SC403B datasheet(20/32 Pages) SEMTECH

20 / 32 page

SC403B

PGOOD will transition low if the V

FB pin exceeds +20% of

nominal, which is also the over-voltage shutdown thresh-

old. PGOOD also pulls low if the EN/PSV pin is low when

DD voltage is present.

Output Over-Voltage Protection

Over-voltage protection becomes active as soon as the

device is enabled. The threshold is set at 20% above the

feedback reference voltage. 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/PSV input is toggled or VDD 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 for eight

consecutive switching 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/PSV is toggled

or VDD is cycled.

VDD UVLO, and POR

UVLO (Under-Voltage Lock-Out) circuitry inhibits switch-

ing and tri-states the DH/DL drivers until V

DD rises above

3.0V. An internal POR (Power-On Reset) occurs when V

exceeds 3.0V, which resets the fault latch and soft-start

counter to prepare for soft-start. The SC403B then begins

a soft-start cycle. The PWM will shut off if VDD falls below

2.4V.

LDO Regulator

SC403B has an option to bias the switcher by using an inter-

nal LDO from V

IN. The LDO output is connected to VDD

internally. The output of the LDO is programmable by using

external resistors from the VDD pin to AGND. The feedback

pin (FBL) for the LDO is regulated to 750mV (see Figure 9).

RLDO1

To FBL pin

VDD

RLDO2

Figure 9 — LDO Voltage Divider

The LDO output voltage is set by the following equation.

LDO

750

VLDO

A minimum 1μF capacitor referenced to AGND is required

along with a minimum 1.0μF capacitor referenced to

PGND to filter the gate drive pulses. The capacitance to

PGND will need to increase to 10uF if VLDO is set to a

voltage lower than 4.5V. Refer to the layout guidelines

section for component placement suggestions.

LDO ENL Functions

The ENL input is used to control the internal LDO. When

ENL is low (grounded), the LDO is off. When ENL is above

the V

IN UVLO threshold, the LDO is enabled and the

switcher is also enabled if EN/PSV and VDD meet the

thresholds.

The ENL pin also acts as the switcher UVLO (under-voltage

lockout) for the V

IN supply. The VIN UVLO voltage is pro-

grammable via a resistor divider at the VIN, ENL and AGND

pins.

If the ENL pin transitions from high to low within 2 switch-

ing cycles and is less than 1V, then the LDO will turn off

but the switcher remains on. If the ENL goes below the V

UVLO threshold and stays above 1V, then the switcher will

turn off but the LDO remains on. The V

IN UVLO function

has a typical threshold of 2.6V on the V

IN rising edge. The

falling edge threshold is 2.4V.

Note that it is possible to operate the switcher with the

LDO disabled, but the ENL pin must be below the logic

low threshold (0.4V maximum). In this case, the UVLO

function for the input voltage cannot be used. The table

below summarizes the function of the ENL and EN pins,

with respect to the rising edge of ENL.

ENL

LDO status Switcher status

low

low, < 0.4V

off

high

low, < 0.4V

off

low

high, < 2.6V

off

high

high, < 2.6V

off

low

high, > 2.6V

off

high

high, > 2.6V

Applications Information (continued)

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