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MM1248

Control of Lithium Ion Battery Charging and Discharging

Mitsumi 

MITSUMI

Control of Lithium Ion Battery Charging and Discharging MM1248

2.2. Charging signal off

When the charging signal is off (CON pin "H"), the high-precision overvoltage sensing unit is in standby

state. A rough overcharging sensing unit is provided internally to sense the overvoltage in this state.

The rough overcharging sensing unit monitors the voltages of each cell, and if the voltage of any cell rises

to or exceeds 4.2V, triggering overvoltage sensing (rough), the high-precision overcharging sensing unit

enters the operating state.

Thereafter operation is the same as in "(2-1) Charging signal on" above.

2-3. Discharge off

In an overdischarge state, if the rough overvoltage detection voltage for any cell reaches or exceeds 4.2V,

an overvoltage signal is output.

Hence in this case a dead time is not set. Further, reset depends on the hysteresis voltage at the time of

rough overvoltage detection.

3. Discharge control

Voltage monitoring for discharge control is performed for each cell; if the voltage of any cell falls to 2.4V or

less, the dead time setting pin (CDC pin 8) is discharged, and if the voltage drops to the discharge-off

threshold voltage, an overdischarge state is detected and the discharge switch gate pin (pin 1) is set "H"

and the battery module monitor unit input gate (pin 10) is also set "H"; the battery module monitor output

impedance goes high.

At this time the monitoring circuit switches to a low consumption current (1µA typ. when all cell voltages

are 2.4V), and the state switches from overdischarge to the reset monitoring state (with reset occurring

when all cell voltages reach 2.65V).

Reset from the overdischarge state occurs when the CDC pin is charged under constant current after all

cell voltages have reached 2.65V; when this pin reaches the threshold voltage for overdischarge reset,

discharge is again turned on.

The dead time setting block of the discharge control unit has a circuit configuration such that if either the

overdischarge or the reset state continues for longer than the set time, the state is switched. Hence when

the state changes within the set time, the CDC pin undergoes rapid charging or discharging.

4. Battery module voltage monitor output

This block outputs a voltage equal to 1/3 the voltage of the battery module; within the IC, it is used as the

charge control input. One-third the battery module voltage is supplied by a bleeder resistance, but in order

to reduce the consumption current on overdischarge, a switch is provided such that current does not flow

through this resistance when overdischarge occurs. The charging voltage and overvoltage detection are

both highly precise, and so this switch is designed to use an external FET.

When battery voltage information is to be input to a microcomputer, this feature may be used, but with the

following caveats regarding use.

1. The output upon overdischarge is at high impedance.

2. The output sink current is small.

In order to reduce the consumption current, the output sink current is held low; but depending on the

external components used, response may be slow, affecting charging control and resulting in oscillation

under constant-voltage control. In such cases, a resistance (of about 300kΩ) should be inserted between

the monitor output and GND, to boost the sink capacity.

Pin Assignment

16 15 14 13 12 11 10 9

Discharge control cell

voltage monitoring

Overcharge

detection unit

(high-precision)

12345678

1

DCHG

9

GND

2

CHG

10

FET (G)

3

VIN

11

BATL

4

CON

12

OV2

5

OV1

13

BATM

6

VBAT

14

FET (D)

7

CCC

15

BATH

8

CDC

16

VCC

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