BQ24180_1 Datasheet, PDF (31/41 Pages) Texas Instruments – Fully Integrated Switch-Mode One-Cell Li-Ion Charger with Full USB Compliance and Accessory Power Connection

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SLUSA02 A – FEBRUARY 2010 – REVISED FEBRUARY 2010
POWER TOPOLOGIES
System Load After Sensing Resistor
One of the simple high-efficiency topologies connects the system load directly across the battery pack, as shown
in Figure 37. The input voltage has been converted to a usable system voltage with good efficiency from the
input. When the input power is on, it supplies the system load and charges the battery pack at the same time.
When the input power is off, the battery pack powers the system directly.
VBUS
SW
Isns
Isys
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-
C1
L1
bq24180
PMID
C2
C4
PGND
Rsns
C3
Ichg
+
BAT
System
Load
Figure 37. System Load After Sensing Resistor
The advantages:
• When the AC adapter is disconnected, the battery pack powers the system load with minimum power
dissipations. Consequently, the time that the system runs on the battery pack can be maximized.
• It saves the external path selection components and offers a low-cost solution.
• Dynamic power management (DPM) can be achieved. The total of the charge current and the system current
can be limited to a desired value by adjusting charge current. When the system current increases, the charge
current drops by the same amount. As a result, no potential over-current or over-heating issues are caused
by excessive system load demand.
• The total of the input current can be limited to a desired value by setting input current limit value. So USB
specifications can be met easily.
• The supply voltage variation range for the system can be minimized.
• The input current soft-start can be achieved by the generic soft-start feature of the IC.
Design considerations and potential issues:
• If the system always demands a high current (but lower than the regulation current), the charging never
terminates. Thus, the battery is always charged, and the lifetime may be reduced.
• Because the total current regulation threshold is fixed and the system always demands some current, the
battery may not be charged with a full-charge rate and thus may lead to a longer charge time.
• If the system load current is large after the charger has been terminated, the voltage drop across the battery
impedance may cause the battery voltage to drop below the refresh threshold and start a new charge. The
charger would then terminate due to low charge current. Therefore, the charger would cycle between
charging and terminating. If the load is smaller, the battery has to discharge down to the refresh threshold,
resulting in a much slower cycling.
• In a charger system, the charge current is typically limited to about 10mA, if the sensed battery voltage is
below 2V short circuit protection threshold. This results in low power availability at the system bus. If an
external supply is connected and the battery is deeply discharged, below the short circuit protection threshold,
the charge current is clamped to the short circuit current limit. This then is the current available to the system
during the power-up phase. Most systems cannot function with such limited supply current, and the battery
supplements the additional power required by the system. Note that the battery pack is already at the
depleted condition, and it discharges further until the battery protector opens, resulting in a system shutdown.
• If the battery is below the short circuit threshold and the system requires a bias current budget lower than the
short circuit current limit, the end-equipment will be operational, but the charging process can be affected
depending on the current left to charge the battery pack. Under extreme conditions, the system current is
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