SC171_10 Datasheet, PDF (13/27 Pages) Semtech Corporation – 1A EcoSpeedTM Synchronous Step-Down Regulator with Optional Ultrasonic Power Save

SC171
Applications Information (continued)
Ultrasonic Power Save Operation
The SC171 provides ultrasonic power save operation at
light loads, with the minimum operating frequency fixed
at 25kHz. This is accomplished using an internal timer
that monitors the time between consecutive high-side
gate pulses. If the time exceeds 40µs, DL drives high to
turn the low-side MOSFET on. This draws current from
V_OUT through the inductor, forcing both V_OUT and V_FB to
fall. When V_FB drops to the 750mV threshold, the next DH
on-time is triggered. After the on-time is completed the
high-side MOSFET is turned off and the low-side MOS-
FET turns on. The low-side MOSFET remains on until the
inductor current ramps down to zero, at which point the
low-side MOSFET is turned off.
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 power save opera-
tion.
minimum f_SW ~ 25kHz
FB Ripple
Voltage (V_FB)
FB threshold
(750mV)
Inductor
(0A)
Current
power save enabled, this can force V_OUT to slowly rise
and reach the over-voltage threshold, resulting in a hard
shutdown. Smart power save prevents this condition.
When the FB voltage exceeds 10% above nominal (ex-
ceeds 825mV), the device immediately disables power-
save, and DL drives high to turn on the low-side MOSFET.
This draws current from V_OUT through the inductor and
causes V_OUT to fall. When V_FB drops back to the 750mV trip
point, a normal T_ON switching cycle begins. This method
prevents a hard OVP shutdown and also cycles energy
from V_OUT back to V_IN. Figure 6 shows typical waveforms
for the Smart Power Save feature.
V_OUT drifts up to due to leakage
current flowing into C_OUT
Smart Power Save
Threshold (825mV)
FB
threshold
DH and DL off
V_OUT discharges via inductor
and low-side MOSFET
Normal V_OUT 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 6 — Smart Power Save
Current Limit Protection
On-time
(T_ON)
DH On-time is triggered when
V_FB reaches the FB Threshold
DH
40µs time-out
DL
After the 40µsec time-out, DL drives high if V_FB
has not reached the FB threshold.
Figure 5 — Ultrasonic 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
© 2010 Semtech Corporation
The device features fixed current limiting, which is ac-
complished by using the R_DS(ON) of the lower MOSFET for
current sensing. While the low-side MOSFET is on, the
inductor current flows through it and creates a voltage
across the R_DS(ON). During this time, the voltage across the
MOSFET is negative with respect to ground. During this
time, If this MOSFET voltage drop exceeds the internal
reference voltage, the current limit will activate. The cur-
rent 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 drop below the
internal reference voltage once more. This method regu-
lates the inductor valley current at the level shown by I_LIM
in Figure 7.
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