SC412AEVB datasheet(10/22 Pages) SEMTECH

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SC412A

POWER MANAGEMENT

The following over-current equation can be used for both

RDS

ON or resistive sensing. For RDSON sensing, the MOSFET

RDS

ON rating is used for the value of RSENSE.

ILIM

OC(Valley) = 10μA • ———

SENSE

Power Good Output

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

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

is 12% below its nominal voltage (660mV), PGD is pulled

low. It is held low until the output voltage returns above

approximately -11% of nominal. PGD is held low during

start-up and will not be allowed to transition high until

soft-start is completed (when FB reaches 0.75V). There

is a 5μs delay built into the PGD circuit to prevent false

transitions.

PGD also transitions low if the FB pin exceeds +20% of

nominal, which is also the over-voltage shutdown point. If

EN is low with VCC supplied, PGD is also pulled low.

Output Over-Voltage Protection

In steady state operation, when FB exceeds 20% of nomi-

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

DL stays high and the SMPS stays off until the EN input is

toggled or VCC is recycled. There is a 5μs delay built into

the OVP detector to prevent false transitions. PGD is also

low after an OVP.

Output Under-Voltage Protection

When FB falls 30% below its trip point for eight consecutive

clock cycles, the output is shut off; the DL/DH drives are

pulled low to tri-state the MOSFETS, and the SMPS stays

off until the EN input is toggled or VCC is recycled.

POR and UVLO

Under-voltage lockout circuitry (UVLO) inhibits switching

and tri-states the DH/DL drivers until VCC rises above 4.4V.

An internal power-on reset (POR) occurs when VCC exceeds

4.4V, which resets the fault latch and soft-start counter, to

prepare the PWM for switching. At this time the SC412A

will come out of UVLO and begin the soft-start cycle.

Applications Information (continued)

The RDS

ON sensing circuit is shown in Figure 4 with RILIM =

R1 and RDS

ON of Q2.

VOUT

+5V

VBAT

GND

VDD

ILIM

BST

SC412A

Figure 4.

The resistor sensing circuit is shown in Figure 5 with R

ILIM

= R1 and R

SENSE = R4.

Resistive sensing operates similar to MOSFET sensing,

except that a resistor is used to improve accuracy. The

resistor connects between the MOSFET source and GND,

and the RILIM connects from the ILIM pin to the sense

resistor, as in Figure 5.

Vout

+5V

GND

VDD

ILIM

BST

VBAT

SC412A

Figure 5.

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