SC4911 Datasheet, PDF (8/14 Pages) Semtech Corporation – Bias/Primary Side PWM Controller with Ultra Low Start-up Current

SC4911
POWER MANAGEMENT
Typical Applications (Cont.)
Isolated feedback 48V to 12V application
Figure 3 gives an example of isolated feedback and low
power flyback application of the SC4911. T1 is a three
winding transformer. The winding 1-2 is a bias winding
which supplies the primary circuitry after the SC4911
starts up. During the startup, the input source trickle
charges the capacitor C1 through the start-up resistor
R1. As long as voltage across C1 reaches the Vcc turn-
on threshold, the SC4911 turns on abruptly and start
driving the gate of MOSFET Q1. As Q1 starts switching,
regenerative feedback from the bias winding will provide
current via D1 to capacitor C1.
In this configuration, the input voltage is in the range of
36V to 72V for telecom applications, and the LUVLO pin
monitors the input voltage via resistor R2 and R5 to
determine turn-on and turn-off of the converter. Capacitor
C5 helps filter out the noise from the input line.
Timing resistor R13 sets switching frequency. R3 and
C6 are used to suppress the voltage spikes across the
switch due to leakage inductance of T1. R11 is current
sense resistor to set current limit of the convertor. R10
and C9 filter spikes caused by parasitic capacitance
during the turning on of Q1. The U3 SC431 is shunt
regulator integrating an accurate 2.5V bandgap
reference, op amp and driver into a single device. It
compares sensed output voltage divided by R7 and R9
to its internal reference and drives the amplified error
signal to opto-coupler U1. C7 keeps ripple and noise
from over driving the device. U1 delivers the isolated
feedback signal to the SC4911. C8 and R12 are for
compensation. An additional small capacitor could be
used from COMP to GND if needed.
Vin = 48V
C3
10uF
C5
100pF
R1
100K
D1 1N5819 _T1
2
C1
10uF
D2 1N5819
1
8
R2
2.7MEG
R5
100K
3
10
R3
68
5
C4
0.1uF
C6
150pF
1
8 U2 SC4911
LUVLO VCC
3 _FB
OUT ⁷
Q1
Si2320DS
4 _COMP
ILIM ⁵
C8
47nF
R12
4.7K
RT ^GND
2
6
R13
100K
R10
51
C9
1000pF
R11
1.00
U1
NEC2501
U3
SC431
R8
1K
R4
330
R6
1K
R7
C7
10K
220pF
R9
10K
Vo = 5V
C2
100uF
 2006 Semtech Corp.
Figure 3. Isolated 48V to 12V Flyback Converter
8
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