BQ24296_15 Datasheet, PDF (42/55 Pages) Texas Instruments – bq2429x I2C Controlled 3-A Single Cell USB Charger With Narrow VDC Power Path Management and Adjustable Voltage USB OTG

bq24296, bq24297
SLUSBP6B – SEPTEMBER 2013 – REVISED NOVEMBER 2014
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10.2.2.2 Input Capacitor
Input capacitor should have enough ripple current rating to absorb input switching ripple current. The worst case
RMS ripple current is half of the charging current when duty cycle is 0.5. If the converter does not operate at
50% duty cycle, then the worst case capacitor RMS current I_CIN occurs where the duty cycle is closest to 50%
and can be estimated by the following equation:
I_CIN = I_CHG ´ D ´ (1- D)
(6)
For best performance, VBUS should be decouple to PGND with 1-μF capacitance. The remaining input capacitor
should be place on PMID.
Low ESR ceramic capacitor such as X7R or X5R is preferred for input decoupling capacitor and should be
placed to the drain of the high side MOSFET and source of the low side MOSFET as close as possible. Voltage
rating of the capacitor must be higher than normal input voltage level. 25-V rating or higher capacitor is preferred
for 15-V input voltage. 22-μF capacitance is suggested for typical of 3-A to 4-A charging current.
10.2.2.3 Output Capacitor
Output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current I_COUT is given:
I_COUT
I_RIPPLE
=
» 0.29 ´ I_RIPPLE
2´ 3
(7)
The output capacitor voltage ripple can be calculated as follows:
DV_O
=
V_OUT
8LC¦s²
æ
ççè_1-
V_OUT
V_IN
ö
÷
÷ø
(8)
At certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The charger device has internal loop compensator. To get good loop stability, the resonant frequency of the
output inductor and output capacitor should be designed between 15 kHz and 25 kHz. The preferred ceramic
capacitor is 6 V or higher rating, X7R or X5R.
42
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