S-8425AAAFT-TB-X Datasheet, PDF (11/45 Pages) Seiko Instruments Inc – BATTERY BACKUP SWITCHING IC

Rev.3.1_01
BATTERY BACKUP SWITCHING IC
S-8425 Series
 Operation
The internal configuration of the S-8425 Series is as follows.
• Voltage regulator 1, which stabilizes input voltage VIN and outputs it to VRO
• Voltage regulator 2, which stabilizes input voltage VIN and outputs it to VOUT
• Voltage regulator 3, which stabilizes input voltage VIN and outputs it to VCH
• CS voltage detector, which monitors input voltage VIN
• RESET voltage detector, which monitors output voltage VOUT
• Switch unit
The functions and operations of the above-listed elements are described below.
1. Voltage Regulators
The S-8425 Series features on-chip voltage regulators with a small dropout voltage. The voltage of the
VRO, VOUT, and VCH pins (the output pins of the voltage regulator) can separately be selected for the
output voltage in 0.1 V steps between the range of 2.3 to 5.4 V.
[Dropout voltage V_drop1, V_drop2, V_drop3]
Assume that the voltage output from the VRO pin is V_RO(E) under the conditions of output voltage 1
described in the electrical characteristics table. VIN1 is defined as the input voltage at which the output
voltage from the VRO pin becomes 98% of VRO(E) when the input voltage VIN is decreased. Then, the
dropout voltage Vdrop1 is calculated by the following expression.
V_drop1 = V_IN1 − V_RO(E) × 0.98
Similarly, assume that the voltage of the VOUT pin is V_{OUT(E), and} V_{CH(E) respectively} under the conditions
of output voltage 2 and 3 described in the electrical characteristics table. VIN2 and VIN3 are defined as the
input voltages at which the output voltage from the VOUT pin becomes 98% of VOUT(E) and VCH(E),
respectively. Then, the dropout voltages Vdrop2 and Vdrop3 are calculated by the following expression.
V_drop2 = V_IN2 − V_OUT(E) × 0.98
V_drop3 = V_IN3 − V_CH(E) × 0.98
2. Voltage Detector
The S-8425 Series incorporates two high-precision, low power consuming voltage detectors with hysteresis
characteristics. The power of the CS voltage detector is supplied from the VIN and VBAT pins.
Therefore, the output is stable as long as the primary or backup power supply is within the operating voltage
range (1.7 to 16 V). All outputs are Nch open-drain, and need pull-up resistors of about 100 kΩ.
2.1 CS Voltage Detector
The CS voltage detector monitors the input voltage V_IN (VIN pin voltage). The detection voltage can be
selected from between 2.4 and 5.3 V in 0.1 V steps. The result of detection is output at the CS pin: “Low”
for lower voltage than the detection level and “High” for higher voltage than the release level (however,
when the VOUT pin voltage is the CS output inhibit voltage (VSW2), a low level is output).
Input voltage
Release voltage
Detection voltage
Output voltage
Figure 6 Definition of Detection and Release Voltages
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