AN3093

Modes of operation

Doc ID16539 Rev 1

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Figure 23.

Constant current operation: switching cycle waveforms

A.3

Current comparator speed

The propagation delay of the internal current comparator used in the CC control block

causes the MOSFET to be switched off with a peak current higher than the theoretical value:

this current overshoot is dependent on the input voltage and introduces an error in the

calculated CC setpoint, rendering Equation 5 no longer valid.

In order to make the cycle-by-cycle current limitation and the CC setpoint almost

independent of the input voltage, a feed-forward function has been implemented, according

the schematic in Figure 24.

During the MOSFET's ON-time, the current sourced from the ZCD/FB pin is mirrored inside

This feed-forward current is proportional to the input voltage and provides an offset on the

current sense to compensate the propagation delay of the current comparator.

following equation:

Equation 6

R

ZCD

n

AUX

n

PRI

-------------

L

P

R

FF

×

T

d

R

SENSE

×

----------------------------------

×

=