5

MB3790

s FUNCTIONAL DESCRIPTION

1.

Battery Backup Function

2.

Power Supply Voltage Level Detect Function

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.

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

µA), this function determines

the reset pulse width. The calculation formula for this is given below:

≅ 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

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

the constant-voltage method of charging. Note that the typical value of the device's internal recharging resistor is

500 ohms.