SI9730 Datasheet, PDF (11/14 Pages) Vishay Siliconix – Dual-Cell Lithium Ion Battery Control IC

Si9730
Vishay Siliconix
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.
R_sense = V_ILIMIT/I_ILIMIT  28 mV/I_ILIMIT
Of course, the resistor must be rated to take the power
dissipated in it as well:
P_RSENSE = I_ILIMIT* V_ILIMIT  28 mV * I_ILIMIT
For example, suppose that the maximum current the battery
will see is 1.8 A. Then, I_LIMIT might be chosen to be 2 A. We
would then select a resistor of
R_SENSE = 28 mV/2 A = 14 mW.
The power dissipation in this resistor is
P_RSENSE = 28 mV * 2 A = 56 mW
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
ǒ_{1 oz. Copper}Ǔ
Document Number: 70658
S-40135—Rev. F, 16-Feb-04
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
1 W = (2 A)² * R_ON
or R_ON = 0.25 W; each MOSFET can be allotted half of this,
R_ON = 125 mW. Account must also be taken of the fact that
MOSFETs’ on-resistance is a function of temperature; a
conservative approach would give a discount of 1/3, R_ON =
125 mW * (2/3) = 80 mW per MOSFET.
A list of recommended MOSFETs, which Vishay Silicoix
supplies, follows.
www.vishay.com
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