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Document Number: 71321

S-60869-Rev. B, 22-May-06

Vishay Siliconix

Si9731

DETAIL OPERATIONAL DESCRIPTION

Si9731 is a chemistry independent battery charger designed

for use with a system processor. For example, Si9731 can be

integrated within a cellular phone whereby the fast charge

and trickle (slow) charge modes can be software controlled

by the DSP. The device is designed to charge 1-cell Li-ion or

1-cell to 3-cell NiCd/NiMH batteries. A regulated or unregu-

lated external dc power source such as a wall adapter rated

input pin. Note that a typical low cost wall adaptor is com-

prised of a transformer, bridge rectifier and a reservoir

capacitor. The wall adaptor’s output voltage decreases lin-

early with increase in output current. When Si9731 is fast

charging the battery, the wall adaptor’s output voltage tracks

the battery voltage plus the voltage drop across Q1 (charging

features of Si9731 are described below.

Since the under voltage lock out (UVLO) point of Si9731 is

2.6 V (typical), it is essential to keep the charger voltage

above this level under all conditions, especially for fast

charging of single cell NiCd/NiMH. One simple solution is

which creates extra voltage drop to elevate the charger volt-

age. The value of the resistor is affected by the output V-I

characteristic of the ac charger.

Trickle Charge

The charge path is via N-Channel MOSFETs Q1 or Q2 (see

applications circuit of Figure 1). Si9731 defaults to trickle

(slow) charge mode if the battery voltage is too low to power

the main processor. With the main processor unable to drive

the MAINCHARGEEN pin as well as the TRICKLECHAR-

GEEN pins, Q1 is turned "OFF" preventing fast charging.

Meanwhile N-Channel MOSFET Q2 turns "ON" and estab-

lishes a trickle charge path from the external power source

voltage charges up to minimum battery operating voltage

3.4 V,

the

internal

latch

is

triggered

and

the

CHARGERPOWER_ON output changes state to wake up

the processor. The processor is now able to disable trickle

charge mode by taking the TRICKLECHARGEEN pin high

while taking control of fast charging via the MAINCHAR-

GEEN pin.

Fast Charge

Fast charging is accomplished by the low "ON" resistance

MOSFET, Q1. The application microprocessor is able to

"Pulse Charge" the battery via the MAINCHARGEREN con-

trol input of Si9731. The processor monitors the battery volt-

age via the system A/D converter and varies the pulse

charging duty cycle accordingly to maintain fast charging.

Note that even though charging current may be sufficiently

high, pulse charging with short "ON" time and long "OFF"

time ensures that heat generation due to thermal heating is

reduced.

In the case of NiCd or NiMH batteries, one of several charge

termination schemes may be used to terminate charge. For

example, the processor may disable fast charging by sens-

ing

ΔV or dV/dt at the V

perature differential

ΔT of the battery. Following fast charge,

trickle charge may be enabled to "top off" the battery.

When charging a 1-cell Li-ion battery, fast charging will oper-

ate in two modes, constant current mode followed by con-

stant voltage mode. In the constant current mode, a

discharged Li-ion battery is charged with constant current

available from the external dc source. The MOSFET pass

transistor (Q1) may be pulsed "ON" and "OFF" at varying

duty cycle by the control signal present at the MAINCHAR-

GEEN input pin. Once the battery voltage reaches it’s termi-

nation voltage of 4.1 V or 4.2 V (depending on the connection

age" charging mode by taking the CVMODE pin high. Taking

CVMODE pin high disables trickle charging and enables the

internal battery voltage divider by turning ON Q5. Then the

internal precision 1.3 V bandgap reference voltage (see Fig-

ure 1). The output of the error amplifier drives the pass tran-

voltage. This operation is same as a linear regulator.

True Load Disconnect

Both the fast charge FET (Q1 in Figure 1) and trickle charge

FET (Q2 in Figure 1) incorporate a floating body diode. In

their "OFF" state both FETs block current bidirectionally.

Note that because of the reverse blocking switches, a Schot-

is not required.