BQ24180_1 Datasheet, PDF (32/41 Pages) Texas Instruments – Fully Integrated Switch-Mode One-Cell Li-Ion Charger with Full USB Compliance and Accessory Power Connection

bq24180
SLUSA02 A – FEBRUARY 2010 – REVISED FEBRUARY 2010
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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 38.
VBUS
+ ^VIN
-
C1
bq24180
PMID
C2
SW
L1
Isns
Rsns
C4
C3
PGND
Isys
Ichg
+
BAT
System
Load
Figure 38. 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 host 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, or the thermal regulation
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.
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:
L_OUT =
VBAT ´ (VBUS - VBAT)
VBUS ´ f ´ DI_L
, the worst case is when battery voltage is as close as to half of the input
voltage.
32
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