Battery Backup IC
Battery Backup IC
1. Background of Development
Around 1985 when began the sales of IC for
microcomputer peripheral equipment and system
reset IC, a set manufacture inquired of us about IC for
control of memory. It was a good change for us to
decide a joint development of general purpose IC.
This resulted in the development of PST531 and
PST532. In 1989 they were improved into the backup
IC series dedicated to cards, games (popular-priced)
and to 1-chip microcomputers. Our IC series now
includes low current consumption type products.
2. What is Battery Backup IC?
Such IC memories as S-RAM and D-RAM all volatilize
when power supply is cut off. Usually we use
batteries not to let these data disappear and to store
them for a longer time (of the order of several years).
The battery backup IC can monitor main supply when
storing data by means of battery and the like, control
memory so that there should not arise any data
trouble in the memory, change the main supply over
to battery, and monitor the residual voltage of the
battery, to take a few examples.
3. Basic Configuration
Fig. 1 below illustrates the basic configuration of the
We have PST531, PST532 and MM1026 as IC of the
3-1 Battery/Main Supply Changeover Circuit
1. 3.3V detect circuit ··· Main supply detection
2. Transistor switch ····· Main supply ON/OFF switch
3. Schottky diode ········ For supplying backup power
3-2 Configuration of CS (CE) Control circuit
1. 4.2V detect circuit ········ Detection of main supply
2. Output reverse circuit ··· Reverses CS signal into
We have MM1025, MM1027 and MM1028 as the
backup IC having the following additional functions.
3-3 Detection of Residual Voltage of Battery
1. 2.70V detect circuit ··· Equivalent to yellow alarm
for residual voltage of battery
2. 2.55V detect circuit ··· Equivalent to red alarm for
residual voltage of battery
3-4 Battery Control Circuit
1. This circuit controls so that battery power should
not be supplied even if the battery is replaced
when there remains no main power supply when
replacing the battery.
4. Principli of Operation
4-1 Battery/Main Supply Changeover Circuit
The 3.3V detect circuit turns off the transistor switch
when VCC is 3.3V or less and turns it on when the
same exceeds 3.3V. The transistor ON will output the
voltage of main supply to VOUT and transistor OFF will
output the battery voltage to VOUT.
4-2 CS (CE) Control Circuit
The 4.2V voltage detect circuit makes CS signal to
low level and CS to high level when VCC is 4.2V or
lower, and CS to high level and CS to low level when
the same exceeds 4.2V. The CS signal level will
change over the S-RAM mode as indicated in Fig.1
When the CS signal is at low level (high level with CS
signal), that is when VCC is 4.2V or lower, then S-RAM
will enter into the backup mode and data read/write is
forbidden thus preventing data trouble.
Fig.1 Basic Configuration