sc424evb Semtech Corporation, sc424evb Datasheet - Page 18

• Manufacturer Part Number: sc424evb
• Description: 6a Integrated Fet Regulator With 5v Ldo
• Manufacturer: Semtech Corporation

Applications Information (continued)
Current Limit Protection
The device features programmable current limiting, which
is accomplished by using the RDS
for current sensing. The current limit is set by R
The R
which is also the drain of the low-side MOSFET. When the
low-side MOSFET is on, an internal ~8μA current flows
from the ILIM pin and through the R
voltage drop across the resistor. While the low-side
MOSFET is on, the inductor current fl ows through it and
creates a voltage across the RDS
MOSFET is negative with respect to ground. If this MOSFET
voltage drop exceeds the voltage across R
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 to zero. This method regulates the induc-
tor valley current at the level shown by ILIM in Figure 8.
Threshold (825mV)
PSAVE threshold is reached
Smart Power Save
Drive (DL)
threshold
Drive (DH)
High-side
Low-side
DL turns on when Smart
FB
ILIM
V
OUT
current flowing into C
DL turns off when FB
resistor connects from the ILIM pin to the LXS pin
threshold is reached
drifts up to due to leakage
Single DH on-time pulse
Figure 7 — Smart Power-save
after DL turn-off
DH and DL off
OUT
ON
ON
. The voltage across the
ILIM
of the lower MOSFET
V
OUT
and low-side MOSFET
resistor, creating a
Normal DL pulse after DH
discharges via inductor
ILIM
Normal V
on-time pulse
, the voltage
ILIM
OUT
resistor.
ripple
Setting the valley current limit to 6A results in a peak
inductor current of 6A plus peak ripple current. In this situ-
ation, the average (load) current through the inductor is
6A plus one-half the peak-to-peak ripple current.
The internal 8μA current source is temperature compen-
sated at 4100ppm in order to provide tracking with the
RDS
The R
When selecting a value for R
maximum voltage value for the ILIM pin. Note that because
the low-side MOSFET with low RDS
sensing, the PCB layout, solder connections, and PCB con-
nection to the LX node must be done carefully to obtain
good results. R
(pin 24).
Soft-Start of PWM Regulator
Soft-start is achieved in the PWM regulator by using an
internal voltage ramp as the reference for the FB compara-
tor. The voltage ramp is generated using an internal
charge pump which drives the reference from zero to
750mV in ~1.8mV increments, using an internal ~500kHz
oscillator. When the ramp voltage reaches 750mV, the
ramp is ignored and the FB comparator switches over to a
fixed 750mV threshold. During soft-start the output
voltage tracks the internal ramp, which limits the start-up
inrush current and provides a controlled soft-start profi le
for a wide range of applications. Typical soft-start ramp
time is 1.7ms.
R
ON
ILIM
.
ILIM
value is calculated by the following equation.
= 1250 x I
Figure 8 — Valley Current Limit
ILIM
should be connected directly to LXS
LIM
x [0.088 x (5V - V5V) + 1]
Time
ILIM
do not exceed the absolute
SC414/SC424
ON
is used for current
I
I
PEAK
LOAD
I
LIM
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