Si9730

Vishay Siliconix

Document Number: 70658

S-40135—Rev. F, 16-Feb-04

www.vishay.com

11

Output Capacitor

Depending on the MOSFET selected, the Si9730 can open the

switch quite rapidly, in a matter of a few microseconds.

However, the various monitoring operations take 10-100 times

longer than this, and the basic period of the Si9730’s oscillator

is 4 msec. In order to prevent false readings by the Si9730, it

is necessary to attach a capacitor across the output of the

battery charger/load (this is not in parallel with the battery,

because of the switch). A 10-mF capacitor is recommended for

this purpose; see Figure 8.

Selecting a Current Sense Resistor

The current sense resistor should be selected based on the

maximum current the battery can source or charge at; above

this current, the Si9730 will open the switch, disconnecting the

battery from its load or charger.

Of course, the resistor must be rated to take the power

dissipated in it as well:

For example, suppose that the maximum current the battery

would then select a resistor of

The power dissipation in this resistor is

and so a 100mW surface mount resistor would be suitable.

Another possibility is to use a thin copper trace as the sense

resistor. The copper has a temperature coefficient of

0.39%/_C, but this is partially compensated for by the

temperature coefficient of the current limit comparator in the

Si9730, which is 0.18%/_C. A simple formula for selecting a

trace to act as a current sensor is:

R + 0.5 mW

length

width

1 oz. Copper

For example, to get a 14-mW. resistor, we need length/width

= 28; with a trace width of 0.01”, the length of the trace should

be 0.28”.

MOSFET Selection

Two MOSFETs in series, with their sources and gates

connected together, are used as the switch. This prevents

current from flowing in either direction when the gate is low; if

only one MOSFET were used, the body diode could conduct

current in the opposing direction.

LITTLE FOOT MOSFETs are recommended for this

application, because of their size, performance and cost

benefits. SO-8 and TSSOP-8 MOSFETs allow for space

efficient designs with performance equal to or better than their

DPAK and TO-220 predecessors. Further, their availability

from multiple sources permits a cost effective solution.

There are two important parameters to consider in MOSFET

selection: gate threshold voltage; and on-resistance, which

determines power dissipation.

Even when the DCO pin of the Si9730 is low, the specification

allows its value to be as high as 0.4 V. If this voltage were close

to the gate threshold voltage, leakage current through the

MOSFETs could be hundreds of microamps, which would

result in the battery quickly becoming discharged. To ensure

that leakage is minimized, n-channel MOSFETs with a

minimum gate threshold voltage of 0.8 V should be chosen.

On resistance of the MOSFETs needs to be selected to limit

power dissipation into the MOSFETs’ package. For example,

a dual MOSFET SO-8 package is rated at 2 W, and a dual

MOSFET TSSOP-8 package is rated at 1 W (both at 25_C; if

the ambient temperature is higher, the allowable power

dissipation in these packages is less). For example, if the

maximum current is 2 A, and a dual MOSFET SO-8 package

is being used, the maximum on-resistance of the two

MOSFETs in series must not exceed

MOSFETs’ on-resistance is a function of temperature; a

125 mW * (2/3) = 80 mW per MOSFET.

A list of recommended MOSFETs, which Vishay Silicoix

supplies, follows.