Adjust The Output Of An Inverting Buck-Boost Regulator Without Level Shifting

Focus:

Switching power applications sometimes require external adjustment of the output voltage setpoint. A common way to adjust the setpoint is to use a microcontroller to generate a variable voltage through either a D-A converter or an averaged PWM signal. This is straightforward when the dc control voltage, input voltage, output voltage and regulator share the same reference—typically the system ground reference (GND). But things get more complicated when trying to adjust the output of an inverting buck-boost regulator, where the output voltage is negative and the regulator GND reference is not the same as the system GND. Typically, a level-shifting circuit (with a current mirror) is required, which adds several extra components. However, under certain operating conditions, which are explained here, the level shifter can be eliminated and a very simple voltage-adjustment scheme (the injection resistor method) can be applied. The circuit used for the injection resistor method is simple, but the math to determine the necessary feedback component values is not. Equations required to select these values are derived here.

What you’ll learn:

• How to adjust the output voltage setpoint of an inverting buck-boost regulator using the injection resistor method

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Author & Publication:

Hrag Kasparian, Texas Instruments, Santa Clara, Calif. and David Baba, Texas Instruments, Phoenix, Ariz. , How2Power Today, May 13 2022

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