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SAA1502ATS

Safety IC for Li-ion

Philips Electronics 

Philips Semiconductors

Safety IC for Li-ion

Preliminary specification

SAA1502ATS

PINNING

SYMBOL

n.c.

VCC

Cext

LF

VSS2

Vd

VM

VSS1

ST

PIN

1, 16

2

3

4, 13

5, 6

7, 8, 9, 10

11, 12

14

15

DESCRIPTION

not connected

positive battery sense input

external delay capacitor

leadframe connection

control circuit

negative battery input and

power ground

drain voltage of SW1 and

SW2

negative sense input

ground for the control circuit

status output

handbook, halfpage

n.c. 1

16 n.c.

VCC 2

15 ST

Cext 3

14 VSS1

LF 4

13 LF

SAA1502ATS

VSS2 5

12 VM

VSS2 6

11 VM

Vd 7

10 Vd

Vd 8

9 Vd

MGM308

Fig.2 Pin configuration.

FUNCTIONAL DESCRIPTION

Figure 3 gives the connection diagram of a Li-ion battery

pack. All that is contained within the solid perimeter is the

safety IC SAA1502ATS. It is a Multichip Module (MCM),

containing two separate but interconnected chips, one is

the control IC and the other contains two vertical power

NMOS transistors which are connected in anti series. Both

transistors have their backgate connected to their source,

resulting in two backgate diodes in anti series.

The basic function of the SAA1502ATS is to protect a

single Li-ion cell against overcharge and overdischarge for

reasons of lifetime and safety. The voltage across the cell

terminals is monitored continuously and compared to an

accurate internal reference voltage. For battery voltages

between 3.6 and 4.18 V and a (dis)charge current below

the current protection level, the safety unit is in normal

operating mode (see Fig.4). In this state both switches are

driven with an elevated supply voltage (with a charge

pump) which guarantees a low resistance in the main

current path. This is important for fully utilizing the high

energy density of Li-ion battery technology.

The discharge PowerMOS transistor SW2 is disabled to

block further discharge, when the battery is discharged

below 2.3 V. The battery voltage will increase stepwise,

because of the sudden disconnection of the load. The unit

will not re-enter the normal operation mode at this event

unless the battery voltage exceeds the voltage restarting

level of 3.6 V.

When no charger is present in the discharge inhibit mode,

the system will switch to the Power-down mode.

The current consumption of the unit (SAA1502ATS and

the Li-ion cell) is then reduced to a typical value of 0.1 µA

for minimizing the discharge of the battery pack.

Connecting a charger in the Power-down mode is detected

by a voltage difference between VCC and VM of more than

3 V. The system will then return to the discharge inhibit

mode. After a short transition phase characterized by

conduction of the backgate diode between the drain and

source leads of SW2, the system goes to the normal

operating mode and SW2 is powered again.

At zero voltage start-up, the system will start at the reset

mode. A special circuit keeps the charge transistor SW1

on as much as possible.

When the battery is charged to a voltage level of 4.18 V it

will enter the charge inhibit mode and the charge

PowerMOS transistor SW1 is switched off, disabling

charging. Connecting a load is then detected by the

reversal of the voltage across SW1 (Idch > 1.5 mA) and will

immediately reactivate SW1, entering the discharge

enable state.

1998 Jan 15

4

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