AAT3680
Lithium-Ion/Polymer
Linear Battery Charge Controller
5. Calculate the required threshold voltage to
deliver ICHARGE(REG)
Where ICHARGE(REG) is the desired typical charge cur-
rent during constant current charge mode. VP-VCSI
is the voltage across RSENSE, shown in the Electrical
Characteristic table as VCS. 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:
:
VGS = (VCS + VOL@DRV) - VP(MIN)
= (0.1V + 0.1V) - 4.5V
= -4.3V
where VGS is the available gate-to-source voltage
provided by the AAT3680, VCS is the voltage
across the sense resistor, VOL@DRV is the rated
low voltage at the DRV pin, and VP(MIN) is the
worst case input voltage (assuming 10% toler-
ance on the 5V supply). Choose a MOSFET
device with sufficiently low VGS(TH) so the device
(VCS)2
P =
RSENSE
(0.1)2
=
0.2
= 50mW
will conduct the desired ICHARGE(REG)
.
6. Calculate the worst case maximum allowable
RDS(ON) at worst case VGS voltage:
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.
(VP(MIN) - VCS(MAX) - VBAT(MAX)
)
RDS(ON)
=
=
ICHARGE(REG)
(4.5V - 0.11V - 4.242V)
0.75A
= 197mΩ
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 VTS1 and VTS2. Please see the equations
below for specifying resistors:
Select a P-channel power MOSFET with RDS(ON)
lower than 197mΩ at VGS = -4.3V.
In summary, select a P-channel MOSFET with ratings
VDS ≥ 12V, RθJA ≤ 79°C/W and RDS(ON) ≤ 197mΩ at
VGS = -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, RSENSE, programs the
charge current according to the following equation:
RT1 and RT2 for use with NTC Thermistor
5 ⋅ RTH ⋅ RTC
3 ⋅ (RTC - RTH)
RT1
RT2
=
=
5 ⋅ RTH ⋅ RTC
(2 ⋅ RTC) - (7 ⋅ RTH)
(VP - VCSI
)
RSENSE
=
ICHARGE(REG)
12
3680.2006.03.1.6
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
