BQ24130_15 Datasheet, PDF (19/29 Pages) Texas Instruments – 600-kHz Synchronous Switch-Mode Host-ControlledBattery/Supercapacitor Charger With 4-A Integrated MOSFETs

bq24130
www.ti.com
APPLICATION INFORMATION
SLUSAN2C – JULY 2011 – REVISED JUNE 2012
Inductor Selection
The bq24130 has 600 kHz 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) ≥ I_(CHG) + (1/2) I_(RIPPLE)
(6)
The inductor ripple current depends on input voltage (V_IN), duty cycle (D = V_OUT/V_IN), switching frequency (fs)
and inductance (L):
I_{(RIPPLE )}
=
V_IN ´D´(1-D)
f s´L
(7)
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 Equation 8:
I_(CIN) = I_(CHG) ´ D´(1- D)
(8)
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. 20 μF capacitance is suggested for typical of 3 A - 4 A charging current.
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 )}
=
2´ 3
» 0.29´I_(RIPPLE)
(9)
The output capacitor voltage ripple can be calculated as follows:
DV_O
=
V
OUT
8LCf s²
æçççè_1-
V
OUT
V
IN
ö÷÷ø÷÷
(10)
At certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The bq24130 has internal loop compensator. To get good loop stability, the resonant frequency of the output
inductor and output capacitor should be designed between 12 kHz and 17 kHz. The preferred ceramic capacitor
is 25 V or higher rating, X7R or X5R
Input Filter Design
During adapter hot plug-in, the parasitic inductance and input capacitor from the adapter cable form a second
order system. The voltage spike at AVCC/PVCC pin may be beyond IC maximum voltage rating and damage IC.
The input filter must be carefully designed and tested to prevent overvoltage event on AVCC/PVCC pin.
There are several methods to damping or limit the overvoltage spike during adapter hot plug-in. An electrolytic
capacitor with high ESR as an input capacitor can damp the overvoltage spike well below the IC maximum pin
voltage rating. A high current capability TVS Zener diode can also limit the over voltage level to an IC safe level.
However, these two solutions may not have low cost or small size.
Copyright © 2011–2012, Texas Instruments Incorporated
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