AN1539 STMicroelectronics, AN1539 Datasheet - Page 5

• Manufacturer Part Number: AN1539
• Description: VIPOWER: LOW COST UNIVERSAL INPUT SMPS FOR DIGITAL SET-TOP BOX BASED ON VIPER50
• Manufacturer: STMicroelectronics

AN1539 - APPLICATION NOTE
where V
is the considered output voltage, V
is maximum input DC voltage, N
is the number of
o
DCmax
o
turns and N
is the primary turns.
p
1.3 OUTPUT FILTER CAPACITORS
Output capacitors values depend on the output current, voltage ripple and switching frequency according
to (5).
I
Ok
max
C
Ok
(5)
f
V
sw
Ok
_
ripple
1.4 VOLTAGE FEEDBACK
The voltage feedback is provided using a TL431 on the 2.5V output and an optocoupler. The voltage at
the collector of the optocoupler sets the peak drain current of the VIPer device. Feedback compensation
circuit is connected across TL431 cathode and reference pin.
For a given current sank from COMP pin of the VIPer in order to perform proper voltage regulation, the
LED current is calculated considering the optocoupler transfer ratio C
of 200%, then the limiting resistor
trr
of the LED current R
can be calculated according to (6).
lim
5
V
V (
V
LED
R
lim
(6)
I
LED
In order to improve the regulation of 5V output, the output voltage sensing is split using two resistors, as
shown in Figure 3. In fact, this output is usually connected to voltage sensitive circuits such as
microprocessors and other low voltage logic devices.
5/15