Datasheet

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Summary
bq29312A
SLUS629A – JANUARY 2005 – REVISED AUGUST 2005
APPLICATION INFORMATION (continued)
The three types of 0-V charge options available with the bq29312A are summarized in Table 4 .
Table 4. Charge Options
CHARGE MODE TYPE HOST CHARGE CAPABILITIES KEY APPLICATION CIRCUIT NOTES
PMS = GND
1) 0-V Charge FET Fast charge and precharge ZVCHG: Drives 0-V charge FET (ZVCHG-FET)
OD: Not used
PMS = PACK
2) Common FET Fast charge and precharge ZVCHG: Not used
OD: Not used
PMS = GND
3) Precharge FET Fast charge but no precharge function 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 as follows:
• 0-V Charge FET (1) vs Common FET (2)
When the charger has both precharge and charging functions, two types of circuit configuration are available.
1. 0-V Charge FET – The bq29312A 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 bq29312A and the host are not active. Therefore, they cannot protect
the cells and cannot update the battery status to the system.
– The bq29312A 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 bq29312A 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, a
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 overdischarged 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 overdischarge 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 ensure a supply voltage for the bq29312A during 0-V charging, the resistance of a series resister (R
PCHG
) must
be high enough. This may result in a high V
PACK
, and some chargers may detect it as an abnormal condition.
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