SC402B
19
tion mode operation. Figure 6 shows typical waveforms
for the Smart Power Save feature.
FB
threshold
High-side
Drive (DH)
Low-side
Drive (DL)
V
OUT
drifts up to due to leakage
current flowing into C
OUT
DH and DL off
DL turns on when Smart
PSAVE threshold is reached
Smart Power Save
Threshold
DL turns off when FB
threshold is reached
Single DH on-time pulse
after DL turn-off
V
OUT
discharges via inductor
and low-side MOSFET
Normal DL pulse after DH
on-time pulse
Normal V
OUT
ripple
Figure 6 Smart Power Save
SmartDrive
TM
For each DH pulse the DH driver initially turns on the high-
side MOSFET at a lower 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 switching losses while
maintaining high efficiency and also avoids the need for
snubbers for the power MOSFETs.
Current Limit Protection
The device features programmable current limiting, which
is accomplished by using the RDS
ON
of the lower MOSFET
for current sensing. The current limit is set by R
ILIM
resistor.
The R
ILIM
resistor connects from the ILIM pin to the LXS pin
which is also the drain of the low-side MOSFET. When the
low-side MOSFET is on, an internal ~10糀 current flows
from the ILIM pin and through the R
ILIM
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
ON
. The voltage across the
MOSFET is negative with respect to ground. If this MOSFET
voltage drop exceeds the voltage across R
ILIM
, the voltage
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 back up to zero. This method regulates
the inductor valley current at the level shown by ILIM in
Figure 7.
Time
I
PEAK
I
LOAD
I
LIM
Figure 7 Valley Current Limit
Setting the valley current limit to 10A results in a peak
inductor current of 10A plus peak ripple current. In this
situation, the average (load) current through the inductor
is 10A plus one-half the peak-to-peak ripple current.
The internal 10糀 current source is temperature compen-
sated at 4100ppm in order to provide tracking with the
RDS
ON
.
The R
ILIM
value is calculated by the following equation.
R
ILIM
= 670 x I
LIM
x [0.0647 x (5V - VDD) +1]
When selecting a value for R
ILIM
be sure not to exceed the
absolute maximum voltage value for the ILIM pin. Note
that because the low-side MOSFET with low RDS
ON
is used
for current sensing, the PCB layout, solder connections,
and PCB connection to the LX node must be done care-
fully to obtain good results. R
ILIM
should be connected
directly to LXS (pin 28).
Soft-Start of PWM Regulator
SC402B has a programmable soft-start time that is con-
trolled by an external capacitor at the SS pin. After the
controller meets both UVLO and EN/PSV thresholds, the
controller has an internal current source of 3礎 flowing
Applications Information (continued)
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