SC410 SEMTECH [Semtech Corporation], SC410 Datasheet - Page 15

• Manufacturer Part Number: SC410
• Description: 3A EcoSpeed TM Step-Down Regulator with LDO and Ultrasonic Power Save
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information (continued)
Current Limit Protection
Programmable current limiting is accomplished by using
the RDS
current limit is set by an internal resistor R
connects from an ILIM node to the LX node which is also
the drain of the low-side MOSFET. When the low-side
MOSFET is on, an internal ~10μA current fl ows from the
ILIM Node and through the R
drop across the resistor. While the low-side MOSFET is on,
the inductor current fl ows through it and creates a voltage
across the RDS
tive with respect to ground. If this MOSFET voltage drop
exceeds the voltage across R
node will be negative and current limit will activate. The
current limit then keeps the low-side MOSFET on and will
not allow another high-side on-time, until the current in the
low-side MOSFET reduces enough to bring the ILIM voltage
back up to zero. This method regulates the inductor valley
current at the level shown by ILIM in Figure 7.
Threshold (825mV)
Smart Power Save
Drive (DH)
threshold
PSAVE threshold is reached
High-side
Drive (DL)
Low-side
DL turns on when Smart
FB
V
OUT
current flowing into C
ON
DL turns off when FB
threshold is reached
drifts up to due to leakage
of the lower MOSFET for current sensing. The
Single DH on-time pulse
Figure 7 — Valley Current Limit
Figure 6 — Smart Power Save
after DL turn-off
ON
DH and DL off
. The voltage across the MOSFET is nega-
OUT
Time
ILIM
ILIM
resistor, creating a voltage
, the voltage at the ILIM
V
OUT
and low-side MOSFET
discharges via inductor
Normal DL pulse after
DH on-time pulse
ILIM
Normal V
. The resistor
OUT
I
I
ripple
PEAK
LOAD
I
LIM
In the SC410, the valley current limit is set to 3A. This results
in a peak inductor current of 3A plus the peak-to-peak
ripple current. In this situation, the average (load) current
through the inductor is 3A plus one-half the peak-to-peak
ripple current.
The internal 10μA current source is temperature compen-
sated at 2500ppm in order to provide tracking with the
RDS
Peak Inductor Current
The peak current through the inductor and switching FETs
must be less than 5A. The only way to meet this require-
ment is to select the switching frequency and inductor
value so that the peak inductor current is less than or equal
to 5A when the trough of the inductor current is 3A.
Soft start of PWM Regulator
Soft start is achieved in the PWM regulator by using an
internal voltage ramp as the reference for the FB com-
parator. The voltage ramp is generated using an internal
charge pump which drives the reference from zero to
750mV in ~1.8mV increments, using an internal ~500kHz
oscillator. When the ramp voltage reaches 750mV, the
ramp is ignored and the FB comparator switches over to
a fixed 750mV threshold. During soft start the output
voltage tracks the internal ramp, which limits the start-up
inrush current and provides a controlled soft start profi le
for a wide range of applications. Typical soft start ramp
time is 850μs.
During soft start the regulator turns off the low-side
MOSFET on any cycle if the inductor current falls to zero.
This prevents negative inductor current, allowing the
device to start into a pre-biased output.
Power Good Output
The PGOOD (power good) output is an open-drain output
which requires a pull-up resistor. When the output voltage
is 10% below the nominal voltage, PGOOD is pulled low. It
is held low until the output voltage returns to the nominal
voltage. PGOOD is held low during soft start and activated
approximately 1ms after V
ON
.
OUT
reaches regulation.
SC410
15