bq24296
bq24297
SLUSBP6A –SEPTEMBER 2013–REVISED OCTOBER 2013
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APPLICATION INFORMATION
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 (ICHG) plus half the ripple current (IRIPPLE):
ISAT ³ ICHG
+ 1/ 2 I
( )
RIPPLE
(4)
The inductor ripple current depends on input voltage (VBUS), duty cycle (D = VBAT/VVBUS), switching frequency
(fs) and inductance (L):
V ´D´(1- D)
IN
IRIPPLE
=
¦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.
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 ICIN occurs where the duty cycle is closest to 50%
and can be estimated by the following equation:
ICIN = ICHG ´ D´(1- D)
(6)
For best performance, VBUS should be decouple to PGND with 1μF 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. 25V rating or higher capacitor is preferred
for 15V input voltage. 22μF capacitance is suggested for typical of 3-4A charging current.
Output Capacitor
Output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current ICOUT is given:
IRIPPLE
ICOUT
=
» 0.29´IRIPPLE
2´ 3
(7)
The output capacitor voltage ripple can be calculated as follows:
æ
ç
ö
÷
VOUT
VOUT
DVO
=
1-
8LC¦s2
ç
÷
V
IN
è
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 15kHz and 25kHz. The preferred ceramic
capacitor is 6V or higher rating, X7R or X5R.
38
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