SC427 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC427' PDF : 29 Pages View PDF
SC417/SC427
Applications Information (continued)
FB Ripple
Voltage
(VFB)
Inductor
Current
Dead time varies
according to load
FB threshold
(500mV)
Zero (0A)
On-time (TON)
DH On-time is triggered when
DH
VFB reaches the FB Threshold.
DL
DL drives high when on-time is completed.
DL remains high until inductor current reaches zero.
VOUT drifts up to due to leakage
current flowing into COUT
Smart Power Save
Threshold (550mV)
FB
threshold
DH and DL off
VOUT discharges via inductor
and low-side MOSFET
Normal VOUT ripple
High-side
Drive (DH)
Single DH on-time pulse
after DL turn-off
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 7 — Smart Power-save
Figure 6 — Power-save Operation
Smart Power-save Protection
Active loads may leak current from a higher voltage into
the switcher output. Under light load conditions with
power-save enabled, this can force V to slowly rise and
OUT
reach the over-voltage threshold, resulting in a hard shut-
down. Smart power-save prevents this condition. When
the FB voltage exceeds 10% above nominal (exceeds
550mV), the device immediately disables power-save, and
DL drives high to turn on the low-side MOSFET. This draws
current from V through the inductor and causes V to
OUT
OUT
fall. When V drops back to the 500mV trip point, a normal
FB
T switching cycle begins. This method prevents a hard
ON
OVP shutdown and also cycles energy from V back to
OUT
V . It also minimizes operating power by avoiding forced
IN
conduction mode operation. Figure 7 shows typical wave-
forms for the Smart Power-save feature.
Current Limit Protection
The device features programmable current limiting, which
is accomplished by using the RDSON of the lower MOSFET
for current sensing. The current limit is set by R resistor.
ILIM
The RILIM resistor connects from the ILIM pin to the LX 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 RDS(ON). The voltage across the
MOSFET is negative with respect to ground. If this MOSFET
voltage drop exceeds the voltage across R , the voltage
ILIM
at the ILIM pin will be negative and current limit will acti-
vate. 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 8.
16
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