ALTAIR04-900 Datasheet, PDF (15/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

ALTAIR04-900
Application information
5.4
Constant voltage operation
The IC is specifically designed to work in primary regulation and the output voltage is
sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier
diode.
Figure 13 shows the internal schematic of the constant voltage mode and the external
connections.
Figure 13. Voltage control principle: internal schematic
R zcd ZCD/FB
Rf b
Aux
S/ H
EA
2. 5V
D EMAG
LOGI C
To PWM Logic
CV
F rom Rsense
COMP
R
C
Due to the parasitic wires resistance, the auxiliary voltage is representative of the output just
when the secondary current becomes zero. For this purpose, the signal on ZCD/FB pin is
sampled-and-held at the end of transformer’s demagnetization to get an accurate image of
the output voltage and it is compared with the error amplifier internal reference.
The COMP pin is used for the frequency compensation: usually, an RC network, which
stabilizes the overall voltage control loop, is connected between this pin and ground.
The output voltage can be defined according the formula:
R_FB
=
---------------------^V----^R----^E---^F----------------------
N_{-----A---U----X--}
N_SEC
⋅
V_OUT
–
V_REF
⋅
R_ZCD
(1)
Where N_SEC and N_AUX are the secondary and auxiliary turn’s number respectively.
The R_ZCD value can be defined depending on the application parameters (see “Section 5.6:
Voltage feedforward block”).
Doc ID 18211 Rev 2
15/29