STCH02 Datasheet, PDF (16/24 Pages) STMicroelectronics – Offline PWM controller for ultra-low standby adapters

Application information
STCH02
3.7
Voltage feedforward block
The current control structure uses the voltage V_C to define the output current, according to
Equation 1. Actually, the CC comparator will be affected by an internal propagation delay
Td, which will switch off the MOSFET with a peak current than higher the foreseen value.
The STCH02 device implements a line feedforward function, which solves the issue by
introducing an input voltage dependent offset on the current sense signal, in order to adjust
the cycle-by-cycle current limitation.
The external schematic configuration is shown in Figure 9.
Figure 9. Feedforward compensation: internal schematic
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The R_ZCD resistor can be calculated as follows:
Equation 2
R
ZCD

N
AUX
N

LR
T ^{P R} R^
FF
P RI
^D
SENSE
where R_FF is an internal parameter, defined in Table 5 on page 7.
In this case the peak drain current does not depend on input voltage anymore.
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