SC4911 Datasheet, PDF (9/14 Pages) Semtech Corporation – Bias/Primary Side PWM Controller with Ultra Low Start-up Current

SC4911
POWER MANAGEMENT
Typical Applications (Cont.)
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.
Vin = 48V
C2
10uF
R5
383K
R7
100K
D1 1N5819
R1
100K
R2
2.7MEG
C4
100pF
R4
100K
C3
10uF
1
8 ^{U1 SC4911}
LUVLO VCC
3 _FB
OUT ⁷
R3
68
C5
150pF
4 _COMP
ILIM ⁵
C6
47nF
R9
4.7K
RT GND
2
6
R10
100K
R6
51
C7
1000pF
T1
D2 1N5819
Q1
Si2320DS
R8
1.00
C1
47uF
Vo = 12V
Figure 4. Non-isolated 48V to 12V Flyback Converter
 2006 Semtech Corp.
9
www.semtech.com