Philips Semiconductors

Product data

NE57607

Two-cell Lithium-ion battery protection with

overcurrent, over- and under-voltage protection

2001 Oct 03

8

The R-C filters around the NE57607

One needs to place R-C filters on the positive input pins of the

NE57607. These are primarily to shield the IC from electrostatic

occurrences and spikes on the terminals of the battery pack. A

secondary need is during the occurrence of a short-circuit across

the battery pack terminals. Here, the Li-ion cell voltage could

collapse and cause the IC to enter an unpowered state. The R-Cs

then provide power during the first instance of the short circuit and

allow the IC to turn OFF the discharge MOSFET. The IC can then

enter an unpowered state. Lastly, the R-C filter on the node between

the two cells filters any noise voltage caused by noisy load current.

The values shown in Figure 6 are good for these purposes.

Selecting the Optimum MOSFETs:

For a 2-cell battery pack, a logic-level MOSFET should be used.

These MOSFETs have turn-on thresholds of 0.9 V and are

considered full-on at 4.5 V VGS. The total pack voltage will be a

maximum of 8.6 V which is within safe operating range of the gate

voltage which is typically more than two times the full-on voltage.

The MOSFETs should have a voltage rating greater than 20 V and

should have a high avalanche rating to survive any spikes

generated across the battery pack terminals.

The current rating of the MOSFETs should be greater than four

times the maximum “C-rating” of the cells. The current rating,

though, is more defined by the total series resistance of the battery

pack. The total resistance of the battery pack is given by Equation 1.

Rbat(tot) = 2(RDS(ON)) + 2Rcell

(Equation 1)

The total pack resistance is typically determined by the system

requirements. The total pack resistance directly determines how

much voltage droop will occur during pulses in load current.

Another consideration is the forward-biased safe operating area of

the MOSFET. During a short-circuit, the discharge current can easily

reach 10–15 times the “C-rating” of the cells. The MOSFET must

survive this current prior to the discharge MOSFET can be turned

OFF. So having an FBSOA envelope that exceeds 20 amperes for

5 ms would be safe.

The Charge MOSFET Circuit.

The NE57607 uses an isolated charge MOSFET drive arrangement.

This is to help keep ESD charges from entering the IC. The charge

MOSFET is normally ON until turned off by the IC. The CF pin uses

a current source to drive an external NPN transistor to turn OFF the

charge FET. If a charge has poor “compliance” or the no load voltage

of the charge can rise significantly above the rating of the battery

pack. This condition causes the source of the charge FET to go very

negative compared to the cell GND voltage after the charge FET

opens. This design allows the charge FET gate drive to “float” down

to this very negative voltage without upsetting the operation of the IC.

PACKING METHOD

SL01305

TAPE DETAIL

COVER TAPE

CARRIER TAPE

REEL

ASSEMBLY

TAPE

GUARD

BAND

BARCODE

LABEL

BOX

Figure 7. Tape and reel packing method.