SC401B datasheet(19/32 Pages) SEMTECH

19 / 32 page

SC401B

tion mode operation. Figure 6 shows typical waveforms

for the Smart Power Save feature.

SmartDriveTM

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 or series resistors in the gate drive.

threshold

High-side

Drive (DH)

Low-side

Drive (DL)

VOUT drifts up to due to leakage

current flowing into COUT

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

VOUT discharges via inductor

and low-side MOSFET

Normal DL pulse after DH

on-time pulse

Normal VOUT ripple

Figure 6 — Smart Power Save

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 ~10μA 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

IPEAK

ILOAD

ILIM

Figure 7 — Valley Current Limit

Setting the valley current limit to 15A results in a peak

inductor current of 15A plus peak ripple current. In this

situation, the average (load) current through the inductor

is 15A plus one-half the peak-to-peak ripple current.

The internal 10μA current source is temperature compen-

sated at 4100ppm in order to provide tracking with the

RDS

ON.

The

ILIM value is calculated by the following equation.

ILIM = 495 x ILIM 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

SC401B 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 3µA flowing

through the SS pin to charge the capacitor. During the

Applications Information (continued)

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