BQ24072T_2 Datasheet, PDF (26/36 Pages) Texas Instruments – 1.5A USB-Friendly Li-Ion Battery Charger and Power-Path Management IC

bq24072T
bq24075T, bq24079T
SLUS937A – DECEMBER 2009 – REVISED APRIL 2010
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APPLICATION INFORMATION
bq2407xT CHARGER DESIGN EXAMPLE
Refer to Typical Application Circuits for Schematics of the Design Example.
Supply voltage = 5V
Fast charge current of approximately 800 mA; ISET – pin 16
Input Current Limit =1.35A; ILIM – pin 12
Safety timer duration, Fast-Charge = 6.25 hours; TMR – pin 14
Battery Temperature Sense = 10kΩ; NTC (103AT-2), 0°C to 50°C Operation
Program the Fast Charge Current (ISET):
R_ISET = K_ISET / I_CHG
K_ISET = 890 AΩ; from the electrical characteristics table
R_ISET = 890AΩ / 0.8A = 1.1125 kΩ
Select the closest standard value, which for this case is 1.13kΩ. Connect this resistor between ISET (pin 16)
and V_SS.
Program the Input Current Limit (ILIM)
R_ILIM = K_ILIM / I_IN(MAX)
K_ILIM = 1600 AΩ; from the electrical characteristics table.
R_ISET = 1600AΩ / 1.35A = 1.19 kΩ
Select the closest standard value, which for this case is 1.18 kΩ. Connect this resistor between ILIM (pin 12)
and V_SS.
Program 6.25-hour Fast-Charge Safety Timer (TMR)
R_TMR = t_MAXCHG / (10 × K_TMR)
K_TMR = 45 s/kΩ from the electrical characteristics table.
R_TMR = (6.25 hr × 3600 s/hr) / (10 x 45 s/kΩ) = 46.8kΩ;
Select the closest standard value, which for this case is 46.4 kΩ. Connect this resistor between TMR (pin 2)
and V_SS.
TS Function
Using a 10kΩ NTC thermistor in the battery pack (103AT-2). Connect a resistor divider from V_IN to V_SS with the
thermistor and TS connected to the center tap (R6 and R7 in typical application circuits).
R_HOT = 4.086kΩ; 50°C threshold from NTC data sheet
R_COLD = 28.16 kΩ; 0°C threshold from NTC data sheet
V_COLD = 0.25 X V_IN = 0.25 X 5V = 1.25V
V_HOT = 0.125 X V_IN = 0.125 X 5V = 0.625V
R7
=
V_IN
´ RCOLD ´ RHOT
é₁
´ _{êëVCOLD}
1
- _VHOT
ù
ú
û
RHOT
´
é _VIN
êë_VHOT
-
1_úù
û
-
RCOLD
´
é _VIN
êë_VCOLD
- 1_úù
û
=
5´
28160
´
4086
´
é1
ê
ë
1.25
-
1
0.625
ù
ú
û
4086 ´
é5
ê
ë
0.625
-
1^ù
ú
û
-
28160 ´
é5
êë_1.25
-
1^ù
ú
û
=
8.236k W
(3)
R6 =
V_IN
V_COLD
-1
1
1
=
5
1.25 ^{- 1}
1
1
= 19.14kW
R7 ⁺ RCOLD 8250 ⁺ 28160
(4)
Since the calculated values for R6 is less than 100kΩ, a 100kΩ resistor for R8 must be used. Choose the closest
standard values, which for this case are R6=8.25kΩ and R7 = 19.1kΩ.
For applications that do not require the TS monitoring function, set R6 = 200kΩ and R7 = 49.9kΩ to set the TS
voltage at a valid level and maintain charging.
26
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