VREF

RT/CT

CS

0.1 µF

Copyright © 2016, Texas Instruments Incorporated

Figure 8-8. Slope Compensation Circuit

8.3.10 Voltage Mode

In certain applications, voltage mode control may be a preferred control strategy for a variety of reasons. Voltage

mode control is easily executable with any current mode controller, especially the UCCx8C5x family members.

Implementation requires generating a 0-V to 0.9-V sawtooth shaped signal to input to the current sense pin

(CS) which is also one input to the PWM comparator. This is compared to the divided down error amplifier

output voltage at the other input of the PWM comparator. As the error amplifier output is varied, it intersects the

sawtooth waveform at different points in time, thereby generating different pulse widths. This is a straightforward

method of linearly generating a pulse whose width is proportional to the error voltage.

Implementation of voltage mode control is possible by using a fraction of the oscillator timing capacitor

The oscillator timing components must be selected to approximate as close to a linear sawtooth waveform

as possible. Although exponentially charged, large values of timing resistance and small values of timing

capacitance help approximate a more linear shaped waveform. A small transistor is used to buffer the oscillator

timing components from the loading of the resistive divider network. Due to the offset of the oscillator’s lower

timing threshold, a DC blocking capacitor is added.

VREF

RT/CT

R

RT

C

CT

CS

2N2222

Figure 8-9. Current Mode PWM Used as a Voltage Mode PWM

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UCC28C53, UCC28C54, UCC28C55

UCC28C56H, UCC28C57H, UCC28C57L, UCC28C59

SLUSER8 – JUNE 2022

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