BQ24298_15 Datasheet, PDF (38/51 Pages) Texas Instruments – bq24298 I2C Controlled 3A Single Cell USB Charger With Narrow VDC Power Path Management and Adjustable Voltage USB OTG

bq24298
SLUSC59 – APRIL 2015
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9.2.2 Detailed Design Procedure
9.2.2.1 Inductor Selection
The device has 1.5-MHz switching frequency to allow the use of small inductor and capacitor values. The
Inductor saturation current should be higher than the charging current (I_CHG) plus half the ripple current (I_RIPPLE):
( ) I_SAT ³ I_CHG + 1/ 2 I_RIPPLE
(4)
The inductor ripple current depends on input voltage (VBUS), duty cycle (D = V_BAT/V_VBUS), switching frequency
(fs) and inductance (L):
I_RIPPLE
=
V_IN
´ D ´ (1- D)
¦s ´ L
(5)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. Usually inductor ripple is designed in
the range of (20 – 40%) maximum charging current as a trade-off between inductor size and efficiency for a
practical design.
9.2.2.2 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.
9.2.2.3 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.
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