STw4102

Low drop-out regulator

21/34

7

Low drop-out regulator

The purpose of the low drop-out regulator (LDO) is to supply the phone directly from the wall

adapter when the battery is low or not present. When the wall adapter is present, MOSFET

is off and the phone supply is regulated at 4.7 V minus the drop voltage across D1. When

the MAIN pin is not connected, MOSFET M2 is on (the gate is pulled down) and the D1

diode avoids any reverse current. The LDO output current is limited by The M1 MOSFET

schematics are illustrated in Figure 21.

When the battery is fully discharged, the regulator can charge the battery from the main

supply and at the same time supply the phone, which avoids waiting for the battery to be

charged up to the minimum operating voltage before using the phone.

Figure 21.

Regulator

the LDO is turned off to protect the external PMOS against high power dissipation as shown

enabled, but can be disabled at factory level by OTP.

Figure 22.

Supply voltage

LDODRV

LDO 4.7V

LDOFB

To phone

M2

M1

D1

Main supply

Battery power supply

Wall adapter power supply

Co >= 15uF

Main supply voltage

4.7V

7.2V

ON

ON

6.5V

Main detect