BQ24085_17 Datasheet, PDF (26/42 Pages) Texas Instruments – 750-mA Single-Chip Li-Ion and Li-Pol Charge Management IC With Thermal Regulation

bq24085, bq24086, bq24087, bq24088
SLUS784E – DECEMBER 2007 – REVISED DECEMBER 2015
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Typical Applications (continued)
9.2.1.2.2 Using Adapters With Large Output Voltage Ripple
Some low cost adapters implement a half rectifier topology, which causes the adapter output voltage to fall below
the battery voltage during part of the cycle. To enable operation with low cost adapters under those conditions
the bq2408x family keeps the charger on for at least 25 msec (typical) after the input power puts the part in sleep
mode. This feature enables use of external low cost adapters using 50-Hz networks.
The backgate control circuit prevents any reverse current flowing from the battery to the adapter terminal during
the charger off delay time.
NOTE
The PG pin is not deglitched, and it indicates input power loss immediately after the input
voltage falls below the output voltage. If the input source frequently drops below the output
voltage and recovers, a small capacitor can be used from PG to VSS to prevent PG
flashing events.
9.2.1.2.3 Calculations
Program the charge current for 400 mA:
R_(ISET) = [V_(SET) × K_(SET) / I_(OUT)]
where
• from the Electrical Characteristics table. . . V_(SET) = 2.5 V
• from the Electrical Characteristics table. . . K_(SET) = 182
(8)
R_(ISET) = [2.5 V × 187 / 0.4 A] = 1137 Ω
(9)
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)]
where
• from the Electrical Characteristics table. . . K_(CHG) = 0.1 hr / kΩ
(10)
K_(TMR) = [5 hrs / (0.1 hr / kΩ)] = 50 kΩ
(11)
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 V_TS1 and V_TS2 on the TS input disables the temp sense function.
from the Electrical Characteristics table. . . V_(TS1) = 30% × V_IN
from the Electrical Characteristics table. . . V_(TS2) = 61% × V_IN
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 to 45°C range with a Semitec 103AT thermistor, the thermistor values are 4912 at 45°C and 27.28 k at
0°C. RT1 (top resistor) and RT2 (bottom resistor) are calculated in Equation 12:
R_T2
=
2.5 R_TCR_TH
R_TC - 3.5R_TH
=
2.5 (27.28k) (4.912k)
27.28k - 3.5(4.912k)
= 33.2k
R_T1 =
3
7 R_THR_T2
(R_TH + R_T2 )
=
7 (4.921k) (33.2k)
3 (4.921k + 33.2k)
= 10k
(12)
26
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