SC403 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC403' PDF : 32 Pages View PDF
SC403
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.
Minimum fSW ~ 20kHz
FB Ripple
Voltage
(VFB)
Inductor
Current
FB threshold
(750mV)
(0A)
On-time
(TON)
DH On-Time is triggered when
VFB reaches the FB Threshold
DH
50µsec time-out
DL
After the 50µsec time-out, DL drives high if VFB
has not reached the FB threshold.
Figure 5 — 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 VOUT to slowly rise and reach
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 VOUT through the induc-
tor and causes VOUT to fall. When VFB drops back to the
750mV trip point, a normal tON switching cycle begins. This
method prevents a hard OVP shutdown and also cycles
energy from VOUT back to VIN. It also minimizes operating
power by avoiding forced conduction mode operation.
Figure 6 shows typical waveforms for the Smart PSAVE
feature.
VOUT drifts up to due to leakage
current flowing into COUT
Smart Power Save
Threshold (825mV)
FB
threshold
DH and DL off
High-side
Drive (DH)
Single DH on-time pulse
after DL turn-off
VOUT discharges via inductor
and low-side MOSFET
Normal VOUT ripple
Low-side
Drive (DL)
DL turns on when Smart
PSAVE threshold is reached
DL turns off when FB
threshold is reached
Normal DL pulse after
DH on-time pulse
Figure 6 — Smart PSAVE
SmartDriveTM
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 RDSON of the lower MOSFET for current sensing. The
current limit is set by the RILIM resistor. The RILIM resistor con-
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 RILIM resistor, creating a voltage drop across the
resistor. While the low-side MOSFET is on, the inductor
current flows through it and creates a voltage across the
RDSON. The voltage across the MOSFET is negative with
respect to ground. If this MOSFET voltage drop exceeds the
voltage across RILIM, the voltage at the ILIM pin will be nega-
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.
18
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