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AAT3680 Datasheet, PDF (11/18 Pages) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller
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AAT3680
Lithium-Ion Linear Battery Charge Controller
PD = (VP(MAX) - VCS - VD - VMIN) · ICHARGE(REG)
PD = (5.5V - 0.1V - 0.4V - 3.1V) · 750mA
PD = 1.4W
2. The next step is to determine which size package
is needed to keep the junction temperature below its
rated value, TJ(MAX). Using this value, and the maxi-
mum ambient temperature inside the system TA(MAX),
calculate the thermal resistance RθJA required:
RθJA
=
(TJ(MAX) - TA(MAX))
PD
RθJA
=
(150 -
1.4
40)
RθJA = 79°C/W
It is recommended to choose a package with a lower
RθJA than the number calculated above. A SOT223
package would be an acceptable choice, as it has an
RθJA of 62.5°C/W when mounted to a PCB with ade-
quately sized copper pad soldered to the heat tab.
3. Choose a drain-source (VDS) voltage rating
greater than the input voltage. In this example, VP
is 5.0V, so a 12V device is acceptable.
4. Choose a MOSFET with a drain current rating at
least 50% greater than the programmed
ICHARGE(REG) value. In this example we would
select a device with at least 1.125A rating.
5. Calculate the required threshold voltage to deliv-
er ICHARGE(REG):
VGS = (VCS + VOL@DRV) - VP(MIN)
VGS = (0.1V + 0.1V) - 4.5V
VGS = -4.3V
where VGS is the available gate to source voltage pro-
vided 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% tolerance on the 5V supply). Choose
a MOSFET device with sufficiently low VGS(TH) so the
device will conduct the desired ICHARGE(REG).
6. Calculate the worst case maximum allowable
RDS(ON) at worst case VGS voltage:
RDS(ON)
=
(VP(MIN) - VCS(MAX) -
ICHARGE(REG)
VBAT(MAX))
RDS(ON)
=
(4.5V
-
0.11V -
0.75A
4.242V)
RDS(ON) = 197mΩ
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.
Q1
RSENSE
RFD10P03L
0.2Ω
VP
R4
100k
R1
1k
BATT+
C2
10µF
DRV
T2X
VP
BATT-
CSI
BAT
RT1
AAT3680
VP
TS
C1
4.7µF
VSS
STAT
D1
TEMP
Battery
RT2
Pack
R2
1k
Figure 5: Typical Applications Schematic Using P-Channel Power MOSFET
3680.2003.4.0.91
11
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