SC427MLTRT【PDF 20/33 页】IC REG DL BUCK/LINEAR 32MLPQ

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Sheet目录46

16页 17页 18页 19页 [20页]21页 22页 23页 24页

SC417/SC427

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

mart Power Save

hreshold (550mV)

DL turns off when FB

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 LX 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

. 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 activate. 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 8.

Tim

PEAK

LOAD

LIM

Figure 8 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

The R

ILIM

value is calculated by the following equation.

ILIM

= 735 x I

LIM

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

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

Soft-start is achieved in the PWM regulator by using an

internal voltage ramp as the reference for the FB

Comparator. The voltage ramp is generated using an

internal charge pump which drives the reference from

zero to 500mV in ~1.2mV increments, using an internal

~500kHz oscillator. When the ramp voltage reaches

500mV, the ramp is ignored and the FB comparator

switches over to a fixed 500mV 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 850約.

Pre-Bias Startup

SC417/427 can start up into a pre-biased output voltage.

The start up time is approximately 850約 from enable to

regulation. The output voltage starts to ramp up when

the internal ramp meets the pre-charged FB voltage level.

Pre-bias startup is achieved by turning off the lower gate

when the inductor current falls below zero. This method

prevents output voltage discharge.

Applications Information (continued)

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