SC418ULTRT Semtech, SC418ULTRT Datasheet - Page 16

• Manufacturer Part Number: SC418ULTRT
• Description: IC BUCK SYNC ADJ 20MLPQ
• Manufacturer: Semtech
• Series: EcoSpeed™, SmartDrive™r
• Type: Step-Down (Buck)r

Specifications of SC418ULTRT

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.5 ~ 5.5 V
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -
• Other names: SC418ULTRTTR

Applications Information (continued)
The equation for determining the R
shown next. The desired minimum frequency is f
Note: the Power-save and Ultrasonic Power-save features
are not supported for VDDA less than 4.5V.
Power-Save Mode Operation
The device provides power-save operation at light loads
with no minimum operating frequency, selected by float-
ing the PSV pin (no connection). In this mode of opera-
tion, the zero cross comparator monitors inductor current
via the voltage across the low-side MOSFET during the
off-time. If the inductor current falls to zero for 8 consecu-
tive switching cycles, the controller enters power-save
operation. It will then turn off the low-side MOSFET on
each subsequent cycle, provided that the current crosses
zero. After the low-side MOSFET is off, both high-side and
low-sides MOSFETs remain off until V
threshold. While the MOSFETs are off the load is supplied
by the output capacitor. If the inductor current does not
reach zero on any switching cycle, the controller immedi-
ately exits power-save and returns to forced continuous
Voltage (V
Inductor
DH
DL
Current
FB Ripple
R
Figure 5 — Ultrasonic Power-Save Operation
PSV
high if V
After the programmable time-out, DL drives
FB
350
programmable time-out
)
FB
pF
has not reached the FB threshold.
minimum frequency
On-time
1
(T
f
SWMIN
ON
)
V
DH On-time is triggered when
FB
reaches the FB Threshold
FB
PSV
drops to the 500mV
resistor value is
FB threshold
(500mV)
(0A)
SWMIN
.
mode. Figure 6 shows power-save operation at light loads.
Note: the Power-save and Ultrasonic Power-save features
are not supported for VDDA less than 4.5V.
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
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
fall. When V
T
OVP shutdown and cycles energy from V
also minimizes operating power by avoiding forced con-
duction mode operation. Figure 7 shows typical wave-
forms for the Smart Power-save feature.
DH
DL
ON
Inductor
Current
FB Ripple
Voltage
switching cycle begins. This method prevents a hard
(V
FB
)
DL remains high until inductor current reaches zero.
Figure 6 — Power-Save Operation
FB
DL drives high when on-time is completed.
drops back to the 500mV trip point, a normal
OUT
On-time (T
through the inductor and causes V
V
DH On-time is triggered when
FB
reaches the FB Threshold.
ON
according to load
Dead time varies
)
Zero (0A)
OUT
FB threshold
to slowly rise and
(500mV)
OUT
back to V
SC418
OUT
IN
. It
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to