BQ25895_16 Datasheet, PDF (51/67 Pages) Texas Instruments – Adjustable Voltage 3.1-A Boost Operation

www.ti.com
9.2.2 Detailed Design Procedure
bq25895
SLUSC88A – MARCH 2015 – REVISED MAY 2016
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_BAT ³ I_CHG + (1/2) I_RIPPLE
(5)
The inductor ripple current depends on input voltage (V_BUS), duty cycle (D = V_BAT/V_VBUS), switching frequency (fs)
and inductance (L):
I_RIPPLE
=
V_BUS x D x (1-D)
fs x L
(6)
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 Buck 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_PMID occurs where the duty cycle is closest to 50%
and can be estimated by Equation 7:
I_PMID = I_CHG x D x (1 - D)
(7)
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 up to 14-V input voltage. 8.2-μF capacitance is suggested for typical of 3 A – 5 A charging current.
9.2.2.3 System 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_CSYS = ^IRIPPLE » 0.29 x I_RIPPLE
2x 3
(8)
The output capacitor voltage ripple can be calculated as follows:
DV_O
=
8
V_SYS
LC_SYS
f
s²
çæçççè1-
V_SYS
V_BUS
öø÷÷÷÷
(9)
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, 1-µH and minimum
of 20-µF output capacitor is recommended. The preferred ceramic capacitor is 6V or higher rating, X7R or X5R.
Copyright © 2015–2016, Texas Instruments Incorporated
Product Folder Links: bq25895
Submit Documentation Feedback
51