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MC33348D-1

Lithium Battery Protection Circuit for One Cell Battery Packs

ON Semiconductor 

MC33348

Current Sensing

Discharge current limit protection is internally provided by the

MC33348. As the battery pack discharges, Pins 5 and 3 sense the

voltage drop across MOSFETs Q1 and Q2. A discharge current limit

fault is detected if the voltage at Pin 5 is greater than Pin 3 by 400 mV

for −1, −3 and −5 suffix devices, or 200 mV for −2, −4 and −6 suffix

devices. The fault information is stored in a data latch and discharge

MOSFET Q2 is turned off, disconnecting the battery pack from the

load. As a result of the discharge current fault, the battery pack is

available for charging only. The discharge current limit is given by:

ILim(dschg)

+

Vth(dschg)

RLim(dschg)

+

Vth(dschg)

RDS(on)Q1 ) RDS(on)Q2

The discharge current fault is reset by either disconnecting the load

from the battery pack, or by connecting the battery pack to the charger.

When the voltage on Pin 5 no longer exceeds Pin 3 by approximately

Vth(dschg), the Sense Enable circuit will turn on discharge MOSFET

Q2.

Figure 13. Power Supply Decoupling

1

Cell

C

10

R

5.1 k

7

MC33348

3

In order to guarantee proper discharge current limit operation when

the battery pack output is shorted, power must be made available to the

MC33348. This can be accomplished by decoupling the VCC input

with the R/C component values shown above. The capacitor value

must be increased to 100 μF if the discharge current limit shutdown

delay time is extended to 40 ms as shown in

Figure 12. A small signal schottky diode can be used in place of R for

enhanced short circuit operation. The diode cathode is connected to

Pin 7 and C, and the anode is connected to the positive terminal of the

cell. The schottky diode solution may be a better choice in applications

that have a charger with a relatively high open circuit voltage. These

components can be deleted if operation of the discharge current limit is

not required when the battery pack output is shorted.

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