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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.
It
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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