SC174MLTRT Semtech, SC174MLTRT Datasheet - Page 13

• Manufacturer Part Number: SC174MLTRT
• Description: IC BUCK SYNC ADJ 4A MLPD-10
• Manufacturer: Semtech
• Series: EcoSpeed™r
• Type: Step-Down (Buck)r

Specifications of SC174MLTRT

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 5.23 V
• Current - Output: 4A
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MLPD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC174MLTR

Applications Information (continued)
Ultrasonic Power Save Operation
The SC174 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
fall. When V
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.
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
Inductor
Voltage (V
DH
DL
Current
OUT
FB Ripple
through the inductor, forcing both V
After the 40µsec time-out, DL drives high if V
Figure 5 — Ultrasonic Power Save Operation
FB
has not reached the FB threshold.
)
FB
drops to the 750mV threshold, the next DH
40µs time-out
minimum f
On-time
(T
ON
)
SW
~ 25kHz
DH On-time is triggered when
V
FB
reaches the FB Threshold
FB
OUT
FB threshold
(750mV)
and V
(0A)
FB
to
Threshold (825mV)
PSAVE threshold is reached
power save enabled, this can force V
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
causes V
point, a normal T
prevents a hard OVP shutdown and also cycles energy
from V
for the Smart Power Save feature.
Smart Power Save
threshold
Drive (DH)
Drive (DL)
Current Limit Protection
The device features fixed current limiting, which is ac-
complished by using the R
current sensing. While the low-side MOSFET is on, the
inductor current flows through it and creates a voltage
across the R
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
in Figure 7.
High-side
Low-side
DL turns on when Smart
FB
V
OUT
OUT
current flowing into C
DL turns off when FB
threshold is reached
OUT
drifts up to due to leakage
back to V
Single DH on-time pulse
to fall. When V
DS(ON)
Figure 6 — Smart Power Save
after DL turn-off
DH and DL off
. During this time, the voltage across the
ON
IN
switching cycle begins. This method
. Figure 6 shows typical waveforms
OUT
FB
DS(ON)
OUT
drops back to the 750mV trip
through the inductor and
of the lower MOSFET for
V
OUT
and low-side MOSFET
Normal DL pulse after DH
OUT
discharges via inductor
Normal V
on-time pulse
to slowly rise
SC174
OUT
ripple
13
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