SC3303 datasheet(15/26 Pages) SEMTECH

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SC3303

Applications Information (continued)

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 6 — Smart Power Save

Current Limit Protection

Programmable current limiting is accomplished by using

the RDS

of the lower MOSFET for current sensing. The

current limit is set by an internal resistor R

ILIM

. The resistor

connects from an ILIM node to the LX node 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 Node 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 nega-

tive with respect to ground. If this MOSFET voltage drop

exceeds the voltage across R

ILIM

, the voltage at the ILIM

node 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 7.

Time

IPEAK

ILOAD

ILIM

Figure 7 — Valley Current Limit

In the SC3303, the valley current limit is set to 3A. This

results in a peak inductor current of 3A plus the peak-to-

peak ripple current. In this situation, the average (load)

current through the inductor is 3A plus one-half the peak-

to-peak ripple current.

The internal 10μA current source is temperature compen-

sated at 2500ppm in order to provide tracking with the

RDS

Peak Inductor Current

The peak current through the inductor and switching FETs

must be less than 5A. The only way to meet this require-

ment is to select the switching frequency and inductor

value such that the peak inductor current is less than or

equal to 5A when the trough of the inductor current is

3A.

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 com-

parator. 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 850μs.

During soft start the regulator turns off the low-side

MOSFET on any cycle if the inductor current falls to zero.

This prevents negative inductor current, allowing the

device to start into a pre-biased output.

Power Good Output

The PGOOD (power good) output is an open-drain output

which requires a pull-up resistor. When the output voltage

is 10% below the nominal voltage, PGOOD is pulled low. It

is held low until the output voltage returns to the nominal

voltage. PGOOD is held low during soft start and activated

approximately 1ms after V

OUT

reaches regulation.

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