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MAX775EPA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX775EPA Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page connected to the output voltage (-5V, -12V, -15V). In nonbootstrapped operation, OUT is connected to ground, and EXT now swings from V+ to ground. At high input-to-output differentials, it may be neces- sary to use nonbootstrapped mode to avoid the 21V V+ to VOUT maximum rating. Also, observe the VGS maxi- mum rating of the external transistor. At intermediate voltages and currents, the advantages of bootstrapped vs. nonbootstrapped operation are slight. When input voltages are less than about 4V, always use the boot- strapped circuit. Shutdown and Quiescent Current The MAX774/MAX775/MAX776 are designed to save power in battery-powered applications. A TTL/CMOS logic-level shutdown input (SHDN) has been provided for the lowest-power applications. When shut down (SHDN = V+), most internal bias current sources and the reference are turned off so that less than 5µA of current is drawn. In normal operation, the quiescent current will be less than 100µA. However, this current is measured by forc- ing the external switch transistor off. Even with no load, in an actual application, additional current will be drawn to supply the feedback resistors’ and the diode’s and capacitor’s leakage current. Under no-load condi- tions, you should see a short current pulse at half the peak current approximately every 100ms (the exact period depends on actual circuit leakages). EXT Drive Voltages EXT swings from OUT to V+ and provides the drive out- put for an external power MOSFET. When using the on- chip feedback resistors for the preset output voltages, the voltage at OUT equals the output voltage. When using external feedback resistors, OUT may be tied to GND or some other potential between VOUT and GND. Always observe the V+ to OUT absolute maximum rat- ing of 21V. For V+ to output differentials greater than 21V, OUT must be tied to a potential more positive than the output and, therefore, the output voltage must be set with an external resistor divider. In nonbootstrapped operation with low input voltages (<4V), tie OUT to a negative voltage to fully enhance the external MOSFET. Accomplish this by creating an inter- mediate voltage for VOUT with a zener diode (Figure 5). __________________Design Procedure Setting the Output Voltage The MAX774/MAX775/MAX776 are preset for -5V, -12V, and -15V output voltages, respectively; however, they may also be adjusted to other values with an external voltage divider. For the preset output voltage, connect FB to REF and connect OUT to the output (Figure 3). In this case, the output voltage is sensed by OUT. For an adjustable output (Figures 3 and 4), connect an external resistor divider from the output voltage to FB, and from FB to REF. In this case, the divided-down out- put voltage is sensed via the FB pin. There are three reasons to use the external resistor divider: 1) An output voltage other than a preset value is desired. 2) The input-to-output differential exceeds 21V. 3) The output voltage (VOUT to GND) exceeds -15V. See Figures 3 and 4 for adjustable operation. The impedance of the feedback network should be low enough that the input bias current of FB is not a factor. For best efficiency and precision, allow 10µA to flow through the network. Calculate (VREF -VFB) / R1 = 10µA. Since VREF = 1.5V and VFB = 0V, R1 becomes 150k Ω. Then calculate R2 as follows: R2 VOUT ___ = _______ R1 VREF (or, VOUT = 10 µA) ______ R2 Choosing an Inductor Practical inductor values range from 10µH to 50µH. The maximum inductor value is not particularly critical. For highest current at high VOUT to V+ ratios, the -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-to-DC Controllers ______________________________________________________________________________________ 11 Figure 5. Connection Using Zener Diode to Boost Base Drive R1 MAX774 MAX775 MAX776 OUT GND FB R2 8 2 1 REF 4 0.1 µF 0.1 µF VOUT RZ 6V ≤ VZ + VIN ≤ 10V VOUT – VZ > IZ RZ IZ = ZENER BREAKDOWN CURRENT VZ = ZENER BREAKDOWN VOLTAGE VIN = INPUT SUPPLY VOLTAGE |
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