SC414EVB datasheet(18/29 Pages) SEMTECH

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SC414/SC424

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 (825mV)

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 7 — Smart Power-save

Current Limit Protection

The device features programmable current limiting, which

is accomplished by using the RDS

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 ~8μA 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

. 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 to zero. This method regulates the induc-

tor valley current at the level shown by ILIM in Figure 8.

Time

IPEAK

ILOAD

ILIM

Figure 8 — Valley Current Limit

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

ILIM value is calculated by the following equation.

ILIM = 1250 x ILIM x [0.088 x (5V - V5V) + 1]

When selecting a value for R

ILIM

do not exceed the absolute

maximum voltage value for the ILIM pin. Note that because

the low-side MOSFET with low RDS

is used for current

sensing, the PCB layout, solder connections, and PCB con-

nection to the LX node must be done carefully to obtain

good results. R

ILIM

should be connected directly to LXS

(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 profile

for a wide range of applications. Typical soft-start ramp

time is 1.7ms.

Applications Information (continued)

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