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BQ24085 Datasheet, PDF (24/34 Pages) Texas Instruments – 750 mA SINGLE-CHIP Li-Ion/Li-Pol CHARGE MANAGEMENT IC WITH THERMAL REGULATION
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bq24085, bq24086
bq24087, bq24088
SLUS784B – DECEMBER 2007 – REVISED MAY 2008 .................................................................................................................................................... www.ti.com
APPLICATION INFORMATION
SELECTING INPUT AND OUTPUT CAPACITOR
In most applications, all that is needed is a high-frequency decoupling capacitor on the input power pin. A 1-µF
ceramic capacitor, placed in close proximity to the IN pin and GND pad, works fine. In some applications,
depending on the power supply characteristics and cable length, it may be necessary to increase the input filter
capacitor to avoid exceeding the IN pin maximum voltage rating during adapter hot plug events.
The bq2408x, at low charge currents, requires a small output capacitor for loop stability. A 0.1 µF ceramic
capacitor placed between the BAT and ISET pad is typically sufficient.
bq2408x CHARGER DESIGN EXAMPLE
Requirements
• Supply voltage = 5 V
• Safety timer duration of 5 hours for fast charge
• Fast charge current of approximately 400 mA
• Battery temp sense is not used
Calculations
Program the charge current for 400 mA:
R(ISET) = [V(SET) × K(SET) / I(OUT)]
from electrical characteristics table. . . V(SET) = 2.5 V
from electrical characteristics table. . . K(SET) = 182
R(ISET) = [2.5 V × 187 / 0.4 A] = 1137 Ω
Selecting the closest standard value, use a 1.13 kΩ resistor connected between ISET (pin 6) and ground.
Program 5-hour safety timer timeout:
R(TMR) = [T(CHG) / K(CHG)]
from the electrical characteristics table. . . K(CHG) = 0.1 hr / kΩ
K(TMR) = [5 hrs / (0.1 hr / kΩ)] = 50 kΩ
Selecting the closest standard value, use a 49.9 kΩ resistor connected between TMR (pin 2) and ground.
Disable the temp sense function:
A constant voltage between VTS1 and VTS2 on the TS input disables the temp sense function.
from electrical characteristics table. . . V(TS1) = 30% × VIN
from electrical characteristics table. . . V(TS2) = 61% × VIN
A constant voltage of 50% × Vin disables the temp sense function, so a divide-by-2 resistor divider
connected between Vin and ground can be used. Two 1-MΩ resistors keeps the power dissipated in this
divider to a minimum.
For a 0–45°C range with a Semitec 103AT thermistor, the thermistor values are 4912 at 45°C and 27.28k at 0°C.
RT1 (top resistor) and RT2 (bottom resistor) are calculated as follows:
RT2
=
2.5 RTCRTH
RTC - 3.5RTH
=
2.5 (27.28k) (4.912k)
27.28k - 3.5(4.912k)
= 33.2k
RT1 =
3
7 RTHRT2
(RTH + RT2 )
=
7 (4.921k) (33.2k)
3 (4.921k + 33.2k)
= 10k
(6)
24
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