SC418ULTRT Semtech, SC418ULTRT Datasheet - Page 17

• 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)
SmartDrive
For each DH pulse, the DH driver initially turns on the
high-side MOSFET at a slower speed, allowing a softer,
smooth turn-off of the low-side diode. Once the diode is
off and the LX voltage has risen 0.5V above PGND, the
SmartDrive circuit automatically drives the high-side
MOSFET on at a rapid rate. This technique reduces switch-
ing noise while maintaining high efficiency, reducing the
need for snubbers or series resistors in the gate drive.
Enable Input for Switching Regulator
The EN input is a logic level input. When EN is low
(grounded), the switching regulator is off and in its lowest
power state. When EN is low and VDDA is above the VDDA
UVLO threshold, the output of the switching regulator
soft-discharges into the VOUT pin through an internal
15Ω resistor. When EN is a logic high (>1V) the switching
regulator is enabled.
The EN input has internal resistors — 2MΩ pullup to
VDDA, and a 1MΩ pulldown to AGND. These resistors will
normally cause the EN voltage to be near the logic high
trip point as VDDA reaches the VDDA UVLO threshold.
To prevent undesired toggling of EN and erratic startup
performance, the EN pin should not be allowed to float as
open-circuit.
Threshold (550mV)
PSAVE threshold is reached
Smart Power Save
threshold
Drive (DH)
Drive (DL)
High-side
Low-side
DL turns on when Smart
FB
V
OUT
current flowing into C
DL turns off when FB
threshold is reached
drifts up to due to leakage
Single DH on-time pulse
Figure 7 — Smart Power-Save
TM
after DL turn-off
DH and DL off
OUT
V
OUT
and low-side MOSFET
Normal DL pulse after DH
discharges via inductor
Normal V
on-time pulse
OUT
ripple
Note that the LDO enable pin (ENL) can also disable the
switching regulator through the V
to the ENL Pin and V
Current Limit Protection
The SC418 features programmable current limiting, which
is accomplished using the RDS
current sensing. The current limit is set by R
which connects from the ILIM pin to 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 R
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
voltage across the MOSFET is negative with respect to
PGND. If this MOSFET voltage drop exceeds the voltage
across R
current limit will activate. The current limit then keeps the
low-side MOSFET on and prevents another high-side on-
time, until the current in the low-side MOSFET reduces
enough to bring the I
regulates the inductor valley current at the level shown by
I
The current limit schematic with the R
in Figure 9.
LIM
in Figure 8.
ILIM
, the voltage at the ILIM pin will be negative and
Figure 8 — Valley Current Limit
IN
LIM
UVLO section.
voltage up to zero. This method
Time
(ON)
of the lower MOSFET for
IN
UVLO function. Refer
ILIM
resistor is shown
ILIM
(ON)
resistor
I
I
LOAD
PEAK
I
LIM
. The
ILIM
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