AIC1821

ONE-CELL LITHIUM-ION BATTERY PROTECTION IC

SPEC NO: DS-1821-P1 06/07/00

ANALOG INTEGRATIONS CORPORATION

www.analog.com.tw

4F, 9 Industry E. 9th Road, Science-based IndustrialPark,Hsinchu,Taiwan, R.O.C.

TEL: (8863)577-2500

FAX:(8863)577-2510

5

l THE OPERATION

Overcharge Protection

When the voltage of the battery cell exceeds the

inhibited by the turning-off of the charge control

MOSFET M2. The overcharge delay time is fixed to

100mS by IC internal circuit. The overcharge

condition is released in two cases:

1. The voltage of the battery cell becomes lower

2. The voltage of the battery cell falls below the

is connected.

overcharge condition is never released even a load

is connected to the pack.

Overdischarge Protection

When the voltage of the battery cell goes below the

is inhibited by the turning-off of the discharge

control MOSFET M1. The overdischarge delay time

defaults to 100mS. Inhibition of discharging is

immediately released when the voltage of the

battery cell becomes higher than overdischarge

Overcurrent Protection

In normal mode, the AIC1821 continuously monitors

the discharge current by sensing the voltage of CS

pin. If the voltage of CS pin exceeds the overcurrent

circuit operates and discharging is inhibited by the

turning-off of the discharge control MOSFET M1.

The overcurrent condition returns to the normal

mode when the load is released and the impedance

between the BATT+ and BATT- terminals is 20M

Ω

or higher.

The AIC1821 is provided with the two overcurrent

detection

levels

(0.2V

and

1V)

and

the

two

to each overcurrent detection level.

Load Detection after Overcharge

The load detection function after overcharge is

implemented by detecting the CS pin voltage. Once

a load is connected to the battery pack after

overcharge, discharge current flows through the

parasitic diode of MOSFET M2 and there is a diode

voltage drop between CS and GND. Load is

determined to be connected to the pack if the CS

pin voltage is above load detection threshold

Power-Down after Overdischarge

When overdischarge occurs, the AIC1821 will go

into power-down mode, turning off all the timing

generation and detection circuitry to reduce the

quiescent current to 0.4

µA (V

time, the CS pin is pull-high to VCC through a high

resistance resistor.

Charge Detection after Overdischarge

When overdischarge occurs, the discharge control

MOSFET M1 turns off and discharging is inhibited.

However, charging is still permitted through the

parasitic

diode

of

M1.

Once

the

charger

is

connected

to

the

battery

pack,

the

AIC1821

immediately turns on all the timing generation and

detection circuitry. Charging is determined to be in

progress if the voltage between CS and GND is

l DESIGN GUIDE

Selection of External Control MOSFETs

Because

the

overcurrent

protection

voltage

is

preset,

the

threshold

current

for

overcurrent

detection is determined by the turn-on resistance of

the discharge control MOSFET M1. The turn-on

resistance of the external control MOSFETs can be

the overcurrent threshold current). For example, if

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

MOSFETs must be 33m

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,

C1

to

VCC

pin

is

recommended.

Protection at CS pin

R2 is used for latch-up protection when charger is