MB3790 Fujitsu Media Devices, MB3790 Datasheet - Page 5

• Manufacturer Part Number: MB3790
• Description: Battery Backup IC
• Manufacturer: Fujitsu Media Devices

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DataSheet
4
1. Battery Backup Function
2. Power Supply Voltage Level Detect Function
When the power supply voltage drops below V
following (note that the detection voltage level has the hysteresis characteristics listed in ELECTRICAL CHARAC-
TERISTICS in this data sheet):
The power supply voltage detection level can be adjusted by fitting an external resistor to the V
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 capacitor. 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 capacitor connected to the C
the reset pulse width. The calculation formula for this is given below:
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 V
Note that the voltage level detection comparator has the hysteresis characteristics listed in ELECTRICAL CHAR-
ACTERISTICS in this data sheet.
6. Secondary Battery Recharging Function
When the power supply voltage is greater than or equal to V
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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FUNCTIONAL DESCRIPTION
• When the power supply voltage exceeds the voltage detection level (i.e., V
• When the power supply voltage is less than or equal to V
• 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.
• When the primary battery voltage is 2.65 V [Typ] or less:
• When the primary battery voltage is 2.37 V [Typ] or less:
INL
up to 200 mA from the V
from V
Reset pulse width t
(When C
The ALARM1 output pin is forced to a Low level to issue an alarm indicating that it’s time to replace the primary
battery.
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)
, the device outputs an alarm signal (Low level) from the CMOS output pin, ALARM1 or ALARM2.
IN
to the primary or secondary battery for backup purposes.
T
= 1000 pF, t
PO
(s)
PO
IN
power supply to the load circuit via the V
1 ms [Typ])
C
T
(F)
10
6
INL
T
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pin with constant current (approx. 3 A), this function determines
, the voltage level detection comparator is actuated to perform the
INL
INL
, the device recharges the secondary battery using
, the device switches the source of power for V
OUT
pin.
INH
), the device outputs a current of
MB3790
SENSE
pin. When
OUT
5