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NEGATIVE_BUCK_REGULATOR

2015-03-23 19:03  
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NEGATIVE_BUCK_REGULATOR
This circuit adopts a step-up (boost) dc-to-dc controller for use in a negative buck-regulator application. The supply voltage must be negative, and it must deliver 160 to 750 mA. Although the boost-regulator IC operates in a buck-regulator circuit, its standard connections enable proper control of Q1. The output voltage, however, must be inverted by an op amp for proper voltage feedback: The load is referred to the most positive supply rail instead of to 101's ground terminal, so the controller must increase its duty cycle as Vout (referred to that terminal) increases. The op amp there-fore inverts the feedback Signal and shifts it to match the 1.5-V threshold internal to IC1. IC1 is configured in its nonbootstrapped mode, which provides an adequate gate-drive signal (ground to -5.2 V) for the external MOSFET (Q1). With Vout set to -3 V and the output current ranging from 160 mA to above 700 mA, the circuit's conversion efficiency ranges from 84 percent to as high as 87.5 percent.