5.2 Pulse-Width Modulation

For Pulse-Width Modulation (PWM) duty cycles that exceed 50%, the control system can become bimodal, where a wide pulse followed by a short pulse repeats instead of the desired fixed pulse width. To prevent this mode of operation, an internal compensating ramp is summed into the current shown in Figure 5-1.

The internal oscillator starts the switching period, which, for MCP16367/8/9, occurs at a 2.2 MHz frequency. With the integrated switch turned on, the inductor current ramps up until the sum of the current sense and slope compensation ramp exceeds the integrated error amplifier output. The error amplifier output slews up or down to increase or decrease the inductor peak current feeding into the output LC filter. If the regulated output voltage is lower than its target, the inverting error amplifier output rises. This results in an increase in the inductor current to correct for errors in the output voltage. The fixed frequency duty cycle is terminated when the sensed inductor peak current summed with the internal slope compensation exceeds the output voltage of the error amplifier. The PWM latch is set by turning off the internal switch and preventing it from turning on until the beginning of the next cycle. An overtemperture signal or bootstrap capacitor undervoltage can also reset the PWM latch to asynchronously terminate the switching cycle.