SC4910A_16 Datasheet, PDF (8/19 Pages) Semtech Corporation – High Performance Secondary Side Controller with Synchronous Rectifier

SC4910A/B
POWER MANAGEMENT
Applications Information
The SC4910A/B is a secondary side PWM controller work- SYNC/EN
ing either in current mode or voltage mode mainly for
applications of forward converters with synchronous rec- The enable function looks at the SYNC/EN pin and an
tification. While the OUTA drives the primary MOSFET internal timing capacitor. If the SYNC/EN pin is low and
through transformer and the secondary forward recti- the internal timing capacitor voltage is high, then the
fier, the OUTB drives the secondary freewheeling recti- SC4910 is disabled with OUTA, OUTB and SS pulled low.
fier. The switching frequency and maximum duty cycle When the SYNC/EN pin is held high, the device is enabled
can be programmable with two resistors. The delay time and runs off of the internal oscillator. When a rising signal
from the falling edge of OUTA to the rising edge of OUTB is detected on the SYNC/EN pin a one-shot is triggered
is adaptive by monitoring the phase node voltage. The and discharges the internal timing capacitor. As long as
delay time from the falling edge of OUTB to the rising the internal timing capacitor is below an internal reference
edge of OUTA is determined by a programming resistor level, the device will synchronize with the external pulse.
from the DELAY pin to ground. The ISHARE pin allows for If the internal timing capacitor is allowed to charge up to
current sharing among the parallel operating units to the internal reference level before another SYNC pulse
make current equally distribute load. The -SENSE pin is detected, the device will switch back to the internal
separated from GND pin provides true output voltage oscillator.
remote sense capability. Other features include soft start,
sychronization or enable/disable by user, provided 5V Soft Start
reference voltage.
The SS pin is connected to the internal reference, 0.75V,
Oscillator
through an internal 6K ohm resistor. The SS pin is also
connected to the non-inverting input of the error amplifier.
The frequency and duty cycle of the oscillator is controlled With an external capacitor connected to this pin, the soft
by placing two resistors from the RT1 and RT2 pins to start timing will be determined by this RC time constant.
ground. The resistor at RT1 controls the maximum “on” During start-up, the SS pin is held low until the
duty cycle and the resistor at RT2 controls the “off” undervoltage lockout threshold is reached. Once the
portion of a cycle. When the resistor at RT1 is equal to UVLO threshold is reach, the SS pin is released and the
that at RT2, the maximum duty cycle will be approximately device will regulate to the voltage on this pin.
50%. The following formula is used to determine the
time duration of the “on” and “off” portions:
Undervoltage Lockout
t = RT × 20 ×10^−12
Current Sense and Current Limit
When the supply voltage V_CC is below the undervoltage
lockout threshold, both OUTA and OUTB are held low. The
SS pin and the COMP pin are also held low. Once the
The CS pin has an input impedance of 20K ohms and ^{undervoltage lockout threshold has been surpassed,}
swings from 1.0V to 2.5V. With a 5K ohm resistor from ^{OUTA, OUTB, SS and COMP are released for normal}
CS to ground, the device operates in voltage mode with ^operation.
a ramp that will swing from 0.2V to 0.5V. When the 5K
resistor is connect to a voltage that is proportional to
the primary side current, the device will operate in current
mode. The cycle-by-cycle current limit is triggered when
the CS pin voltage rises above 1V. If CS exceeds 1.25V,
the faulty latch will be set and the outputs will be driven
low. The soft start capacitor is then discharged by the
internal current sink. No outputs are allowed until the
soft start capacitor is fully discharged to 0.15V. At this
point the fault latch will be reset and the SC4910 will
begin a soft start process. This results in a hiccup current
limit mode for continuous fault conditions.
 2005 Semtech Corp.
8
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