SC424MLTRT【PDF 18/29 页】IC REG DL BUCK/LINEAR 28MLPQ

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Sheet目录46

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SC414/SC424

threshold

High-side

Drive (DH)

Low-side

Drive (DL)

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 (825mV)

DL turns off when FB

threshold is reached

Single DH on-time pulse

after DL turn-off

OUT

discharges via inductor

and low-side MOSFET

Normal DL pulse after DH

on-time pulse

Normal V

OUT

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 ~8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 fl ows 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

PEAK

LOAD

LIM

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 8糀 current source is temperature compen-

sated at 4100ppm in order to provide tracking with the

RDS

The R

ILIM

value is calculated by the following equation.

ILIM

= 1250 x I

LIM

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 profi le

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

time is 1.7ms.

Applications Information (continued)

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