BQ29312_15 Datasheet, PDF (31/39 Pages) Texas Instruments – FOUR CELL LITHIUM-ION

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bq29312
SLUS546E – MARCH 2003 – REVISED MARCH 2005
APPLICATION INFORMATION (continued)
Summary
The three types of 0-V charge options available with the bq29312 are summarized in Table 4.
CHARGE MODE TYPE
1) 0-V Charge FET
2) Common FET
3) Precharge FET
Table 4. Charge Options
HOST CHARGE CAPABILITIES
Fast charge and precharge
Fast charge and precharge
Fast charge but no precharge function
KEY APPLICATION CIRCUIT NOTES
PMS = GND
ZVCHG: Drives 0-V charge FET (ZVCHG-FET)
OD: Not used
PMS = PACK
ZVCHG: Not used
OD: Not used
PMS = GND
ZVCHG: Not used
OD: Drives the precharge FET (PCHG-FET)
There a number of tradeoffs between the various 0-V charge modes which are discussed below.
• 0-V Charge FET (1) vs Common FET (2)
When the charger has both of precharge and charging functions, there are two types of circuit configuration
available.
1. 0-V Charge FET – The bq29312 is active even during precharge. Therefore, the host can update the battery
status to the system and protect the battery pack by detecting abnormal conditions.
– A high voltage is applied on the 0-V charge FET at 0-V cell voltage. In order to avoid excessive heat
generation the 0-V charge current must be limited.
2. Common FET – During 0-V charge the bq29312 and the host are not active. Therefore, they cannot protect
the cells and cannot update the battery status to the system.
– The bq29312 can tolerate high 0-V charge current as heat generation is not excessive.
– A dedicated FET for the 0-V charge is not required.
• 0-V Charge FET (1) vs Precharge FET (3)
The current paths of the 0-V charge FET (1) and Precharge FET (3) modes are the same. If the 0-V charge FET
(1) mode is used with chargers without precharge function, the bq29312 consumes extra current of up to 1 mA in
order to turn ON the ZVCHG output.
1. If the charger has a precharge function - ZVCHG-FET is turned ON only during 0-V charging. In this case, 1
mA increase is not a concern because the charger is connected during the 0-V charging period.
2. If the charger does not have precharge function - The ZVCHG-FET must be turned ON during 0-V charging
and also precharging. When the battery reaches an over discharged state, it must turn OFF DSG-FET and
CHG-FET and turn ON ZVCHG-FET. The reason for this is the battery must keep the 0-V charge path while
waiting for a charger to be connected to limit the current.
– Consuming 1 mA, while waiting for a charger to be connected in over discharge state, is significant if
compared to current consumption of other modes.
• Precharge FET (3)
If the precharge FET (3) mode is used with a charger with precharge function, care must be taken as limiting the
0-V charge current with resistance may cause some issues. The charger may start fast charge immediately, or
detect an abnormal condition.
When the charger is connected, the charger may raise the output voltage to force the precharge current. In order
to assure a supply voltage for the bq29312 during 0-V charging, the resistance of a series resister (R_PCHG) must
be high enough. This may result in a very high V_PACK, and some chargers may detect it as an abnormal
condition.
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