BQ24075T Datasheet, PDF (23/29 Pages) Texas Instruments – 1.5A USB-Friendly Li-Ion Battery Charger and Power-Path Management IC

www.ti.com
bq24075T
bq24079T
SLUS937 – DECEMBER 2009
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
é
´ê
1
ë^VCOLD
-¹
V_HOT
ù
ú
û
=
5´
28160
´
4086
´
é1
êë_1.25
-
1
0.625
ù
úû
= 8.236kW
é
RHOT ´ _ê
V_IN
ù
é
- 1_ú - RCOLD ´ _ê
V_IN
ù
- 1_ú
ë^VHOT û
ë^VCOLD û
4086 ´
é5
êë _0.625
-
ù
1_úû
-
28160 ´
é5
êë_1.25
-
ù
1_úû
(3)
V_{IN - 1}
R6 =
V_COLD
1
1
=
5
-1
1.25
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.
CHG and PGOOD LED Status: connect a 1.5kΩ resistor in series with a LED between OUT and CHG to
indicate charging status. Connect a 1.5kΩ resistor in series with a LED between OUT and PGOOD to indicate
when a valid input source is connected.
Processor Monitoring Status: connect a pullup resistor (on the order of 100 kΩ) between the processor power
rail and CHG and PGOOD
System ON/OFF (SYSOFF): Connect SYSOFF high to disconnect the battery from the system load. Connect
SYSOFF low for normal operation.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): bq24075T bq24079T
Submit Documentation Feedback
23