SC418 datasheet(18/30 Pages) SEMTECH

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SC418

VOUT

VIN

CIN

RILIM

CBST

PGND

ILIM

BST

COUT

Figure 9 — 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 current through the inductor is 10A

plus one-half the peak-to-peak ripple current.

The R

ILIM value is calculated by the next equation.

LIM

DSON

ILIM

The internal 10μA current source is temperature compen-

sated at 4100ppm in order to provide tracking with the

RDS

ON.

Note that MOSFET RDS

(ON) increases significantly if the

VDDP voltage is 3.3V compared to 5.0V. When selecting

the R

ILIM value, use the RDS(ON) value that corresponds to

the VDDP voltage used in the application.

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

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.

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,

regardless of the psave setting. 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 voltage at the

FB pin is 10% below the nominal voltage, PGOOD is pulled

low. It remains low until the FB voltage returns above -8%

of nominal. During start-up PGOOD is held low and will

not be allowed to transition high until the PGOOD start-

up delay fime has passed and soft-start is completed

(when V

FB reaches 500mV). The delay time starting from

EN going high is typically 2ms for VDDA = 5V and 1ms for

VDDA = 3.3V.

PGOOD will transition low if the FB voltage exceeds +20%

of nominal (600mV), which is also the over-voltage shut-

down threshold. PGOOD also pulls low if the EN pin is low

when VDDA is present.

Output Over-Voltage Protection

OVP (Over-voltage protection) becomes active as soon as

the device is enabled. The OVP threshold is set at 500mV

+ 20% (600mV). When V

FB exceeds the OVP threshold, DL

latches high and the low-side MOSFET is turned on. DL

remains high and the controller remains off until the EN

input is toggled or VDDA is cycled. There is a 5μs delay

built into the OVP detector to prevent false transitions.

PGOOD is also low after an OVP event.

Output Under-Voltage Protection

When V

FB falls 25% below its nominal voltage (falls to

375mV) for eight consecutive clock cycles, the switcher is

shut off and the DH and DL drives are pulled low to tri-

state the MOSFETs. The controller stays off until EN is

toggled or VDDA is cycled.

VDDA UVLO and POR

The VDDA Under-Voltage Lock-Out (UVLO) circuitry inhib-

its switching and tri-states the DH/DL drivers until VDDA

rises above 2.9V. When VDDA exceeds 2.9V, an internal

POR (Power-On Reset) resets the fault latch and the soft-

start counter and then the SC418 begins the soft-start

Applications Information (continued)

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