Power efficiency improvement solutions

　　If a designer wants to use the Linear Technology LT1072 switching regulator in buck mode and needs to handle high input voltages, getting the highest efficiency becomes an issue. For example, if you need to convert a device from 20V to 5V at a lower power level of 1.25W, the quiescent current of the device (typically 6 mA) will become a significant portion of the circuit's power consumption.

　　Quiescent current is relatively unaffected by input voltage, so IC power consumption is proportional to its supply voltage. If your system has an external low-voltage supply available, you can use it as the IC power supply—the LT1052 can operate at voltages as low as 2.6V. If you don't have such an auxiliary supply, you can add a conversion circuit to run the IC itself from its output (Figure 1). Adding this feature can increase the overall efficiency of the power supply from 77% to 83%.

　　When the power supply is first powered up, the regulator has no output: R8 and D7 keep C6 uncharged and the gate of MOSFET Q4 connected to ground. Since Q4 is off, the increased voltage pulls the gate of Q3 higher through R5. As the supply voltage increases, Q3 turns on, applying the full input voltage to the IC, and the regulator begins to operate.

　　Once the regulator starts, C6 begins to charge through R8 as the output voltage rises. When the gate voltage of Q4 reaches approximately 2.5V, Q4 turns on, pulling the gate voltage of Q3 to ground, turning it off. This turn-off action removes the input voltage from the IC. As C5 discharges into the IC, D5 becomes forward biased, providing the supply voltage from the output voltage to the IC.

　　As C5 discharges into the IC, D5 becomes forward biased, providing the supply voltage from the output voltage to the IC.

　　If a power failure or temporary short circuit causes the output voltage to drop below the minimum value for normal operation of the LT1072, diode D7 will quickly discharge C6 and supply the input voltage to the IC again. When the voltage rises again, normal operation is restored.

Figure 1 After the power supply starts up, this conversion circuit powers the regulator IC from the output of the power supply. Powering the IC with the lower voltage output rather than the input can increase the power supply efficiency from 77% to 83%.