ALTAIR04-900TR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ALTAIR04-900TR' PDF : 29 Pages View PDF
ALTAIR04-900
Application information
5.4
Constant voltage operation
The IC is specifically designed to work in the 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 wire 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 the transformer 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 to the following formula:
Equation 1
RFB = -NN---------AS--------UE------CX-------⋅---V-V---O-R---U-E---TF----–-----V----R----E---F-- ⋅ RZCD
where NSEC and NAUX are the numbers of secondary and auxiliary turns respectively.
RZCD value depends on the application parameters (see “Section 5.6: Voltage feed-forward
block”).
DocID18211 Rev 3
15/29
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