SS6804BCSTR datasheet(10/12 Pages) SSC

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SS6804

The delay time can also be approximately

calculated by the following equations (if CTC,

CTD ≤ 82nF) :

TOC(mS) = 11.8 x CTC(nF)

TOD(mS) = 11.8 x CTD(nF)

Setting the Overcurrent Delay Time 1

The overcurrent delay time 1 (TOI1) at 0.15V <

VCC-VCS < 0.3V is set by the external capacitor

CTI, while the overcurrent delay time 2 and 3

(TOI2 and TOI3) is fixed by IC internal circuit.The

relationship between capacitance of the external

capacitor and delay time is tabulated as below.

CTI (nF)

TOI (ms)

4.8

2.2

15.0

3.3

18.8

23.6

6.8

31.0

61.8

Selection of External Control

MOSFETs

Because the overcurrent protection voltage is

preset, the threshold current for overcurrent

detection

determined

the

turn-on

resistance of the discharge control MOSFET M1.

The turn-on resistance of the external control

MOSFETs can be determined by the equation:

RON=VOIP/IT (IT is the overcurrent threshold

current). For example, if the overcurrent

threshold current IT is designed to be 5A, the

turn-on resistance of the external control

MOSFETs must be 30m

Ω. Users should 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

a MOSFET increases as the voltage between

gate and source decreases). Once the turn-on

resistance of the external MOSFET changes,

the overcurrent threshold current will change

accordingly.

Suppressing the Ripple and

Disturbance from Charger

To suppress the ripple and disturbance from

charger, connecting R1 to R4 and C1 to C4 is

recommended. Larger R1 will cause larger error

of battery sense voltage.

Controlling the Charge Control

MOSFET

R5, R6, R7 and NPN transistor Q1 are used to

switch the charge control MOSFET M2. If

overcharge does not occur, no current flows into

OC pin and Q1 is turned off, then M2 is turned

on. When overcharge occurs, current flows into

OC pin and Q1 is turned on, which turns off M2

in turn.

Protection at CS Pin

R8 is used for protection of IC when charger is

connected in reverse. The charge detection

function after overdischarge is possibly disabled

by larger value of R8. Resistance of 1K

Ω is

recommended.

12/29/2003 Rev.2.02

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