Datasheet

7.14.1 Cell Undervoltage Detection and Recovery
Undervoltage
DetectComparator
ChargeCurrrent
DetectComparator
bq77PL900
Bat
Battery
Off On
Pack+
Pack
Pack–
S0349-01
Load
bq77PL900
www.ti.com ...................................................................................................................................................... SLUS844B – JUNE 2008 – REVISED JANUARY 2009
When any one of the cell voltages falls below V
UV
for a period of t
UV
, the bq77PL900 enters the undervoltage
mode. At this time, the DSG FET is turned OFF and XALERT driven low. Both V
UV
and t
UV
can be programmed
in the internal EEPROM.
Figure 12. Cell Undervoltage Protection Mode in Host Mode and Stand-Alone Mode (Attaching a Charger)
In Host-Control Mode
Cell undervoltage protection recovery conditions are when:
1. All cell voltages become higher than (V
UV
+ Δ V
UVH
), or
2. All cell voltages are higher than V
UV
AND a charger is connected between PACK+ and PACK – , noting that
PACK+ voltage must be higher than BAT due to the diode forward voltage.
The bq77PL900 monitors the voltage difference between the PACK+ and BAT pins. When a difference higher
than 0.4V (typ.) is seen, it is interpreted that a charger has been connected.
Figure 12 illustrates the circuit schematic in undervoltage protection mode.
In some applications, it is required not to turn OFF the DSG FET suddenly. In these cases, by setting UVLEVLE
[UVFET_DIS] = 1, only XALERT is driven low in response to entering an undervoltage condition. The host can
turn OFF the DSG FET to protect the undervoltage condition. When the bq77PL900 recovery condition is
satisfied, the host must send a sequence of firmware commands to the bq77PL900. The firmware command
sequence to turn ON the DSG FET is as follows:
1. The host must toggle LTCLR from 0 to 1 and back to 0.
2. Then the host must set the DSG ON bit to 1.
3. Then the host can read the status register to reset XALERT high.
Figure 13 and Figure 14 illustrate the timing chart of protection mode.
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