Specifications

Section 43. High-Speed PWM

High-Speed PWM

43.16.5 Current Reset PWM

The Current Reset PWM, illustrated in Figure 43-52, is a variable frequency mode where the

actual PWM period is less than or equal to the specified period value. The independent time base

is reset externally after the PWM signal has been deasserted. The Current Reset PWM mode

can be used in Constant PWM On-Time mode. To operate in PWM Current Reset, the PWM

generator must be in Independent Time Base. If an external Reset signal is not received, the

PWM period uses the PHASEx register value by default.

In Current Reset mode, the PWM frequency varies with the load current. This is different than

most PWM modes because the user-assigned application sets the maximum PWM period and

an external circuit measures the inductor current. When the inductor current falls below a

specified value, the external current comparator circuit generates a signal that resets the PWM

time base counter. The user-assigned application specifies a PWM ON time, and then some time

after the PWM signal becomes inactive, the inductor current falls below a specified value and the

PWM counter is reset earlier than the programmed PWM period. This is called Constant On-Time

Variable Frequency PWM output and is used in Critical Conduction mode PFC applications.

This should not be confused with cycle-by-cycle current-limiting PWM output where the PWM

output is asserted, an external circuit generates a current fault and the PWM signal is turned off

before its programmed duty cycle normally turns it off. Here, the PWM frequency is fixed for a

given time base period.

The advantages of the Current Reset PWM mode in PFC applications are as follows:

• As the PFC boost inductor does not require to store energy at the end of each switching

cycle, a smaller inductor can be used. Usage of the smaller inductor leads to reduced cost.

• Commutation of Boost diode from ON to OFF happens at zero current. Slower diodes can

be used to reduce the cost.

• Inner current feedback loop is much faster, since feedback is received for every cycle.

Note: In the Current Reset mode, the local time base resetting is based on the leading

edge of the current-limit input signal after completion of the PWMxH/L duty cycle.