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SAA1502ATS

Safety IC for Li-ion

Philips Electronics 

Philips Semiconductors

Safety IC for Li-ion

Preliminary specification

SAA1502ATS

A short time is needed to charge the gate of SW1. During

this time the backgate diode between drain and source of

SW1 conducts.

The system will remain in the discharge enable mode

unless:

• The battery voltage drops below 3.95 V, which results in

re-entering normal operation. This transition is not

externally noticeable, because both switches remain low

ohmic.

• A charger is connected which will immediately

deactivate SW1 if Ich > 280 mA. As an additional safety

precaution also VCC > 4.18 V yields the same reaction,

because a small current of a charger may be undetected

with the condition of Ich > 280 mA, leading to

overcharging the Li-ion cell.

Current protection will deactivate both switches and is

detected by a voltage drop or rise of VVM when both

switches are activated. A release of this state can only be

achieved by removing the load (or charger).

The temperature protection overrules all other states and

yields deactivation of both switches. This situation is

activated at a junction temperature of 130 °C and released

at a junction temperature of 60 °C. The temperature

protection is followed by a return to its preceding mode.

Normal mode

In case of correct temperature, battery voltage and

(dis)charge current, the system will be in the normal

operation mode. Both the charge and discharge output will

be active high, so both switches are conducting

(SW1 = SW2 = 1).

Discharge inhibit mode

If the battery drops below 2.3 V, the system will switch to

the discharge inhibit mode. In this mode only charging of

the battery is allowed (SW1 = 1, SW2 = 0). The system will

return to the normal operation mode as soon as the battery

voltage will exceed 3.6 V, or by detection of a charge

current.

The overdischarge detection of 2.3 V has a delay of 40 ms

typical. The voltage detection level 3.6 V has a delay of

50 ms typical. Because a charge current is necessary to

increase the battery voltage, the system will normally

switch to the normal operation mode at VCC = 2.3 V by

detecting a charge current. But if the charge current is too

small to detect, the 3.6 V detection is a backup.

To prevent an instable situation between the normal

operation and the discharge inhibit mode when the battery

is charged at a voltage below 2.3 V, an extra condition of

Vbat > 2.3 V is included going from the discharge inhibit to

the normal operation mode.

Power-down mode

At low battery voltage the supply current is reduced to

100 nA for minimizing the discharge of the battery by the

SAA1502ATS.

At the Power-down mode all analog circuitry, except

circuitry for detecting a charger present (VCC − VVM > 3 V),

is disabled. The Power-down mode is entered when the

system is in the discharge inhibit mode and no charger is

present. The discharge inhibit mode will be entered again

as soon as a charger is connected.

The detection of a charger is accomplished by detecting a

voltage difference of 3 V between VCC and VM. In this

mode the voltage difference (see Fig.5) is:

VCC − VVM = Vbat − VR1 + Vj(DO) + Vds(CO) ≈ Vbat + 0.6 V.

So in the application the battery has to be charged in the

Power-down mode until such a voltage that

VCC − VVM = 3 V.

Reset mode

If the battery voltage is below 1.9 V, the system will be in

the reset mode. Because in this mode the charge pump is

disabled and battery charging should be possible, the

charge FET is switched on with a reduced Vgs voltage.

As soon as the battery voltage exceeds 2.25 V the system

will switch to the discharge inhibit mode and the charge

pump will be activated again.

Zero voltage start-up

The system has to be able to charge the battery at ‘0 Volt’.

This means that when connecting a charger in case of a

complete empty battery, the charge FET has to be active.

In the reset mode the charge FET (SW1) is connected via

a diode to VCC, so that the charge FET will be active when

the VVM voltage is negative. The discharge inhibit mode

will be entered as soon as a battery voltage exceeds

2.25 V.

Charge inhibit mode

If the battery voltage exceeds 4.18 V, the charge inhibit

mode will be entered. At this mode the battery can only be

discharged (SW1 = 0, SW2 = 1). The overcharge

detection has a delay of 40 ms. This delay can be

increased by an external capacitor. The delay time is then

1998 Jan 15

5

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