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19-1724; Rev 0; 5/00
4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter
General Description
The MAX1709 sets a new standard of space savings for high-power, step-up DC-DC conversion. It delivers up to 20W 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 pushbutton switch, as well as conventional on/off logic control. The MAX1709 also features programmable soft-start and current limit for design flexibility and optimum performance with batteries. The maximum RMS switch current rating is 10A. For a device with a lower current rating, smaller size, and lower cost, refer to the MAX1708 data sheet. o On-Chip 10A Power MOSFET o 5V, 4A 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)
Features
MAX1709
________________________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 MAX1709ESE
Ordering Information
TEMP. RANGE -40C to +85C PIN-PACKAGE 16 Narrow SO
Typical Operating Circuit
INPUT 1V TO 5V 1H OUTPUT 3.3V, 5V, OR ADJ UP TO 4A
Pin Configuration
TOP VIEW
ONA 1 LX 2 LX 3 LX 4 GND 5 SS/LIM 6 REF 7 GND 8 16 ONB 15 CLK 14 3.3/5
OFF ON SYNC OR INTERNAL
ONA MAX1709 CLK SS/LIM REF
LX
MAX1709
13 PGND 12 PGND 11 PGND 10 FB 9 OUT
GND OUT
NARROW SO ________________________________________________________________ Maxim Integrated Products 1
For free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800. For small orders, phone 1-800-835-8769.
4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter MAX1709
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 Narrow SO (derate 16.5mW/C above +70C) .....1.3W Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature ........................................-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 FB Input Current Output Voltage Adjust Range Output Undervoltage Lockout 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 IREF = 0 -1A IREF 50A +2.5V VOUT +5.5V ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V Input Low Level (Note 7) CLK, 2.7V < VOUT < 5.5V 1.245 1.260 4 0.2 (Note 3) VOUT = 1.5V IOUT < 1mA (Note 1), TA = +25C (Note 4) (Note 5) VSS/LIM = 1V VFB = 1.5V (Note 6) V ONB = 3.6V VLX = V ONB = VOUT = +5.5V 3.2 VFB < 0.1V (Note 1) CONDITIONS 3.3/5 = GND, ISW = 1A 3.3/5 = OUT, ISW = 1A MIN 3.26 4.92 1.215 2.5 2.0 40 0.9 0.7 4 200 0.1 0.1 22 SS/LIM = open SS/LIM = 150k to GND 7.5 3.5 9 5 5.0 440 5 40 40 12 6.5 10 1.275 10 5 0.2 x VOUT 0.2 x VOUT V TYP 3.34 5.05 -0.25 1.240 1 MAX 3.42 5.17 -0.45 1.265 200 5.5 2.3 400 1.1 UNITS V %/A V nA V V kHz V V A A A A m A ARMS V mV mV
Measured between 1A < ISW < 3A (Note 2) ISW = 1A VFB = +1.5V
2
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4A, Low-Noise, High-Frequency, 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 < 5.5 CLK, 2.7 V< VOUT < 5.5V ONA, ONB, CLK, 3.3/5 520 82 350 (Note 8) (Note 8) 100 50 600 90 MIN 0.8 x VOUT 0.8 x VOUT 1 680 94 1000 TYP MAX UNITS V A kHz % kHz ns ns
MAX1709
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 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 = unconnected SS/LIM = 150k to GND IREF = 0 7.5 3.5 1.24 (Note 1) ISW = 1A VFB = +1.5V Measured between 1A < ISW < 5A (Note 2) SS/LIM = 1V V ONB = 3.6V VFB = 1.5V (Note 6) 3.2 CONDITIONS VFB < 0.1V, VIN = +2.4V 3.3/5 = GND, ISW = 1A 3.3/5 = OUT, ISW = 1A MIN 3.24 4.9 1.21 TYP MAX 3.45 5.2 1.27 200 -0.45 5.2 5 400 40 15 7 1.28 UNITS V V nA %/A A A A m V V
_______________________________________________________________________________________
3
4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter MAX1709
ELECTRICAL CHARACTERISTICS (continued)
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = -40C to +85C, unless otherwise noted.) (Note 9)
PARAMETER CONDITIONS ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V Input Low Level (Note 7) CLK, 2.7V < VOUT < 5.5V ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V Input High Level CLK, VOUT = 5.5V Logic Input Current Internal Oscillator Frequency Maximum Duty Cycle External Clock Frequency Range CLK/SEL Pulse Width CLK/SEL Rise/Fall Time (Note 8) (Note 8) ONA, ONB, CLK, 3.3/5 500 80 350 100 50 0.8 x VOUT 0.8 x VOUT 1 700 95 1000 MIN TYP MAX 0.2 x VOUT 0.2 x VOUT UNITS V
V
A kHz % kHz ns ns
Note 1: Output voltage is specified at 1A switch current ISW, which is equivalent to approximately 1A (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 MAX1709 is bootstrapped and will operate down to a 0.7V input once started. Note 6: Supply current is measured from the OUT pin to the output voltage (+3.3V). 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.
4
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4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 3.3V)
MAX1709 TOC01
MAX1709
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 5V)
MAX1709 TOC02
EFFICIENCY vs. SWITCHING FREQUENCY (VIN = 3.3V, VOUT = 5V, IOUT = 2A)
MAX1709 TOC03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.001 0.01 0.1 1 VIN = 1.2V VIN = 2.5V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 VIN = 3.3V VIN = 2.5V
86.0 85.5 EFFICIENCY (%) 85.0 84.5 84.0 83.5 83.0
10
0.001
0.01
0.1
1
10
0.4
0.5
0.6
0.7
0.8
0.9
1.0
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OPERATING FREQUENCY (MHz)
LOAD REGULATION (VIN = 3.3V, VOUT = 5V)
MAX1709 TOC04
LINE REGULATION (VOUT = 5V, IOUT = 1A)
MAX1709 TOC05
NO-LOAD INPUT CURRENT vs. INPUT VOLTAGE
INPUT VOLTAGE INCREASING INPUT CURRENT (mA) 100 INPUT VOLTAGE DECREASING 10 VOUT = 5V 1
MAX1709 TOC06
2
0.8 0.6 VOUT REGULATION (%) 0.4 0.2 0 -0.2 -0.4 -0.6
1000
VOUT REGULATION (%)
1
0
-1
VOUT = 3.3V 0.1 2.0 2.5 3.0 INPUT VOLTAGE (V) 3.5 4.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 INPUT VOLTAGE (V)
-2 0 1 2 3 4 5 OUTPUT CURRENT (A)
-0.8
STARTUP VOLTAGE vs. LOAD CURRENT
MAX1709 TOC07
SWITCHING FREQUENCY vs. TEMPERATURE
MAX1709 TOC08
NOISE vs. FREQUENCY
25 20 NOISE (mVRMS)
MAX1709 TOC09
2.8 2.6 2.4 STARTUP VOLTAGE (V) 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 TA = +25C TA = +85C 0.001 0.01 0.1 LOAD CURRENT (A) 1 VOUT = 5V TA = -40C VOUT = 3.3V
1.0 0.5 FREQUENCY CHANGE (%) 0 -0.5 -1.0 -1.5 -2.0 VOUT = 3.3V
30
15 10 5 0 -5 -10
10
-40
-15
10
35
60
85
0.1
1 FREQUENCY (MHz)
10
TEMPERATURE (C)
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5
4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter MAX1709
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
SWITCH CURRENT LIMIT vs. SS/LIM RESISTANCE
MAX1709 TOC10
SWITCH CURRENT LIMIT vs. TEMPERATURE
MAX1709-12
HEAVY SWITCHING WAVEFORM
MAX1709-13a
10 9 8 CURRENT LIMIT (A) 7 6 5 4 3 2 50 100 150 200 250
11.5 11.0 CURRENT LIMIT (A) 10.5 10.0
5V 0 VLX 5V/div
5V 9.5 9.0 8.5 8.0 VOUT = 3.3V 4A 2A IOUT = 2A
VOUT 100mV/div
IL 2A/div
300
-40
-15
10
35
60
85
1s/div
SS/LIM RESISTANCE (k)
TEMPERATURE (C)
HEAVY SWITCHING WAVEFORM (WITH LC FILTER)
MAX1709-13b
LINE-TRANSIENT RESPONSE
MAX1709-14
5V 0 VLX 5V/div 3.5V 3V 5V VOUT 100mV/div VOUT 50mV/div IOUT = 1A 1s/div IOUT = 2A L = 12.5nH (COILCRAFT A04T) C = 1F 100s/div VIN 0.5V/div
4A 2A
IL 2A/div
5V
LOAD-TRANSIENT RESPONSE
MAX1709-15
SHUTDOWN WITH SOFT-START (CSS = 0.1F)
5V IOUT 2A/div VOUT 100mV/div 0
MAX1709-16
3A 1A 5V
VONA 5V/div
4A 2A
IIN 2A/div
6A 4A 2A IL 2A/div
0 4V 2V 1ms/div 20s/div CSS = 0.1F ROUT = 5 VONB = VOUT VOUT 2V/div
6
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4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter MAX1709
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
SHUTDOWN WITH SOFT-START (CSS = 0.01F)
5V 0 6A 4A 2A 0 4V 2V 1ms/div CSS = 0.01F ROUT = 5 VONB = VOUT
SHUTDOWN WITHOUT SOFT-START
MAX1709-18
MAX1709-17
VONA 5V/div
5V 0 6A
VONA 5V/div
IIN 2A/div
4A 2A 0
IIN 2A/div
VOUT 2V/div
4V 2V 1ms/div CSS = 0 ROUT = 5 VONB = VOUT
VOUT 2V/div
Pin Description
PIN 1 2, 3, 4 5, 8 NAME ONA LX GND FUNCTION On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1). Drain of N-Channel Power Switch. Connect pins 2, 3, and 4 together. Connect external Schottky diode from LX to OUT. 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, this pin is internally pulled to GND to discharge the soft-start capacitor. 1.26V 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 to PGND with less than 5mm trace length. Connect a 2 series resistor from the output filter capacitor 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 +1.25V (Figure 4). Power Ground, Source of N-Channel Power MOSFET Switch Output Voltage Selection Pin. 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. Optionally, drive with an external clock for external synchronization. Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1).
6
SS/LIM
7
REF
9
OUT
10 11, 12, 13 14
FB PGND 3.3/5
15 16
CLK ONB
_______________________________________________________________________________________
7
4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter MAX1709
KEEP TRACES SHORT AND WIDE 1H L1 D1 VIN
1V TO 5V
C1, C2 2 x 150F
ON-OFF CONTROL
ONA LX LX LX GND SS/LIM MAX1709
ONB CLK 3.3/5 PGND C6, C7 2x 150F
VOUT 5V
GND PGND PGND FB OUT C5 0.1F R2 2
R1 C3 C4 0.22F
REF GND
Figure 1. Standard Operating Circuit
_______________Detailed Description
The MAX1709 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 MAX1709 internal N-channel MOSFET switch is rated for 10A (RMS value) and can deliver loads to 4A, 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 1000kHz.
gle-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 MAX1709 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 frequency can be changed on the fly. The MAX1709 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 MAX1709 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 MAX1709. Figure 3 shows the simplified PWM con-
ONA, ONB
The logic levels at ONA and ONB turn the MAX1709 on or off. When ONA = 1 or ONB = 0, the part is on. When ONA = 0 and ONB = 1, the part is off (Table 1). Logic high on control can be implemented by tying ONB high and using ONA for shutdown. Implement inverted sin8
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4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter MAX1709
Table 1. On/Off Logic Control
ONA 0 0 1 1 ONB 0 1 0 1 MAX1709 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 MAX1709 PWM CONTROLLER EN STARTUP Q OSCILLATOR D SEE FIGURE 3.
ONA ONB REF CLK FB 3.3/5 GND 1.260V
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 MAX1709 enters synchronized current-mode PWM when a clock signal (350kHz < f CLK < 1000kHz) 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 the higher frequencies (see 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 ground. Select capacitor C3 (connected to SS/LIM pin) as: C3 (in F) = 3.2 tSS where tSS is the time (in seconds) it takes the switch current limit to reach full 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 = 312.5k x I1 (R1 312.5k) ILIM
Setting the Output Voltage
The MAX1709 features Dual ModeTM operation. When FB is connected to ground, the MAX1709 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. _______________________________________________________________________________________ _______________________________________________________________________________________ 9
4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter MAX1709
Table 3. Component Selection Guide
PRODUCTION Surface mount INDUCTORS Coilcraft DO3316P-102HC Coiltronics UP2B-1R0 CAPACITORS Panasonic EEFUE0J151R Sanyo 6TPC100M DIODES Motorola MBRD1035CTL STM-Microelectronics STPS8L30B
Table 4. Component Suppliers
SUPPLIER Coilcraft Coiltronics Motorola Panasonic STMMicroelectronics PHONE 847-639-6400 561-241-7876 602-303-5454 714-373-7939 617-259-0300 FAX 847-639-1489 561-241-9339 602-994-6430 714-373-7183 617-259-9442
where I1 is the desired current limit in amperes, and I LIM is the current limit value from the Electrical Characteristics.
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 internal current limit. Note that this current may be reduced through SS/LIM if less than the MAX1709's full load current is needed (see Electrical Characteristics for ratings). For highest efficiency, use a coil with low DC resistance, preferably under 10m. 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 current VOUT = output voltage D' = (VIN) / (VOUT + VD), assuming switch voltage drop is negligible f = switching frequency L1 = inductor value
VIN
__________________Design Procedure
Inductor Selection (L1)
The MAX1709's high switching frequency allows the use of a small-size inductor. Use a 1.0H inductor for 600kHz operation. If the MAX1709 will be synchronized at a different frequency, scale the inductor value with the inverse of frequency (L1 = 1H 600kHz / fSYNC). The PWM design tolerates inductor values within 25% of this calculated value, so choose the closest standard inductor value. For example, use 1.5H for 350kHz and 0.68H for 1MHz).
FB REF SLOPE COMP
LX R Q N
LX VOUT
S
MAX1709
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
10
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4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter
capacitor to GND and a 2 series resistor (R2, as shown in Figure 1).
270k MAX1709 ON/OFF ONB ONA VDD I/O I/O C
MAX1709
MAX1709 IC Power Dissipation
The major components of MAX1709 dissipated power are switch conductance loss (PSW), capacitive loss (PCAP), and switch transition loss (PTRAN). Throughout the formulas, numerical examples are provided in {}, corresponding to the following condition: {VIN = 3.3V, VOUT = 5V, VD = 0.5V, IOUT = 4A} An important parameter to compute the power dissipated in the MAX1709 is the approximate peak switch current (ISW): I ISW = OUT D' VIN D' = VOUT + VD PD (MAX1709) = PSW + PCAP + PTRAN PSW = (1 - D') ISW2 RSW {6.67A } {0.6} {0.83W} {0.59W}
270k 0.1F
Figure 5. Momentary Pushbutton On-Off Switch
ILIM = minimum value of switch current limit from Electrical Characteristics or set by RSET/LIM.
Diode Selection (D1)
The MAX1709's high switching frequency demands a high-speed rectifier. Schottky diodes, such as the MBRD1035CTL or STPS8L30B (Table 3), are recommended. 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.
where: RSW = switch resistance {33m} PCAP = (CDIO + CDSW + CGSW) (VOUT + VD)2f {0.09W} CDIO = catch-diode capacitance {1000pF} CDSW = switch drain capacitance {2500pF} CGSW = switch gate capacitance {1500pF} f = switching frequency {600kHz} PTRAN = (VOUT + VD) ISW tSW f / 3 {0.15W} where tSW = is switch turn-on or turn-off time {20ns}.
Applications Information
Using a Momentary On/Off Switch
A momentary pushbutton switch can be used to turn the MAX1709 on and off. As shown in Figure 5, when ONA is pulled low and ONB is pulled high, the part 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 part will stay on regardless of the subsequent switch state. To turn the regulator off, press 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.
Capacitor Selection
Input Bypass Capacitors (C1, C2) Two 150F, low-ESR tantalum input capacitors 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 50m are acceptable. Smaller ceramic capacitors may also be used for light loads or in applications that can tolerate higher input current ripple. Output Filter Capacitors (C6, C7) The output filter capacitor ESR must be kept under 15m for stable operation. Two parallel 150F polymer capacitors (Panasonic EEFUE0J151R) typically exhibit 5m of ESR. This translates to approximately 35mV of output ripple at 7A switch current. Bypass the MAX1709 IC supply input (OUT) with a 0.1F ceramic
Layout Considerations
Due to high inductor current levels and fast switching waveforms, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground config11
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4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter
uration. 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 capacitors (C6 and C7) 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. Refer to a layout example in the MAX1709EVKIT data sheet.
MAX1709
___________________Chip Information
TRANSISTOR COUNT: 1112
Package Information
SOICN.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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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