SC508A datasheet(21/32 Pages) SEMTECH

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SC508/SC508A

Applications Information (continued)

80mA. When V

LDO reaches 4.5V, the LDO current limit

increases to 200mA and the LDO output rises quickly to

5.0V. The LDO start-up profile is shown in Figure 14.

4.5V

20mA constant current

voltage regulating with

200mA current limit

1.0V

VLDO

increasing current

Figure 14 — LDO Start-Up

LDO Switch-Over Operation

The SC508 includes a switch-over function for the LDO.

The switch-over function is designed to increase efficiency

by using the more efficient DC-DC converter to power the

IC, avoiding the less efficient LDO regulator when possible.

The switch-over function connects the VLDO pin directly

to the VOUT pin through an internal switch. When the

switch-over is complete the LDO is turned off, which

reduces operating power loss. If the LDO output is used to

bias the SC508, then after switch-over the device is self-

powered from the switching regulator with the LDO

turned off.

After the switcher completes soft-start and the PGOOD

delay, the switch-over logic waits for 32 switching cycles

before it starts the switch-over. There are two methods of

completing the switch-over of V

LDO to VOUT.

In the first method, the LDO is already in regulation when

the DC-DC converter is enabled. As soon as the PGOOD

output goes high, the 32 cycle count is started. The volt-

ages at the VLDO and VOUT pins are then compared; if the

two voltages are within ±300mV of each other, the VLDO

pin connects to the VOUT pin using an internal switch,

and the LDO is turned off.

In the second method, the DC-DC converter is already

running and the LDO is enabled. In this case the 32 cycle

count is started as soon as the LDO reaches 90% of its final

value. At this time, the VLDO and VOUT pins are compared,

and if within ±300mV the switch-over occurs and the LDO

is turned off.

Switch-over MOSFET Parasitic Diodes

The switch-over MOSFET contains parasitic diodes that

are inherent to its construction, as shown in Figure 15.

Switchover

MOSFET

Parasitic diode

VDDA

VLDO

VOUT

Switchover

control

Figure 15— Switch-over MOSFET Parasitic Diodes

It is important to prevent forward bias of these diodes.

The following two conditions must be satisfied in order for

the parasitic diodes to stay off.

VDDA > V

LDO

VDDA > V

OUT

If either V

LDO or VOUT is higher than VDDA, the respective

diode will turn on and the SC508 operating current will

then flow through this diode. This has the potential of

damaging the device.

Using the Internal LDO to Bias the SC508

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 4.5V.

When the switch-over feature is used and the VDDA/VDDP

power comes from the VOUT pin, the EN and ENL inputs

must be used carefully. Do not connect the EN pin directly

to VDDA or another supply voltage. If this is done, driving

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