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SS6821

DESIGN GUIDE

Selection of external control MOSFETs

Because the over-current protection voltage is

pre-set, the threshold current for over-current

detection is determined by the on-resistance of

the discharge control MOSFET M1. The

required on-resistance of the external control

MOSFETs can be determined by the equation:

RON=VOIP/ (2 x IT) (IT is the over-current

threshold current). For example, if the over-

current threshold current IT is designed to be

3A, the on-resistance of the external control

MOSFETs must be 33m

Ω. Users should be

aware that on-resistance of the MOSFET

changes with temperature variation due to heat

dissipation.

changes

with

the

voltage

between gate and source as well. (On-

resistance of a MOSFET increases as the

voltage between gate and source decreases).

Once the on-resistance of the external MOSFET

changes, the over-current threshold current will

change accordingly.

Suppressing the ripple and disturbance

from the charger

To suppress the ripple and disturbance from the

charger, connecting R1, C1 to the VCC pin is

recommended.

Protection at CS pin

R2 is used for latch-up protection when the

charger is connected under an over-discharge

condition,

and

over-stress

protection

for

accidental reverse-connection of the charger. A

larger value of R2 reduces the charger leakage

current in over-charge mode, but may possibly

disable the charge detection function after over-

discharge. A resistance value of 100K

Ω is

recommended.

12/6/2004 Rev.2.02

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