S8425 datasheet(13/45 Pages) SII

13 / 45 page

BATTERY BACKUP SWITCHING IC

Rev.3.1_01

S-8425 Series

3.2 VSW2 Detector

The VSW2 detector monitors the VOUT pin voltage and keeps the CS release voltage output low until the

VOUT pin voltage rises to VSW2 voltage. The CS pin output then changes from low to high if the VIN pin

voltage is more than the CS release voltage (

+VDET1) when the VOUT pin voltage rises to 95 ±2% of the

output voltage of voltage regulator 2 (VOUT). The CS pin output changes from high to low regardless of the

VSW2 voltage when the VIN pin voltage drops to less than the CS detection voltage (

−VDET1).

The CS pin output remains high if the VIN pin voltage stays higher than the CS detection voltage

(

−VDET1) when the VOUT pin voltage drops to less than the VSW2 voltage due to an undershoot.

3.3 Switch Controller

The switch controller controls voltage regulator 2 and switch transistor M1. There are two statuses

corresponding to the power supply voltage VIN (or power supply voltage VBAT) sequence: a special

sequence status and a normal sequence status. When the power supply voltage VIN rises and becomes

equal to or exceeds the CS release voltage (

+VDET1), the normal sequence status is entered, but until then

the special sequence status is maintained.

(1) Special sequence status

The switch controller sets voltage regulator 2 ON and switch transistor M1 OFF from the initial status

until the primary power supply voltage VIN is connected and reaches more than the CS release voltage

(

+VDET1) in order to prevent consumption of the backup power supply regardless of the VSW1 detector

status. This status is called the special sequence status.

(2) Normal sequence status

The switch controller enters the normal sequence status from the special sequence status once the

primary power supply voltage VIN reaches more than the CS release voltage (

+VDET1).

Once the normal sequence is entered, the switch controller switches voltage regulator 2 and switch

transistor M1 ON/OFF as shown in Table 4 according to the power supply voltage VIN. The time

required for voltage regulator 2 to be switched from OFF to ON is a few hundred

μs at most. During this

interval, voltage regulator 2 and switch transistor M1 may both switch OFF and the VOUT pin voltage

may drop. To prevent this, connect a capacitor of 10

μF or more to the VOUT pin.

When the VOUT pin voltage becomes lower than the RESET detection voltage, the status returns to the

special sequence status.

Table 4 ON/OFF Switching of Voltage Regulator 2 and Switch Transistor M1

According to Power Supply Voltage VIN

Power Supply Voltage VIN

Voltage Regulator 2

Switch Transistor M1

VOUT Pin Voltage

VIN

> VSW1

OFF

VOUT

VIN

< VSW1

OFF

VBAT

− Vdif

S8425 Datasheet(PDF) 13 Page - Seiko Instruments Inc

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