MB3790 Datasheet, PDF (5/21 Pages) Fujitsu Component Limited. – Battery Backup IC

MB3790
s FUNCTIONAL DESCRIPTION
1. Battery Backup Function
• When the power supply voltage exceeds the voltage detection level (i.e., VINH), the device outputs a current of
up to 200 mA from the VIN power supply to the load circuit via the VOUT pin.
• When the power supply voltage is less than or equal to VINL, the device switches the source of power for VOUT
from VIN to the primary or secondary battery for backup purposes.
2. Power Supply Voltage Level Detect Function
When the power supply voltage drops below VINL, the voltage level detection comparator is actuated to perform the
following (note that the detection voltage level has the hysteresis characteristics listed in ELECTRICAL CHARAC-
TERISTICS in this data sheet):
• The comparator first outputs the RESET signal (High level).
• It switches the source of power for the load circuit to the primary or secondary battery.
The power supply voltage detection level can be adjusted by fitting an external resistor to the VSENSE pin. When
adjusting the detection level, be sure to set it to 4.0 V or higher by considering the power supply voltage for the
internal circuit operation.
In addition, the detection set time can be extended by connecting a capacitator. For this method of adjustment, refer
to APPLICATION in this data sheet.
3. Reference Voltage Circuit
This is a temperature-compensated reference voltage circuit of a band gap type so that it outputs a trimming-
adjusted exact reference voltage.
The reference voltage power supply is used to set the reference voltage/constant current values of the detection
circuit, as well as the secondary battery recharging voltage.
4. Power-on Reset Function
By charging the capacitator connected to the CT pin with constant current (approx. 3 µA), this function determines
the reset pulse width. The calculation formula for this is given below:
Reset pulse width tPO (sec) ≅ CT (F) × 10⁶
(When CT = 1000 pF, tPO ≅ 1 ms [typ])
5. Primary Battery Voltage Detection Function
If the primary battery voltage drops below the detection level when the power supply voltage is greater than or equal
to VINL, the device outputs an alarm signal (Low level) from the CMOS output pin, ALARM1 or ALARM2.
Note that the voltage level detection comparator has the hysteresis characteristics listed in ELECTRICAL CHAR-
ACTERISTICS in this data sheet.
• When the primary battery voltage is 2.65 V [typ] or less:
The ALARM1 output pin is forced to a Low level to issue an alarm indicating that it’s time to replace the primary
battery.
• When the primary battery voltage is 2.37 V [typ] or less:
The ALARM2 output pin is forced to a low level to issue an alarm indicating that the primary battery voltage
is less than the voltage necessary to retain the SRAM data (approx. 2.0 V)
6. Secondary Battery Recharging Function
When the power supply voltage is greater than or equal to VINL, the device recharges the secondary battery using
the constant-voltage method of charging. Note that the typical value of the device's internal recharging resistor is
500 ohms.
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