BQ29312A Datasheet, PDF (29/35 Pages) Texas Instruments – TWO-CELL, THREE-CELL, AND FOUR-CELL LITHIUM-ION OR LITHIUM-POLYMER BATTERY PROTECTION AFE

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bq29312A
SLUS629A – JANUARY 2005 – REVISED AUGUST 2005
APPLICATION INFORMATION (continued)
In this configuration, attention must be paid to high power consumption in the PCHG-FET and the series resistor
R_(PCHG). The highest power is consumed when V_BAT = 0 V, where it is the highest differential between the PACK
and BAT pins. For example, the power consumption in 4-series cells with 17.4 V fast charge voltage and R_(PCHG)
= 188 Ω is expressed below.
• I_PCHG = (17.4 V – 0.0 V)/188 Ω = 92.6 mA
• 17.4 V × 92.6 mA = 1.61 W
An optional solution is to combine a thermistor with a resistor to create R_(PCHG); therefore, as temperature
increases, the current reduces.
Once the lowest cell voltage reaches the fast charge level (typ 3.0 V per cell), the host controller turns ON
CHG-FET and DSG-FET, and turns OFF PCHG-FET.
It is also appropriate to turn on DSG-FET during precharge in order to supply precharge current efficiently, as
shown in Figure 15.
V_(PACK)
REG
Host : Active
Charge CV
0V
3.3 V
0V
OD
OFF _{PCHG FET = ON}
OFF
0V
CHG
DSG
Battery
Voltage
CHG FET = OFF
OFF
DSG FET = ON
”L” (1 V)
”L” (1 V)
Charge
Current
Charge
0V
Fast Charge Current
0 V and Precharge Current
0A
Charge Mode
0 V and
Precharge Mode
Fast Charge
Mode
Figure 15. Signal Timing of Pins During 0-V Charging and Precharging (Precharge FET)
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