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 added to the current, as shown in Figure 5-1.

The internal oscillator initiates each switching period at a fixed frequency of 2.2 MHz for the MCP16367/8/9. When the integrated switch is turned on, the inductor current ramps up until the sum of the current sense signal and the slope compensation ramp exceeds the output of the integrated error amplifier. 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 falls below its target, the error amplifier output increases, resulting in a higher inductor current to correct the output voltage. The fixed frequency duty cycle is terminated when the sensed inductor peak current, combined with the internal slope compensation, exceeds the error amplifier output voltage. 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.