Datasheet

V
IN
+
-
C1
C2
PMID
VBUS
SW
L1
PGND
bq2415xA
C4
Isns
Rsns
C3
Ichg
BAT
+
Isys
System
Load
bq24155
SLUS942 –FEBRUARY 2010 www.ti.com
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.
• For instance, if the battery pack voltage is too low, highly depleted, or 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 very similar to first one; the difference is that the system load is connected before the sense
resistor, as shown in Figure 21.
Figure 21. System Load Before Sensing Resistor
The advantages of system load before sensing resistor to system load after sensing resistor:
• The charger controller is based only on the current goes through the current-sense resistor. So, the constant
current fast charge and termination functions work well, and are not affected by the system load. This is the
major advantage of it.
• A depleted battery pack can be connected to the charger without the risk of the safety timer expiration caused
by high system load.
• 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 run off of the battery via the sense resistor.
Design considerations and potential issues:
• 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 applicable if the system requires a high current.
• Efficiency declines when discharging through the sense resistor to the system.
• No thermal regulation. Therefore, system design should ensure the maximum junction temperature of the IC
is below 125°C during normal operation.
DESIGN EXAMPLE FOR TYPICAL APPLICATION CIRCUITS
Systems Design Specifications:
• VBUS = 5 V
• V
(BAT)
= 4.2 V (1-Cell)
• I
(charge)
= 1.25 A
• Inductor ripple current = 30% of fast charge current
1. Determine the inductor value (L
OUT
) for the specified charge current ripple:
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