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S-8424AAAFT-TB-X Datasheet, PDF (20/46 Pages) Seiko Instruments Inc – BATTERY BACKUP SWITCHING IC
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BATTERY BACKUP SWITCHING IC
S-8424A Series
Rev.3.0_01
2.2 PREEND Voltage Detector
The PREEND voltage detector monitors the input voltage VBAT (VBAT pin voltage). The detection voltage
can be selected from between 1.7 V and 3.4 V in 0.1 V steps. A higher voltage can also be seclected
keeping a constant difference with the RESET voltage. This function enables the warning that the backup
battery is running out. The detection result is output to the PREEND pin: “Low” for lower voltages than
the detection voltage and “High” for higher voltages than the release voltage. The power supply of the
PREEND voltage detector is supplied from the VIN pin. The output is valid only when the voltage is
supplied from the VIN pin to the VOUT pin (VIN ≥ VSW1). The output is the low level when the voltage is
supplied from the VBAT pin to the VOUT pin (VIN < VSW1).
2.3 RESET Voltage Detector
The RESET voltage detector monitors the output voltage VOUT (VOUT pin voltage). The detection
voltage can be selected from between 1.7 V and 3.4 V in 0.1 V steps. The result of detection is output at
the RESET pin: “Low” for lower voltages than the detection level and “High” for higher voltages than
the release level. RESET outputs the normal logic if the VOUT pin voltage is 1.0 V or more.
Caution The PREEND and RESET voltage detectors use the different pins, respectively.
Practically, the current is taken from the VBAT side, and consider the I/O voltage
difference (Vdif) of M1 when M1 is ON.
3. Switch Unit
The switch unit consists of the VSW1 and VSW2 detectors, a switch controller, voltage regulator 2, and switch
transistor M1 (Refer to “Figure 8 Switch Unit”).
VOUT
VIN
REG2
M1
VBAT
Switch
controller
VSW1
detector
VSW2
detector
Figure 8 Switch Unit
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.
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