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MAX761CPA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX761CPA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page used in a bootstrapped circuit (Figure 5), undervoltage lockout prevents start-up at low input voltages; but once started, operation can continue down to a lower voltage that depends on the load. There is no undervoltage lockout when the internal feed- back resistors are used (Figure 2), and special circuitry guarantees start-up at 2.0V. The start-up circuitry fixes the duty cycle at 50% until V+ is driven to 2.5V, above which the normal control system takes over. Shutdown Mode The MAX761/MAX762 enter shutdown mode when SHDN is high. In this mode, the internal biasing circuitry is turned off (including the reference) and VOUT equals V+ minus a diode drop (due to the DC path from the input to the output). In shutdown mode, the supply cur- rent drops to less than 5µA. SHDN is a TTL/CMOS logic level input. Connect SHDN to GND for normal operation. LBO is high impedance during shutdown. Modes of Operation When delivering high output currents, the MAX761/ MAX762 operate in CCM. In this mode, current always flows in the inductor, and the control circuit adjusts the switch’s duty cycle on a cycle-by-cycle basis to maintain regulation without exceeding the switch-current capabili- ty. This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode (DCM), current through the inductor starts at zero, rises to a peak value, then ramps down to zero on each cycle. Although effi- ciency is still excellent, the switch waveforms contain ringing (the inductor's self-resonant frequency). This ringing is normal and poses no operational problems. Low-Battery Detector The MAX761/MAX762 provide a low-battery comparator that compares the voltage on LBI to the 1.5V reference voltage. When the LBI voltage is below VREF, LBO (an open-drain output) goes low. The low-battery compara- tor’s 20mV of hysteresis adds noise immunity, prevent- ing repeated triggering of LBO. Use a resistor-divider network between V+, LBI, and GND to set the desired trip voltage VTRIP (Figure 3). When SHDN is high, LBI is ignored and LBO is high impedance. The value of resistor R3 should be no larger than 500k Ω to ensure the LBI leakage current does not cause inaccuracies in VTRIP. __________________Design Procedure Setting the Output Voltage The MAX761/MAX762’s output voltage can be adjusted from 5V to 16.5V using external resistors R1 and R2 configured as shown in Figures 3 and 5. For adjustable- output operation, select feedback resistor R1 in the 10k Ω to 250kΩ range. Higher R1 values within this range give lowest supply current and best light-load efficiency. R2 is given by: R2 = (R1)( VOUT - 1) VREF where VREF = 1.5V. Note: Tie FB to GND for fixed-output operation (bootstrapped mode only). 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters _______________________________________________________________________________________ 9 Figure 4a. CCM, Heavy Load Current Waveform (500mA/div) Figure 4b. Light/Medium Load Current Waveform (500mA/div) 1A 500mA 1A 500mA 0A |
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