S-8424AAAFT-TB-X Datasheet, PDF (21/46 Pages) Seiko Instruments Inc – BATTERY BACKUP SWITCHING IC

Rev.3.0_01
BATTERY BACKUP SWITCHING IC
S-8424A Series
3.2 V_SW2 Detector
The V_SW2 detector monitors the VOUT pin voltage and keeps the CS release voltage output low until the
VOUT pin voltage rises to V_{SW2 voltage.} The CS pin output then changes from low to high if the VIN pin
voltage is more than the CS release voltage (+V_DET1) when the VOUT pin voltage rises to 95 ±2% of the
output voltage of voltage regulator 2 (V_OUT). The CS pin output changes from high to low regardless of
the V_SW2 voltage when the VIN pin voltage drops to less than the CS detection voltage (−V_DET1).
The CS pin output remains high if the VIN pin voltage stays higher than the CS detection voltage (−V_DET1)
when the VOUT pin voltage drops to less than the V_SW2 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 V_IN (or power supply voltage V_BAT) sequence: a special
sequence status and a normal sequence status. When the power supply voltage V_IN rises and becomes
equal to or exceeds the CS release voltage (+V_DET1), 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 V_IN is connected and reaches more than the CS release
voltage (+V_DET1) in order to prevent consumption of the backup power supply regardless of the V_SW1
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 V_IN reaches more than the CS release voltage (+V_DET1).
Once the normal sequence is entered, the switch controller switches voltage regulator 2 and switch
transistor M1 ON/OFF as shown in Table 14 according to the power supply voltage V_IN. 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 14 ON/OFF Switching of Voltage Regulator 2 and
Switch Transistor M1 According to Power Supply Voltage (V_IN)
Power Supply Voltage (V_IN)
V_IN > V_SW1
V_IN < V_SW1
Voltage Regulator 2
ON
OFF
Switch Transistor M1
OFF
ON
VOUT Pin Voltage
V_OUT
V_BAT − V_dif
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