7 2007 Semtech Corp. www.semtech.com
SC4812
POWER MANAGEMENT
Application Information
Introduction
The SC4812 is a 8 pin peak current mode controlled
PWM controller for isolated or non-isolated DC-DC
switching power supplies. It features an internal startup
regulator, programmable external soft start and
programmable cycle -by-cycle current limit It operates in
a fixed frequency at 260KHz.
The SC4812 can be applied in a flyback or forward
topology with the input voltage ranging from 36V to 75V.
Startup Regulator
The internal startup regulator of the SC4812 makes its
initial startup without a lossy startup resistor or external
startup circuitry to save cost and board space. As input
voltage exceeds 12V, it provides regulated 7.4V to V
CC
which is with the maximum turn-on voltage 6.7V.
Soft-Start/Shutdown and V
CC
Lockout
During power startup, the output voltage has to ramp up
in a controlled manner to avoid overshoot. The SC4812
internal non-inverting terminal to the error amplifier is
connected to the soft-start pin, which forces the internal
2.5V reference to gradually ramp up during power
startup. In a non-isolated application, as the internal error
amplifier is used, output voltage of a converter ramps
up with close loop startup fashion smoothly. In case of
an isolated application in which the internal error amplifier
may not be used, isolated voltage feedback is through
an opto-coupler and connected to the COMP pin. During
power startup, the COMP pin will follow the soft-start pin
voltage through an internal diode.
Soft-start operation begins when SS/SHDN ramps above
0.7V. When soft-start completed, SS/SHDN is regulated
to 2.50V, the internal voltage reference. Pull SS/SHDN
below 0.5V to disable the controller.
Undervoltage lockout shuts down the controller when V
CC
is less than 5.5V. The internal startup regulator and the
reference remain on during shutdown.
Current-Sense
The function of the CS pin is to limit the peak current
through the MOSFET. Current is sensed at CS as a voltage
across a sense resistor between the source of the
MOSFET and GND. An RC filter is recommended to
connect CS to the sense resistor to reduce effect of the
MOSFET turn-on leading edge spike and noise. Select
the current-sense resistor with the following equation:
R
S
= 0.465V/(1.2xI
LIM_PRI
)
where
,
I
LIM_PRI
is the maximum peak primary-side current.
When VCS > 465mV, the power MOSFET turns off after
70nS propagation delay from the switch current reaches
the trip point to the driver.
PWM Comparator and Slope Compensation
The SC4812 runs fixed 260 KHz frequency by an internal
oscillator. At the beginning of each switching cycle, the
OUT pin switches the MOSFET on. the OUT pin switches
off after the 80% maximum duty cycle has been reached,
regardless of the feedback.
Since an artificial slope is required for current-mode
operation when duty cycle is larger than 50%, the SC4812
uses an internal ramp generator for slope compensation.
The internal ramp signal is reset at the beginning of each
cycle and slews at 26mV/
µS.
The PWM comparator compares the instantaneous
primary peak current to the feedback error signal through
the opto-coupler, the internal offset and slope
compensation and determine when to turn off the
MOSFET. In steady state operation, the MOSFET turns
off when:
I
LIM_PRI
· R
S
> V
OPTO
– V
OFFSET
- V
SLOPE
where I
LIM_PRI
is the current on the primary side through
the MOSFET. V
OFFSET
is 1.5V internal offset voltage and
V
SLOPE
is the artificial ramp starting at with slew rate 26mV/
µS.
When selecting an inductor in a forward-converter or
magnetizing inductance in a flyback-converter, the
following condition must be met to avoid control-loop sub-
harmonic oscillations:
S/mV26
L
VRK
N
N
OS
P
S
µ=
××
×
where K = 0.5 TO 1, AND N
S
and N
P
are the turns of the
main transformer on secondary and primary side
respectively. L is the output inductor on secondary side
in a forward-converter or magnetizing inductance on
secondary side in a flyback-converter.
