BQ24140_14 Datasheet, PDF (32/39 Pages) Texas Instruments – Integrated Dual-Input Switch-Mode One-Cell Li-Ion Charger with Full USB Compliance and USB-OTG Support

bq24140
SLUSAO5 – OCTOBER 2011
www.ti.com
depending on the current left to charge the battery pack. Under extreme conditions, the system current is
close to the short circuit current levels and the battery may not reach the fast-charge region in a timely
manner. As a result, the safety timers flag the battery pack as defective, terminating the charging process.
Because the safety timer cannot be disabled, the inserted battery pack must not be depleted to make the
application possible.
6. If the battery pack voltage is too low, highly depleted, totally dead or even shorted, the system voltage is
clamped by the battery and it cannot operate even if the input power is on.
System Load Before Sensing Resistor
The second circuit is similar to first one; the difference is that the system load is connected before the sense
resistor, as shown in Figure 22.
VBUS
+ ^VIN
-
C1
bq2414x
PMID
C2
SW
L1
Isns
Rsns
C4
C3
PGND
Isys
Ichg
+
BAT
System
Load
Figure 22. System Load Before Sensing Resistor
The advantages of system load before sensing resistor to system load after sensing resistor:
1. The charger controller is based only on the current going through the current-sense resistor. So, the constant
current fast charge and termination functions operate without being affected by the system load. This is the
major advantage of having the system load connected before the sense resistor.
2. A depleted battery pack can be connected to the charger without the risk of the safety timer expiration
caused by high system load.
3. The charger can disable termination and keep the converter running to keep battery fully charged; or let the
switcher terminate when the battery is full and then allow the system to run off of the battery through the
sense resistor.
Design considerations and potential issues:
1. The total current is limited by the IC input current limit, or peak current protection, but not the charge current
setting. The charge current does not drop when the system current load increases until the input current limit
is reached. This solution is not recommended if the system requires a high current.
2. Efficiency declines when discharging through the sense resistor to the system.
3. No thermal regulation. Therefore, the system design should ensure the maximum junction temperature of the
IC is below 125°C during normal operation.
32
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