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MM1248

Control of Lithium Ion Battery Charging and Discharging

Mitsumi 

MITSUMI

Control of Lithium Ion Battery Charging and Discharging MM1248

Circuit conventions

• Charge control unit

• Overcharge detection unit

Subtraction amp, buffer amp, voltage comparator, dead time setting circuit

• Discharge control unit

Cell voltage monitoring circuit, dead time setting circuit

• Overvoltage sensing (rough detection) unit

• High-precision reference voltage unit

Operating outline

1. Charge control

Charge control is turned on and off using the CON pin; when CON is low the charge control unit is in

operation. The error amp of the charge control unit takes inputs of 4.155V from the high-precision reference

voltage unit and the battery module voltage monitor output. The battery module voltage monitor outputs a

voltage equal to 1/3 the battery module voltage, so that in charging control the charging control FET (pin 2)

is controlled such that the battery module voltage is 4.155 3 V.

This block performs constant-voltage control only; current limiting (constant-current control) should be

implemented on the charging input side.

2. Overcharge detection

There are two blocks for overcharge detection; three output modes are provided according to input

conditions and cell voltage.

1. Charging signal on (during discharge)

2. Charging signal off (during discharge)

3. Discharge off

2-1. Charging signal on

The high-precision overcharge sensing unit is always in the operating state, and the voltage of each cell is

monitored.

In order to monitor each cell's voltage precisely, subtraction amps are provided for monitoring M and H cell

voltages, and a buffer amp is used to monitor the L cell voltage. By means of these amps the voltage of

each cell is converted to a GND-reference voltage, and compared with the high-precision reference voltage

of 4.335V.

If the voltage of any one of the cells exceeds 4.335V, the overvoltage is sensed, and the dead time setting

pin (CCC pin 7) is charged by a constant current; when the CCC pin is charged to the threshold voltage of

the dead time setting circuit (4.35V), an overvoltage signal is output.

(OV1 pin "H", OV2 pin "L")

When an overcharge signal is output, the overvoltage monitoring unit switches from the overvoltage state

to the reset monitoring state, and an overcharge signal is output until the voltages of all cells fall to 4.16 V.

To summarize the sequence of operations, when the voltage of any of the cells remains in the overcharge

state continuously for the specified time (the dead time), that cell is judged to be in an overcharged state

and is switched to overvoltage output; when all the cell voltages drop to the overcharge reset voltage or

below, the overcharge state is canceled.

On entering the overcharge state, if the charge control unit circuit is turned off, the latter unit will not

operate.

Output logic

Pin

Normal

Overvoltage

Output type

OV1

L

H

PNP output, internal pull-down resistance

OV2

High impedance

L

NPN open-collector output

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