Si9730

Vishay Siliconix

www.vishay.com

8

Document Number: 70658

S-40135—Rev. F, 16-Feb-04

To avoid this, if one or both cells becomes over-discharged

the switch to prevent further discharging, and goes into a

shutdown mode in which it draws minute power from the

Over-Discharged Cell(s) Charging

If one or both cells is over-discharged, and a charger is

present, charging can begin, and so the Si9730 closes the

switch. However, removal of the charger in this condition could

potentially damage the battery if the removal is not recognized

and the cells are discharged. Since the voltage drop across

the switch is small, the Si9730 actually cycles the switch at a

7/8 duty cycle; during the 1/8 time when the switch is open, the

IC checks that the charger is still present.

Once both cells are back into the normal operating range,

normal charging resumes.

Undervoltage Charging

there is insufficient voltage for the Si9730 to properly monitor

fault conditions. Of course, the switch is already open, since

Si9730 recovers and goes into an undervoltage mode. (A

mode, the switch is on at a 1/8 duty cycle, to limit the power

dissipation across the switch, and, again, to detect the

continuing presence of the charger.

continues in the over-discharged state.

Output Short

If too much current is drawn from the battery due to a load

short, the switch must be opened quickly to prevent damage

to the battery. The Si9730 monitors the load current by looking

at the voltage across an external sense resistor (see Figure 8).

mV, the switch is opened. The Si9730 leaves the switch open

until the load is completely removed.

Of course, the IC must have some way of detecting that the

load has been removed. For this purpose, a small current

once the short is detected and the switch is turned off. The

continue to flow until the battery voltage becomes

overdischarged. Once the short is removed, the IC is allowed

to turn the switch back on.

The current limit threshold has a temperature coefficient of

0.18%/_C. This can partially compensate for a copper circuit

board trace being used as the sense resistor.

Open Center Tap

An open center tap is a mechanical failure of the battery pack

point of the two-cell battery. If this connection is open, the IC

opens the switch, as it cannot measure the cell voltages in this

condition. The switch is left open until connection is

re-established.

If the battery is under-voltaged and the

charger is present in this case, the battery is allowed to charge

even with the center tap open. In this state, batteries are

almost impossible to damage by 1/8 duty cycle charging.

Once the battery voltage reaches the over-discharged

voltage, the switch is turned off.

State Transition Table

The number of different states of the Si9730 can seem

overwhelming at first. This state transition table will help to

organize thinking about the different operational conditions of

the IC, by listing each possible transition from one condition to

another.

Reading the table is straightforward. There are two cells

constituting the battery, one with its positive terminal

referred to as the high cell (see Figure 8); and one cell with its

in one of three voltages:

D Normal Operation (NO), where

or