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Charge CV
0 V
Host : Active
3.3 V
0 V
OFF
CHG FET = OFF
OFF
0 V
DSG FET = ON
”L” (1 V)
OFF
”L” (1 V)
0 V
Fast Charge Current
0 V and Precharge Current
0 A
0 V and
Precharge Mode
Fast Charge
Mode
V
(PACK)
REG
CHG
DSG
Battery
Voltage
Charge
Current
Charge
Charge Mode
OD
PCHG FET = ON
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 .
Figure 15. Signal Timing of Pins During 0-V Charging and Precharging (Precharge FET)
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