FS312F-G datasheet(9/13 Pages) FORTUNE

9 / 13 page

12. Design Guide

Selection of External Control MOSFET

Because the overcurrent protection voltage is preset,

the threshold current for overcurrent detection is

determined by the turn-on resistance of the charge

and discharge control MOSFETs. The turn-on

resistance of the external control MOSFETs can be

determined by the equation: RON=VOIP/ (2 x IT) (IT

is the overcurrent threshold current). For example, if

the overcurrent threshold current IT is designed to

be 3A, the turn-on resistance of the external control

MOSFET must be 25mΩ.

Be aware that turn-on

resistance of the MOSFET changes with

temperature variation due to heat dissipation. It

changes with the voltage between gate and source

as well. (Turn-on resistance of MOSFET increases

as the voltage between gate and source decreases).

As the turn-on resistance of the external MOSFET

changes, the design of the overcurrent threshold

current changes accordingly.

Suppressing the Ripple and Disturbance

from Charger

To suppress the ripple and disturbance from charger,

connecting R1 and C1 to VCC is recommended.

Protection the CS pin

R2 is used for latch-up protection when charger is

connected under overdischarge condition and

overstress protection at reverse connecting of a

charger.

FS312F-G データシート(PDF) 9 Page - Fortune Semiconductor Corp.