ALTAIR05T-800 Datasheet, PDF (18/28 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

Application information
ALTAIR05T-800
Figure 16. Feedforward compensation: internal schematic
R zcd ZCD/FB
F eedf orward .
Logic
Rfb
A ux
CC
I FF Block
CC
DRAIN
PWM
LOGI C
Rf f
SOURCE
Rs ense
The R_ZCD resistor can be calculated as follows:
R_ZCD
=
N_AUX
N_PRI
⋅ ^LP⋅RFF
T_d ⋅ R_{S ENSE}
(4)
In this case the peak drain current does not depend on input voltage anymore.
One more consideration concerns the R_ZCD value: during MOSFET’s ON-time, the current
sourced by the ZCD/FB pin, I_ZCD, is compared with an internal reference current I_ZCDON (-50
µA typical).
If I_ZCD < I_ZCDON, the brownout function is activated and the IC is shut-down.
This feature is especially important when the auxiliary winding is accidentally disconnected
and considerably increases the end-product’s safety and reliability.
5.7
Cable drop compensation (CDC)
The voltage control loop regulates the output voltage as seen across the output capacitor.
If an output cable is used to supply the load, the voltage at the externally available terminals
is dependent on the output current value. The CDC function compensates the voltage drop
across the cable, so ideally zero load regulation can be achieved also at the end of the
cable.
18/28
Doc ID 17957 Rev 1