SC461 datasheet(20/31 Pages) SEMTECH

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SC461

Applications Information (continued)

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

off but the LDO will remain on.

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), otherwise the V

IN UVLO

function will disable the switcher.

The next table summarizes the function of the ENL and EN

pins.

ENL

LDO status Switcher status

low

low, < 0.4V

off

high

low, < 0.4V

off

low

high, < 1.24V

off

high

high, < 1.24V

off

low

high, > 1.55V

off

high

high, > 1.55V

Figure 12 shows the ENL voltage thresholds and their

effect on LDO and Switcher operation.

AGND

ENL low

threshold

(min 0.4V)

1.55V

1.24V

LDO on

LDO off

VIN UVLO hysteresis

ENL voltage

Switcher on if EN = high

Switcher off by VIN UVLO

Figure 12 — ENL Thresholds

ENL Logic Control of PWM Operation

When the ENL input exceeds the V

IN UVLO threshold of

1.55V, internal logic checks the PGOOD signal. If PGOOD

is high, the switcher is already running and the LDO will

start without affecting the switcher. If PGOOD is low, the

device disables PWM switching until the LDO output has

reached 80% of its final value. This delay prevents the

additional current needed by the DH and DL gate drives

from overloading the LDO at start-up.

LDO Start-up

Before LDO start-up, the device checks the status of the

following signals to ensure proper operation can be

maintained.

ENL pin

VLDO output

IN input voltage

When the ENL pin is high and V

IN voltage is available, the

LDO will begin start-up. During the initial phase when

LDO is below 1V, the LDO initiates a current-limited start-

up (typically 15mA). This protects the LDO from thermal

damage if the VLDO pin is shorted to ground. As V

LDO

exceeds 1V, the start-up current gradually increases to

96mA. When V

LDO reaches 4V, the LDO current limit

increases to 200mA and the LDO output rises quickly to

5V. The LDO start-up profile is shown in Figure 13.

15mA constant current

voltage regulating with

200mA current limit

VLDO

increasing current

Figure 13 — LDO Start-Up

Using the Internal LDO to Bias the SC461

The following steps must be followed when using the

internal LDO to bias the device.

Connect VDDA and VDDP to VLDO before

enabling the LDO.

During the initial start-up the LDO, when the

LDO output is less than 1V, the external load

should not exceed 10mA. Above 1V, any exter-

nal load on VLDO should not exceed 40mA until

the LDO voltage has reached 4V.

•

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