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MC33348

Lithium Battery Protection Circuit for One Cell Battery Packs

ON Semiconductor 

MC33348

Figure 10. Cell Voltage Limit Sampling

From

Cell Voltage

Sample Switch

Over/Under

Cell Voltage

Detector

&

Reference

Cell Voltage

Discharge Voltage R1

Threshold

+

Charge Voltage

Threshold

R2

Cell

Voltage

−

To

Cell Negative

Terminal

Cell Voltage

R3

Return

The cell charge and discharge voltage limits are controlled by the

values selected for the internal resistor divider string and the

comparator input threshold. As the battery pack reaches full charge,

the Cell Voltage Detector will sense an overvoltage fault condition

when the cell exceeds the designed overvoltage limit. The fault

information is stored in a data latch and charge MOSFET Q1 is turned

off, disconnecting the battery pack from the charging source. An

internal current source pull−up is then applied to lower tap of the

divider, creating a hysteresis voltage. As a result of an overvoltage

fault, the battery pack is available for discharging only.

The overvoltage fault is reset by applying a load to the battery pack.

As the voltage across the cell falls below the hysteresis level, charge

MOSFET Q1 will turn on and the current source pull−up will turn off.

The battery pack will now be available for charging or discharging.

As the load eventually depletes the battery pack charge, the Cell

Voltage Detector will sense an undervoltage fault condition when the

cell falls below the designed undervoltage limit. After three

consecutive faults are detected, discharge MOSFET Q2 is turned off,

disconnecting the battery pack from the load. The protection circuit

will now enter a low current sleepmode state. Refer to Figure 6. As a

result of the undervoltage fault, the battery pack is available for

charging only. The typical cutoff thresholds and hysteresis voltage are

shown in Figure 11.

Figure 11. Cutoff and Hysteresis Limits

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Device

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Suffix

Charging

Cutoff

(V)

Hysteresis

(mV)

Disharging

Cutoff

(V)

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ −1, −2

4.20

300

2.25

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ −3, −4

4.25

300

2.28

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ −5, −6

4.35

300

2.30

The undervoltage logic is designed to automatically reset if less

than three consecutive faults appear. This helps to prevent a

premature disconnection of the load during high current pulses when

the battery pack charge is close to being depleted.

The undervoltage fault is reset by applying charge current to the

battery pack. When the voltage on Pin 3 exceeds Pin 5 by 0.6 V,

discharge MOSFET Q2 will turn on. The battery pack will now be

available for charging or discharging.

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