SC403MLTRT Semtech, SC403MLTRT Datasheet - Page 18

• Manufacturer Part Number: SC403MLTRT
• Manufacturer: Semtech

Specifications of SC403MLTRT

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Applications Information (continued)
Because the on-times are forced to occur at intervals no
greater than 40µs, the frequency will not fall below
~25kHz. Figure 5 shows ultrasonic PSAVE operation.
Smart PSAVE Protection
Active loads may leak current from a higher voltage into
the switcher output. Under light load conditions with
PSAVE enabled, this can force V
the over-voltage threshold, resulting in a hard shutdown.
Smart PSAVE prevents this condition. When the FB voltage
exceeds 10% above nominal, the device immediately dis-
ables PSAVE, and DL drives high to turn on the low-side
MOSFET. This draws current from V
tor and causes V
750mV trip point, a normal t
method prevents a hard OVP shutdown and also cycles
energy from V
power by avoiding forced conduction mode operation.
Figure 6 shows typical waveforms for the Smart PSAVE
feature.
Inductor
Current
FB Ripple
DH
DL
Voltage
(V
After the 50µsec time-out, DL drives high if V
Figure 5 — Ultrasonic PSAVE Operation
FB
)
has not reached the FB threshold.
OUT
50µsec time-out
Minimum f
OUT
back to V
On-time
(T
to fall. When V
ON
)
SW
IN
~ 20kHz
ON
. It also minimizes operating
DH On-Time is triggered when
V
switching cycle begins. This
FB
OUT
reaches the FB Threshold
to slowly rise and reach
OUT
FB
through the induc-
drops back to the
FB
FB threshold
(750mV)
(0A)
SmartDrive
For each DH pulse the DH driver initially turns on the high-
side MOSFET at a lower speed, allowing a softer, smooth
turn-off of the low-side diode. Once the diode is off and
the LX voltage has risen 1V above PGND, the SmartDrive
circuit automatically drives the high-side MOSFET on at a
rapid rate. This technique reduces switching power loss
while maintaining high efficiency and also avoids the
need for snubbers or series resistors in the gate drive.
Current Limit Protection
Programmable current limiting is accomplished by using
the RDS
current limit is set by the R
nects from the ILIM pin to the LXS pin which is also the drain
of the low-side MOSFET. When the low-side MOSFET is on,
an internal ~10μA current flows from the ILIM pin and
through the R
resistor. While the low-side MOSFET is on, the inductor
current flows through it and creates a voltage across the
RDS
respect to ground. If this MOSFET voltage drop exceeds the
voltage across R
tive 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
threshold
Drive (DH)
PSAVE threshold is reached
High-side
Drive (DL)
Low-side
DL turns on when Smart
FB
ON
V
. The voltage across the MOSFET is negative with
OUT
ON
current flowing into C
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
ILIM
TM
after DL turn-off
ILIM
Figure 6 — Smart PSAVE
resistor, creating a voltage drop across the
DH and DL off
, the voltage at the ILIM pin will be nega-
OUT
ILIM
resistor. The R
V
OUT
and low-side MOSFET
discharges via inductor
Normal DL pulse after
DH on-time pulse
ILIM
Normal V
resistor con-
SC403
OUT
ripple
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