AAT3680 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller

5. Calculate the required threshold voltage to
deliver I_CHARGE(REG):
V_GS = (V_CS + V_OL@DRV) - V_P(MIN)
= (0.1V + 0.1V) - 4.5V
= - 4.3V
where V_GS is the available gate-to-source voltage
provided by the AAT3680, V_CS is the voltage
across the sense resistor, V_OL@DRV is the rated
low voltage at the DRV pin, and V_P(MIN) is the
worst case input voltage (assuming 10% toler-
ance on the 5V supply). Choose a MOSFET
device with sufficiently low V_GS(TH) so the device
will conduct the desired I_CHARGE(REG).
6. Calculate the worst case maximum allowable
R_DS(ON) at worst case V_GS voltage:
R_DS(ON) =
(V_P(MIN)
- V_CS(MAX) - V_BAT(MAX))
I_CHARGE(REG)
= ^{(4.5V - 0.11V - 4.242V)}
0.75A
= 197mΩ
Select a P-channel power MOSFET with R_DS(ON)
lower than 197mΩ at V_GS = -4.3V.
In summary, select a P-channel MOSFET with ratings
V_DS ≥ 12V, R_θJA ≤ 79°C/W and R_DS(ON) ≤ 197mΩ at
V_GS = -4.3V in a SOT223 (or better thermal) package.
Choosing a Sense Resistor
The charging rate recommended by lithium-
ion/polymer cell vendors is normally 1C, with a 2C
absolute maximum rating. Charging at the highest
recommended rate offers the advantage of short-
ened charging time without decreasing the battery
lifespan. This means that the suggested fast
charge rate for a 500mAH battery pack is 500mA.
The current sense resistor, R_SENSE, programs the
charge current according to the following equation:
R_SENSE
=
(V_P - V_CSI)
I_CHARGE(REG)
AAT3680
Lithium-Ion/Polymer
Linear Battery Charge Controller
Where I_CHARGE(REG) is the desired typical charge cur-
rent during constant current charge mode. V_P-V_CSI
is the voltage across R_SENSE, shown in the Electrical
Characteristic table as V_CS. To program a nominal
500mA charge current during fast-charge, a 200mΩ
value resistor should be selected. Calculate the
worst case power dissipated in the sense resistor
according to the following equation:
P
=
(V_CS)²
R_SENSE
= ^(0.1)2
0.2
= 50mW
A 500mW LRC type sense resistor from IRC is
adequate for this purpose. Higher value sense
resistors can be used, decreasing the power dissi-
pated in the sense resistor and pass transistor.
The drawback of higher value sense resistors is
that the charge cycle time is increased, so tradeoffs
should be considered when optimizing the design.
Thermistor
The AAT3680 checks battery temperature before
starting the charge cycle, as well as during all
stages of charging. This is accomplished by mon-
itoring the voltage at the TS pin. Either a negative
temperature coefficient thermistor (NTC) or posi-
tive temperature coefficient thermistor (PTC) can
be used because the AAT3680 checks to see that
the voltage at TS is within a voltage window bound-
ed by V_TS1 and V_TS2. Please see the equations
below for specifying resistors:
R_T1 and R_T2 for use with NTC Thermistor
R_T1 =
5 ⋅ R_TH ⋅ R_TC
3 ⋅ (R_TC - R_TH)
R_T2 =
5 ⋅ R_TH ⋅ R_TC
(2 ⋅ R_TC) - (7 ⋅ R_TH)
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