTICS (continued)
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = -40C to +85C, unless otherwise noted.) (Note 9)
PARAMETER Input Low Level (Note 7) CONDITIONS ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V CLK, 2.7V < VOUT < 5.5V Input High Level Logic Input Current Internal Oscillator Frequency Maximum Duty Cycle External Clock Frequency Range CLK Pulse Width CLK Rise/Fall Time (Note 8) (Note 8) ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V CLK, 2.7V < VOUT < 5.5V ONA, ONB, CLK, 3.3/5 = 0, 5.5V 0.8 VOUT 0.8 VOUT -1 500 80 350 100 50 1 700 95 1000 A kHz % kHz ns ns MIN MAX 0.2 VOUT 0.2 VOUT V UNITS V
Note 1: Output voltage is specified at 0.5A switch current ISW, which is equivalent to approximately 0.5A (VIN / VOUT) of load current. Note 2: Load regulation is measured by forcing specified switch current and straight-line calculation of change in output voltage in external feedback mode. Note that the equivalent load current is approximately ISW (VIN / VOUT). Note 3: Until undervoltage lockout is reached, the device remains in startup mode. Do not apply full load until this voltage is reached. Note 4: Startup is tested with Figure 1's circuit. Output current is measured when both the input and output voltages are applied. Note 5: Minimum operating voltage. The MAX1708 is bootstrapped and will operate down to a 0.7V input once started. Note 6: Supply current is measured from the output voltage (+3.3V) to the OUT pin. This correlates directly with actual input supply current but is reduced in value according to the step-up ratio and efficiency. Note 7: ONA and ONB inputs have approximately 0.15V hysteresis. Note 8: Guaranteed by design, not production tested. Note 9: Specifications to -40C are guaranteed by design, not production tested.
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
MAX1708 toc01a
MAX1708
EFFICIENCY vs. OUTPUT CURRENT
MAX1708 toc01b
EFFICIENCY vs. SWITCHING FREQUENCY
MAX1708 toc02
100 VIN = 3.3V VIN = 2.5V
100 VIN = 2.5V
90
80 EFFICIENCY (%)
80 EFFICIENCY (%)
89 60 60 VIN = 1.2V 40 EFFICIENCY (%)
88
40
20 VOUT = 5V 0 0.1 1 10 100 1000 10,000 OUTPUT CURRENT (mA)
20 VOUT = 3.3V 0 0.1 1 10 100 1000 10,000 OUTPUT CURRENT (mA)
87 VIN = 3.3V, VOUT = 5V, IOUT =1A 86 350 450 550 650 750 850 950 SWITCHING FREQUENCY (kHz)
LOAD REGULATION (VIN = 3.3V, VOUT = 5V)
MAX1708 toc03a
LOAD REGULATION (VIN = 2.5V, VOUT = 3.3V)
MAX1708 toc03b
LINE REGULATION (VOUT = 5V)
MAX1708 toc04a
2.0 1.5 VOUT REGULATION (%) 1.0 0.5 0 -0.5 -1.0 -1.5 PLOT NORMALIZED TO IOUT = 500mA -2.0 0.1 1 10 100 1000
2.0 1.5 VOUT REGULATION (%) 1.0 0.5 0 -0.5 -1.0 -1.5 PLOT NORMALIZED TO IOUT = 500mA -2.0 0.1 1 10 100 1000
0.3 0.2 VOUT REGULATION (%) 0.1 0 -0.1 -0.2 PLOT NORMALIZED TO VIN = 3.3V -0.3
IOUT = 1A
IOUT = 500mA
10,000
10,000
2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
LINE REGULATION (VOUT = 3.3V)
0.7 0.6 VOUT REGULATION (%) 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 IOUT = 500mA PLOT NORMALIZED TO VIN = 2.5V 1.50 1.75 2.00 2.25 2.50 2.75 3.00 INPUT VOLTAGE (V) 5 IOUT = 1A
MAX1708 toc04b
NO LOAD INPUT CURRENT vs. INPUT VOLTAGE
MAX1708 toc05a
NO LOAD INPUT CURRENT vs. INPUT VOLTAGE
MAX1708 toc05b
0.8
25
70 60 VOUT = 5V, VIN DECREASING INPUT CURRENT (mA) 50 40 30 20 10 0 VOUT = 3.3V, VIN DECREASING
20 INPUT CURRENT (mA)
VOUT = 5V, VIN INCREASING
15
10 VOUT = 3.3V, VIN INCREASING
0 0 1 2 3 4 5 6 INPUT VOLTAGE (V)
0
1
2
3
4
5
6
INPUT VOLTAGE (V)
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
STARTUP VOLTAGE vs. LOAD CURRENT (VOUT = 5V)
1.6 STARTUP VOLTAGE (V) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 1 10 100 1000 LOAD CURRENT (mA) 10,000 0 TA = -40C TA = +25C TA = +85C
MAX1708 toc06a
STARTUP VOLTAGE vs. LOAD CURRENT (VOUT = 3.3V)
MAX1708 toc06b
SWITCHING FREQUENCY vs. TEMPERATURE
MAX1708 toc07
1.8
2.5
2 PLOT NORMALIZED TO 25C FREQUENCY CHANGE (%) 1
2.0 STARTUP VOLTAGE (V)
1.5 TA = -40C 1.0 TA = +25C TA = +85C
0
0.5
-1 VOUT = 3.3V -2
1
10
100
1000
10,000
-40
-15
10
35
60
85
LOAD CURRENT (mA)
TEMPERATURE (C)
NOISE vs. FREQUENCY
MAX1708 toc08
SWITCH CURRENT LIMIT vs. SS/LIM RESISTANCE
MAX1708 toc09
SWITCH CURRENT LIMIT vs. TEMPERATURE
MAX1708 toc10
4000 3500 3000 NOISE (V) 2500 2000 1500 1000 500 0 0.1 1 FREQUENCY (MHz) RESOLUTION = 1kHz
5
6.0
SWITCH CURRENT LIMIT (A)
SWITCH CURRENT LIMIT (A)
4
5.5
3
5.0
2
1
4.5 VIN = 3.3V, VOUT = 5V
0 10 0 50 100 150 200 250 300 SS/LIM RESISTANCE (k)
4.0 -40 -15 10 35 60 85 TEMPERATURE (C)
HEAVY SWITCHING WAVEFORM
MAX1708 toc11
LINE TRANSIENT RESPONSE
MAX1708 toc12
5V VLX 5V/div 0
3.5V VIN 500mV/div 3V
IL 4A 2A/div 2A 0 VOUT 5V AC-COUPLED 50mV/div 1s/div
VOUT 5V AC-COUPLED 50mV/div
100s/div
6
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX1708
LOAD TRANSIENT RESPONSE
MAX1708 toc13
STARTUP WITHOUT SOFT-START (CSS = 0)
5V VONA 5V/div 0
MAX1708 toc14
4A ISW 2A/div 2A 0 VOUT 5V AC-COUPLED 50mV/div 0 2A IOUT 1A 1A/div 0 40s/div
IIN 2A 1A/div 1A 0 VOUT 4V 2V/div 2V RL = 5
2ms/div
STARTUP WITH SOFT-START (CSS = 0.01F)
5V VONA 5V/div 0 2A IIN 1A 1A/div 0 VOUT 2V/div 4V 2V 0 2ms/div RL = 5
MAX1708 toc15
STARTUP WITH SOFT-START (CSS = 0.1F)
5V VONA 5V/div 0 IIN 1A/div 1A 0 4V VOUT 2V 2V/div 0 RL = 5 2ms/div
MAX1708 toc16
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
Pin Description
PIN 1 2 3, 4, 5 6, 9 NAME ONB ONA LX GND FUNCTION Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1). On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1). Drain of N-Channel Power Switch. Connect pins 3, 4, and 5 together with wide traces. Connect an external Schottky diode from LX to VOUT. (Figure 1) Ground Soft-Start and/or Current-Limit Input. Connect a capacitor from SS/LIM to GND to control the rate at which the device reaches current limit (soft-start). To reduce the current limit from the preset values, connect a resistor from SS/LIM to GND (see Design Procedure). During shutdown, SS/LIM is internally pulled to GND to discharge the soft-start capacitor. Voltage Reference Output. Bypass with a 0.22F capacitor to GND. Maximum REF load is 50A. Output Voltage Sense Input. The device is powered from OUT. Bypass with a 0.1F capacitor to PGND with less than 5mm trace length. Connect a 2 series resistor from the output filter capacitor (0.1F) to OUT (Figure 1). DC-DC Converter Feedback Input. Connect FB to GND for internally set output voltage (see 3.3/5 pin description). Connect a resistor-divider from the output to set the output voltage in the +2.5V to +5.5V range. FB regulates to +1.24V (Figure 4). Power Ground, Source of N-Channel Power MOSFET Switch. Connect pins 12, 13, and 14 together with wide traces. Output Voltage Selection Input. When FB is connected to GND, the regulator uses internal feedback to set the output voltage. 3.3/5 = low sets output to 3.3V; 3.3/5 = high sets output to 5V. If an external divider is used at FB, connect 3.3/5 to ground. Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Drive CLK with an external clock for external synchronization.
7
SS/LIM
8
REF
10
OUT
11
FB
12, 13, 14
PGND
15
3.3/5
16
CLK
8
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
L1 2.2H VIN KEEP TRACES SHORT AND WIDE C1 150F LX ON/OFF CONTROL R1 C3 SS/LIM C4 0.22F REF ONA ONB CLK 3.3/5 MAX1708 OUT C5 0.1F FB GND KEEP TRACES SHORT AND WIDE GND PGND PGND PGND LX LX R2 2 C2 150F GND D1 VOUT +5V
GND
Figure 1. Standard Operating Circuit
_______________Detailed Description
The MAX1708 step-up converter offers high efficiency and high integration for high-power applications. It operates with an input voltage as low as 0.7V and is suitable for single- to 3-cell battery inputs, as well as 2.5V or 3.3V regulated supply inputs. The output voltage is preset to +3.3V or +5.0V or can be adjusted with external resistors for voltages between +2.5V to +5.5V. The MAX1708 internal N-channel MOSFET switch is rated for 5A (RMS value) and can deliver loads to 2A, depending on input and output voltages. For flexibility, the current limit and soft-start rate are independently programmable. A 600kHz switching frequency allows for a small inductor to be used. The switching frequency is also synchronizable to an external clock ranging from 350kHz to 1MHz.
Implement inverted single-line on/off control by grounding ONA and toggling ONB. Implement momentary pushbutton on/off as described in the Applications Information section. Both inputs have approximately 0.15V of hysteresis.
Switching Frequency
The MAX1708 switches at the fixed-frequency internal oscillator rate (600kHz) or can be synchronized to an external clock. Connect CLK to OUT for internal clock operation. Apply a clock signal to CLK to synchronize to an external clock. The MAX1708 will synchronize to a new external clock rate in two cycles and will take approximately 40s to revert to its internal clock frequency once the external clock pulses stop and CLK is driven high. Table 2 summarizes oscillator operation.
Operation
The MAX1708 switches at a constant frequency (600kHz) and modulates the MOSFET switch pulse width to control the power transferred per cycle and regulate the voltage across the load. In low-noise applications, the fundamental and the harmonics generated by the fixed switching frequency are easily filtered out. Figure 2 shows the simplified functional diagram for the MAX1708. Figure 3 shows the simplified PWM con9
ONA, ONB
The logic levels at ONA and ONB turn the MAX1708 on or off. When ONA = 1 or ONB = 0, the device is on. When ONA = 0 and ONB = 1, the device is off (Table 1). Logic high on-control can be implemented by connecting ONB high and using ONA for shutdown.
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
Table 1. On/Off Logic Control
ONA 0 0 1 1 ONB 0 1 0 1 MAX1708 On Off On On
Table 2. Selecting Switching Frequency
CLK 0 1 External clock (350kHz-1000kHz) MODE Not allowed PWM Synchronized PWM
OUT IC POWER 2.15V
UNDERVOLTAGE LOCKOUT MAX1708 PWM CONTROLLER EN STARTUP Q OSCILLATOR D
ONA ONB REF CLK FB 3.3/5 GND 1.26V
ON RDY REFERENCE
EN EN 600kHz OSCILLATOR OSC N FB OUT
LX
PGND
DUAL MODE FB
Figure 2. Simplified Functional Diagram
troller functional diagram. The MAX1708 enters synchronized current-mode PWM when a clock signal (350kHz < fCLK < 1MHz) is applied to CLK. For wireless or noise-sensitive applications, this ensures that switching harmonics are predictable and kept outside the IF frequency band(s). High-frequency operation permits low-magnitude output ripple voltage and minimum inductor and filter capacitor size. Switching losses will increase at higher frequencies (see MAX1708 IC Power Dissipation).
V R3 = R4 OUT - 1 VFB where VFB = 1.24V.
Soft-Start/Current Limit Adjustment (SS/LIM)
The soft-start pin allows the soft-start time to be adjusted by connecting a capacitor from SS/LIM to GND. Select capacitor C3 (see Figure 1): tSS = 4ms + [110 C3 (in F)] where tSS is the time (in milliseconds) it takes output to reach its final value. To improve efficiency or reduce inductor size at reduced load currents, the current limit can be reduced from its nominal value (see Electrical Characteristics). A resistor (R1 in Figure 1) between SS/LIM and ground reduces the current limit as follows: R1 = 312k x I1 ILIM
Setting the Output Voltage
The MAX1708 features Dual ModeTM operation. When FB is connected to ground, the MAX1708 generates a fixed output voltage of either +3.3V or +5V, depending on the logic applied to the 3.3/5 input (Figure 1). The output can be configured for other voltages, using two external resistors as shown in Figure 4. To set the output voltage externally, choose an R3 value that is large enough to minimize load at the output but small enough to minimize errors due to leakage and the time constant to FB. A value of R4 50k is required
Dual Mode is a trademark of Maxim Integrated Products. 10
where I1 is the desired current limit in amperes and R1 312k. ILIM = 5A, if R1 is omitted.
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
Table 3. Component Selection Guide
PRODUCTION Surface mount INDUCTORS Coiltronics UP2B-2R2 Coilcraft DO3316P-222HC CAPACITORS Sanyo 6TPC100M Panasonic EEFUE0J151R DIODES Motorola MBRD1035CTL Central CMSH5-20
Table 4. Component Suppliers
SUPPLIER Central Coilcraft Coiltronics Motorola Panasonic PHONE 631-435-1110 847-639-6400 561-241-7876 602-303-5454 714-373-7939 FAX 631-435-1824 847-639-1489 561-241-9339 602-994-6430 714-373-7183
internal current limit. Note that this current may be reduced through SS/LIM if less than the MAX1708's full load current is needed (see Electrical Characteristics for ratings). For highest efficiency, use a coil with low DC resistance, preferably under 20m. To minimize radiated noise, use a toroid, pot core, or shielded inductor. See Tables 3 and 4 for a list of recommended components and component suppliers. To calculate the maximum output current (in amperes), use the following equation: V + VD - VIN IOUT(MAX) = D' ILIM - D' OUT 2 x x L1 where: VIN = input voltage VD = forward voltage drop of the Schottky diode at ILIM VOUT = output voltage D' = (VIN) / (VOUT + VD), neglecting switch voltage drop f = switching frequency L1 = inductor value ILIM = minimum value of switch current limit from Electrical Characteristics or set by R1 of Figure 1.
__________________Design Procedure
Inductor Selection (L1)
The MAX1708's high switching frequency allows the use of a small-size inductor. Use a 2.2H inductor for 600kHz operation. If the MAX1708 is synchronized at a different frequency, scale the inductor value with the inverse of frequency (L1 = 2.2H 600kHz / fSYNC). The PWM design tolerates inductor values within 25% of this calculated value, so choose the closest standard inductor value. For example, use 3.3H for 350kHz and 1.5H for 1MHz). Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use at high switching frequencies. Ensure the inductor's saturation rating (the current at which the core begins to saturate and inductance falls) exceeds the
VIN FB REF SLOPE COMP
LX R Q N S LX MAX1708 VOUT
R4 SS/LIM 12.5 (LIMITED TO 100mV) OSCILLATOR 11m PGND FB R3
KEEP SHORT
Figure 3. Simplified PWM Controller Functional Diagram
Figure 4. Adjustable Output Voltage
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High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708
MAX1708 IC Power Dissipation
270k MAX1708 ON/OFF ONB ONA VDD I/O I/O C
The major components of MAX1708 dissipated power are switch conductance loss (PSW), capacitive loss (PCAP), and switch transition loss (PTRAN). Numerical examples provided in brackets ({ }) correspond to the following condition: {VIN = 3.3V, VOUT = 5V, VD = 0.5V, IOUT = 2A} An important parameter to compute the power dissipated in the MAX1708 is the approximate peak switch current (ISW): I ISW = OUT {3.33A } D' VIN {0.6} D' = VOUT + VD PD = PSW + PCAP + PTRAN {0.472W} PSW = (1 - D') ISW2 RSW {0.353W} PCAP = (CDIO + CDSW + CGSW) (VOUT + VD)2f {0.045W} PTRAN = (VOUT + VD) ISW tSW f / 3 {0.073W} where: RSW = switch resistance {80m} CDIO = catch-diode capacitance {500pF} CDSW = switch drain capacitance {1250pF} CGSW = switch gate capacitance {750pF} f = switching frequency {600kHz} tSW = switch turn-on or turn-off time {20ns}
270k 0.1F
Figure 5. Momentary Pushbutton On-Off Switch
Diode Selection (D1)
The MAX1708's high switching frequency demands a high-speed rectifier. Use Schottky diodes (Table 3). The diode's current rating must exceed the maximum load current, and its breakdown voltage must exceed VOUT. The diode must be placed within 10mm of the LX switching node and the output filter capacitor. The diode also must be able to dissipate the power calculated by the following equation: PDIODE = IOUT VD where IOUT is the average load current and VD is the diode forward voltage at the peak switch current.
Capacitor Selection
Input Bypass Capacitor (C1) A 150F, low-ESR input capacitor will reduce peak currents and reflected noise due to inductor current ripple. Lower ESR allows for lower input ripple current, but combined ESR values up to 100m are acceptable. Smaller ceramic capacitors may also be used for light loads or in applications that can tolerate higher input current ripple. Output Filter Capacitor (C2) The output filter capacitor ESR must be kept under 30m for stable operation. Polymer capacitors of 150F (Panasonic EEFUE0J151R) typically exhibit 10m of ESR. This translates to approximately 35mV of output ripple at 3.5A switch current. Bypass the MAX1708 IC supply input (OUT) with a 0.1F ceramic capacitor to GND and a 2 series resistor (R2, as shown in Figure 1).
Applications Information
Using a Momentary On/Off Switch
A momentary pushbutton switch can be used to turn the MAX1708 on and off. As shown in Figure 5, when ONA is pulled low and ONB is pulled high, the device is off. When the momentary switch is pressed, ONB is pulled low and the regulator turns on. The switch should be on long enough for the microcontroller to exit reset. The controller issues a logic high to ONA, which guarantees that the device will stay on regardless of the subsequent switch state. To turn the regulator off, depress the switch long enough for the controller to read the switch status and pull ONA low. When the switch is released, ONB pulls high and the regulator turns off.
Layout Considerations
Due to high inductor current levels and fast switching waveforms, proper PC board layout is essential. Protect
12 ______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter
sensitive analog grounds by using a star ground configuration. Connect PGND, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). In addition, minimize trace lengths to reduce stray capacitance and trace resistance, especially from the LX pins to the catch diode (D1) and output capacitor (C2) to PGND pins. If an external resistor-divider is used to set the output voltage (Figure 4), the trace from FB to the resistors must be extremely short and must be shielded from switching signals, such as CLK or LX. To optimize package power dissipation and minimize device heating under heavy loads, expand PC trace area connected to the three PGND pins as much as the layout can allow. This is best accomplished with a large PGND plane on the surface of the board. Also note that outer-layer ground plane area beneath the device provides little heat-sinking benefit. If an outer-layer ground plane is not feasible, the PGND pins should be connected to the inner-layer ground plane with multiple vias (at least three vias per pin is recommended). Since the purpose of these vias is to optimize thermal conductivity to the inner ground plane, be sure that the vias have no gaps in their connections to the ground plane. Refer to a layout example in the MAX1708EVKIT data sheet.
MAX1708
___________________ Chip Information
TRANSISTOR COUNT: 1112 SUBSTRATE: GND PROCESS: BiCMOS
Package Information
QSOP.EPS
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.
13 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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