SC4810B_16 Datasheet, PDF (13/24 Pages) Semtech Corporation – High Performance Current Mode PWM Controller with Complementary Output, Programmable Delay

SC4810B/E
POWER MANAGEMENT
Application Information (Cont.)
VDD and LUVLO
SC4810 features three different input turn-on voltage
thresholds, as specified in the Electrical Characteristics
on page 2. V starts to regulate when the supply voltage
REF
on the VDD pin is above the turn-on voltage threshold.
V drops to ground when VDD is lower than the turn-on
REF
threshold minus the hysteresis value. The soft start cap
remains grounded as long as LUVLO is below the thresh-
old voltage 3V. The soft start cap will be charged up
through an internal 5uA current source when LUVLO is
above the threshold voltage.
Soft Start
The soft-start function is implemented by charging the
soft-start cap through an internal 5uA current source.
Under normal soft-start, the SS pin is discharged below
0.65V and ramps up to 1V, during which time the output
driving signals OUT1 and OUT2 are held low. During the
time when the SS pin is charged from 1V to 2.5V, soft-
start is implemented by an increasing output duty ratio.
The duty ratio is completely under the control of the
feedback after the SS pin is above 2.5V.
When the SS pin is pulled down below 0.5V, OUT1 and
OUT2 will be held low and the VREF pin will be grounded
via an internal FET.
Complementary Driving with Programmable Delays
The SC4810 features dual driving signals to drive two
power switches complementarily. This feature makes the
SC4810 suitable for a variety of applications in which
dual complimentary driving signals are needed. The ^{Over Current Protection}
SC4810 even provides programmable driving delay as
an extra feature for applications such as active-clamp
forward topology. The users can program the driving delay
by adjusting the resistors tied to pin DELAY1 and pin
DELAY2 respectively to achieve the optimum delay for
each output. The delay of OUT1 is controlled by the
resistor tied to pin DELAY1 and the delay of OUT2 is
controlled by the resistor tied to pin DELAY2. For
The SC4810 provides Hiccup mode over-current
protection when the sensed current signals are beyond
0.6V. When the hiccup mode over-current protection is
triggered, the soft-start cap will be discharged
immediately by an internal grounded FET. When the soft-
start pin SS is pulled down below 1V, OUT1 and OUT2 will
be disabled, and a soft re-start sequence will follow.
illustration, see Fig. 8.
SC4810 can also be configured to implement cycle-by-
cycle over-current limit. As illustrated in Fig. 9, cycle-by-
cycle over-current limitation can be achieved by adjusting
the values of R1 and R2 to limit the voltage of FB pin to
less than the threshold voltage (0.6Volt) of the hiccup
over-current protection, using equations (3) and (4).
 2006 Semtech Corp.
13
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