BQ29312_15 Datasheet, PDF (30/39 Pages) Texas Instruments – FOUR CELL LITHIUM-ION

bq29312
SLUS546E – MARCH 2003 – REVISED MARCH 2005
www.ti.com
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)
30