SC4905B Semtech Corporation, SC4905B Datasheet - Page 10

• Manufacturer Part Number: SC4905B
• Description: Sc4905b High-performance Voltage-mode Pwm Controller | Semtech
• Manufacturer: Semtech Corporation

FEED BACK
The error signal from the output of an external Error am-
plifier such as SC431 or SC4431 is applied to the invert-
ing input of the PWM comparator at the FB pin either
directly or via an opto coupler for the Isolated applica-
tions. For best stability keep the FB trace length as short
as possible.
FB
The signal at the FB pin is then compared to the ramp
signal from the RC pin and the OUT gate drive signal is
generated.
Voltages below 600mV at the FB pin, will produce a 0%
duty cycle at the OUT drive. Maximum duty cycle is pro-
duced when VFB-600mV>VFF. The FB signal range is from
600mv to 4V.
OVER CURRENT
A pulse by pulse current limit is provided by the SC4905.
The current information is sensed at the ILIM pin and
compared to a peak current limit level of 180mV. If the
180mV limit is exceeded, the OUT pulse is terminated.
POWER MANAGEMENT
Application Information (Cont.)
2006 Semtech Corp.
REF
R22
1.1k
C30
NA
1
2
3
4
5
8
7
6
5
VDD
FB
VFF
DMAX
RC
SC4905
MOCD207
U1
SYNC
GND
1
2
3
4
REF
OUT
ILIM
4.7nF
R24
3.74k
10
9
8
7
6
0.1u,16V
C36
C34
C22
REF
R29
NA
C17
C26
22n, 16V
VDD
C35
R13
0
5
D15
R17
Secondary Supply
C18
Rsense
1
2
SC1301A
Vref
C32
1nF
1
R30
4
22pF
U6
SC4431
M1
VDD
3
2
R25
100k
5
C31
U2
R11
4
Vout
D7
0
R26
80.2k
R27
9.1k
D8
D11
C33
680pF
R23
5.1k
10
GATE DRIVERS
OUT is a CMOS gate drive output stage that is supplied
from REF and provides a peak source/sink current of
about 1mA. The output stage is capable of driving the
logic input of external MOSFET Drivers and is switched at
the oscillator frequency. When the voltage on the RC pin
is rising,, the output is high.
It should be noted that if high speed/high current drivers
such as the SC1301 are used, careful layout must be
followed in order to minimize stary inductance, which
might cause negative voltages at the output of the driv-
ers. This negative voltage can be clamped to reasonable
level by placing a small Schottky diode directly at the
output of the driver as shown below.
SOFT START
During start up of the converter, the discharged output
capacitor, and the load current demand large supply cur-
rent requirements. To avoid this a soft start scheme is
usually implemented where the duty cycle of the regula-
tor is gradually increased from 0% until the soft start
duration is elapsed.
Programmable soft start duration can implemented ex-
ternally by utilizing a simple external circuitry shown be-
low.
Approximate soft start duration can be calculated as be-
low:
3
4
MOCD207
1
2
3
4
5
VDD
FB
VFF
DMAX
RC
Csoft start
SC4905
6
5
R22
1.1k
C30
U1
SYNC
REF
GND
REF
OUT
ILIM
10
9
8
7
6
56.2K
C22
REF
C17
VDD
R13
C18
SC1301A
1
SC4905A/B
2
3
VDD
1
2
3
4
5
U2
5
VDD
FB
VFF
DMAX
RC
SC4905
Mosfet Gate
4
www.semtech.com
U1
SYNC
D8
D11
GND
REF
OUT
ILIM
9
8
7
6