S-8426AAA-J8T1X Datasheet, PDF (11/37 Pages) Seiko Instruments Inc – BATTERY BACKUP SWITCHING IC

Rev.2.0_02
BATTERY BACKUP SWITCHING IC
S-8426A Series
3. 1 VSW1 Detector
The VSW1 detector monitors the power supply voltage VIN and sends the results of detection to the switch
controller. The detection voltage (VSW1) can be set to 77 ±2% or 85 ±2% of the CS release voltage (+VDET1).
In the products VSW2 > +VDET1, the setting value is 77 ±2% or 85 ±2% of +VDET4 which is calculated with the
following equation.
+VDET4 = (−VDET1) + 15 × { (−VDET1) – 0.8 } ÷ 372
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 6 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 6 ON/OFF Switching of Voltage Regulator 2 and Switch Transistor M1 According to Power Supply Voltage (VIN)
Power Supply Voltage (VIN)
VIN > VSW1
VIN < VSW1
Voltage Regulator 2
ON
OFF
Switch Transistor M1
OFF
ON
VOUT Pin Voltage
VOUT
VBAT − Vdif
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