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MAX1771EPA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1771EPA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 16 page 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller 8 _______________________________________________________________________________________ increases at low input voltages. However, the supply current is also reduced because V+ is at a lower volt- age, and because less energy is consumed while charging and discharging the external MOSFET’s gate capacitance. The minimum input voltage is 3V when using external feedback resistors. With supply voltages below 5V, bootstrapped mode is recommended. Note: When using the MAX1771 in non-boot- strapped mode, there is no preset output operation because V+ is also the output voltage sense point for fixed-output operation. External resistors must be used to set the output voltage. Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2b, 2c) to achieve an overall output volt- age accuracy of ±5%. To achieve highest efficiency, operate in bootstrapped mode whenever possible. External Power-Transistor Control Circuitry PFM Control Scheme The MAX1771 uses a proprietary current-limited PFM control scheme to provide high efficiency over a wide range of load currents. This control scheme combines the ultra-low supply current of PFM converters (or pulse skip- pers) with the high full-load efficiency of PWM converters. Unlike traditional PFM converters, the MAX1771 uses a sense resistor to control the peak inductor current. The device also operates with high switching frequencies (up to 300kHz), allowing the use of miniature external components. As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses the output is out of regulation. However, unlike tradition- al PFM converters, the MAX1771 switch uses the com- bination of a peak current limit and a pair of one-shots that set the maximum on-time (16µs) and minimum off- time (2.3µs); there is no oscillator. Once off, the mini- mum off-time one-shot holds the switch off for 2.3µs. After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. Figure 2a. 12V Preset Output, Bootstrapped Figure 2b. 12V Output, Non-Bootstrapped Figure 2c. 9V Output, Bootstrapped MAX1771 VIN = 5V REF SHDN AGND GND N MTD20N03HDL 7 EXT CS FB L1 22 µH D1 1N5817-22 R1 18k C4 300 µF C5 100pF C3 0.1 µF 5 4 6 1 8 3 2 V+ C1 68 µF VOUT = 12V @ 0.5A R2 127k RSENSE 40m Ω C2 0.1 µF VOUT VREF R2 = (R1) ( -1 ) VREF = 1.5V MAX1771 REF SHDN AGND GND N 7 EXT CS FB C1 47 µF L1 22 µH D1 1N5817-22 R1 28k C4 200 µF C5 100pF C3 0.1 µF 5 4 6 1 8 3 2 V+ VOUT = 9V R2 140k RSENSE 40m Ω C2 0.1 µF VOUT VREF R2 = (R1) ( -1 ) VREF = 1.5V Si9410DY/ MTD20N03HDL VIN = 4V MAX1771 VIN = 5V REF SHDN FB AGND GND N 7 EXT CS C2 0.1 µF C1 68 µF L1 22 µH D1 1N5817-22 Si9410DY/ MTD20N03HDL RSENSE 40m Ω C4 300 µF C3 0.1 µF 5 4 3 6 1 8 2 V+ VOUT = 12V @ 0.5A |
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