ALTAIR04-900 Datasheet, PDF (17/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

ALTAIR04-900
Application information
Figure 15. Constant current operation: Switching cycle waveforms
T
IP
t
Is
t
Q
t
IC
I _CREF
t
I_CREF
=
−
V_C
R
5.6
Voltage feedforward block
The current control structure uses the voltage V_C to define the output current, according to
(2). Actually, the CC comparator is affected by an internal propagation delay Td, which
switches off the MOSFET with a peak current than higher the foreseen value.
This current overshoot is equal to:
Δ I_P
=
V_IN ⋅ T_d
L_P
(3)
where Lp is the primary inductance and it introduces an error on the calculated CC setpoint,
depending on the input voltage.
The 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 internal schematic is shown in Figure 16.
Doc ID 18211 Rev 2
17/29