Datasheet

PACK
CHG
REG2
3.3V
Charge Current
0-VandPrechgCurrent
Quick-ChargeCurrent
BatteryVoltage
DSG
0 A
0V
L
CHG-FET =ON
DSG-FET =ONbyuC
0V
0V
uP: Active
GPOD
0V
ChargeCV
CHG-FET =OFF
PCHGFET =ON
OFFOFF
L
OVChargeand
PrechargeMode
Quick-
Charge
Mode
T0382-01
bq77PL900
SLUS844B – JUNE 2008 – REVISED JANUARY 2009 ...................................................................................................................................................... www.ti.com
During the 0-V/precharge, the CHG FET is turned OFF and the PCHG FET is turned ON. When the host
controller detects that all the cell voltages have reached the fast-charge threshold, it then turns ON the CHG FET
and turns OFF the PCHG FET. The signal timing is shown in Figure 31 .
The CHG, DSG and PCHG FETs are turned OFF when the charger is connected. Then, the charger applies its
maximum output voltage (constant-voltage-mode output voltage) to the PACK+ pin. Then, the bq77PL900 3.3-V
regulator becomes active and supplies power to the host controller. As the host controller starts up, it turns on
the GPOD output and the 0-V/precharge current begins to flow.
In this configuration, attention is needed to control high power consumption at the PCHG FET and the series
resistor (R
P
). The highest power is consumed at 0-V cell voltage (highest voltage between PACK+ and BAT pins)
and it results in highest heat generation. For example, the power consumption in 10 series batteries with 42-V
fast charge voltage and 1-k Ω R
P
is expressed as follows.
I
OV/PCHG
= (42 V – 0 V) /1 k Ω = 42 mA
(Power consumption at R
P
) = 42 V × 42 mA = 1.6 W
It is recommended to combine the resistor (R
P
) and the thermistor to reduce the consumption. Once the cell
voltage reaches the fast-charge threshold, the host controller turns ON the CHG and DSG FETs and also turns
OFF the PCHG FET.
Figure 31. Signal Timing of Pins During 0-V Charging and Precharging (Precharge FET) With
Host-Control Mode
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