BQ24157 Datasheet, PDF (30/39 Pages) Texas Instruments – Fully Integrated Switch-Mode Charger With USB Compliance and USB-OTG Support

bq24157
SLUSB80 – SEPTEMBER 2012
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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.
4. The total input current can be limited to a desired value by setting the input current limit value. USB
specifications can be met easily.
5. The supply voltage variation range for the system can be minimized.
6. The input current soft-start can be achieved by the generic soft-start feature of the IC.
Design considerations and potential issues:
1. If the system always demands a high current (but lower than the regulation current), the battery charging
never terminates. Thus, the battery is always charged, and its lifetime may be reduced.
2. 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.
3. If the system load current is large after the charger has been terminated, the IR drop across the battery
impedance may cause the battery voltage to drop below the refresh threshold and start a new charge cycle.
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.
4. In a charger system, the charge current is typically limited to about 30mA, 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.
5. 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
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.
DESIGN EXAMPLE FOR TYPICAL APPLICATION CIRCUIT
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.
L_OUT
=
5
´
2.5 ´ (5 - 2.5)
(3 ´ 10⁶ ) ´ 1.25
´
0.3
(4)
L_OUT = 1.11 μH
Select the output inductor to standard 1 μH. Calculate the total ripple current with using the 1-μH inductor:
30
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