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SC4911IMSTRT

P1-P3P4-P6P7-P9P10-P12P13-P14
72006 Semtech Corp. www.semtech.com
SC4911
POWER MANAGEMENT
48V TO 12V FLYBACK CONVERTER AS AUXILIARY
SUPPLY
Figure 2 shows a simple bias supply using the SC4911.
This 36 - 72V input flyback power supply employs primary
side regulation from the transformer bias winding, which
eliminates the need of opto coupler and secondary
reference error amplifier. This is best for low cost
applications requiring isolation and narrow load variation
range on the secondary.
During the start-up, the input source trickle charges the
capacitor C1 through 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. After the SC4911 starts up the primary
bias winding supplies the primary circuitry.
Typical Applications
The output of the T1 bias winding is rectified, filtered by
C1 and is fed to the FB through feedback resistor R5
and R7. The output voltage on the secondary is
proportional to the bias voltage by the turns ratio of the
output to the bias winding. C7 and R9 are for frequency
compensation. An additional small capacitor could be
used from COMP to GND if it is needed. The LUVLO pin
monitors the input voltage via resistor R2 and R4 to
determine turn-on and turn-off of the converter. Capacitor
C5 helps filter out the noise from the input line.
Figure 2. 48V to 12V Flyback Converter as Auxiliary Supply
R1
100K
C7
47nF
Vin = 48V
Vpri = 12V
R4
100K
R2
2.7MEG
C5
100pF
R9
4.7K
C8
1000pF
D1 1N5819
Vo = 12V
Q1
Si2320DS
U1 SC4911
1
2
3
4 5
6
7
8
LUVLO
RT
FB
COMP ILIM
GND
OUT
VCC
T1
3
5
8
10
2
1
R8
1.00
C3
10uF
D2 1N5819
R7
100K
R5
383K
R3
68
R6
51
C4
0.1uF
C6
150pF
C1
10uF
C2
100uF
R10
100K
82006 Semtech Corp. www.semtech.com
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.
D2 1N5819
C2
100uF
D1 1N5819
C5
100pF
Q1
Si2320DS
R12
4.7K
R7
10K
C7
220pF
R2
2.7MEG
R10
51
R5
100K
R9
10K
C1
10uF
NEC2501
U1
C9
1000pF
Vin = 48V
C8
47nF
R4
330
C4
0.1uF
R11
1.00
R13
100K
R3
68
R1
100K
Vo = 5V
C3
10uF
U3
SC431
C6
150pF
R6
1K
R8
1K
U2 SC4911
1
2
3
4 5
6
7
8
LUVLO
RT
FB
COMP ILIM
GND
OUT
VCC
T1
3
5
8
10
2
1
Figure 3. Isolated 48V to 12V Flyback Converter
92006 Semtech Corp. www.semtech.com
SC4911
POWER MANAGEMENT
Non-Isolated feedback 48V to 12V application
Figure 4 is an example of low cost, non-isolated feedback,
low power application of the SC4911. T1 is a two winding
transformer. The output voltage is directly fed to the
primary circuitry.
During the start-up, the input source trickle charges the
capacitor C3 through start-up resistor R1. As long as the
voltage across C3 reaches the Vcc turn-on threshold,
the SC4911 turns on abruptly and starts driving the gate
of MOSFET Q1. After the SC4911 starts up the secondary
winding supplies the primary circuitry.
In this 36V to 72V telecom application, LUVLO monitors
the input voltage via resistor R2 and R4 to determine
turn-on and turn-off of the converter. Capacitor C4 helps
filter out the noise from the input line. Voltage feedback
is directly brought from the output to the input error
amplifier of SC4911 through R5 and R7. C6 and R9 are
for frequency compensation. An additional small
capacitor could be used from COMP to GND if needed.
Typical Applications (Cont.)
Vo = 12V
C6
47nF
C4
100pF
D1 1N5819
Q1
Si2320DS
C5
150pF
U1 SC4911
1
2
3
4 5
6
7
8
LUVLO
RT
FB
COMP ILIM
GND
OUT
VCC
C1
47uF
R6
51
C3
10uF
R7
100K
T1
C7
1000pF
R4
100K
D2 1N5819
R8
1.00
C2
10uF
R1
100K
R9
4.7K
R2
2.7MEG
R3
68
R5
383K
Vin = 48V
R10
100K
Figure 4. Non-isolated 48V to 12V Flyback Converter
P1-P3P4-P6P7-P9P10-P12P13-P14

SC4911IMSTRT

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Manufacturer:
Semtech
Description:
Switching Controllers BI/PRI SIDE PWM CNTRL W/LOW ST
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