BQ24610 Datasheet, PDF (25/33 Pages) Texas Instruments – Stand-Alone Synchronous Switch-Mode Li-Ion or Li-Polymer Battery Charger with System Power Selector and Low Iq

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bq24610
bq24617
SLUS892 – DECEMBER 2009
APPLICATION INFORMATION
Inductor Selection
The bq24610/7 has 600kHz switching frequency to allow the use of small inductor and capacitor values. 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
(12)
The inductor ripple current depends on input voltage (V_IN), duty cycle (D=V_OUT/V_IN), switching frequency (f_s) and
inductance (L):
I_RIPPLE =
V_IN
´D
f_S
´ (1
´L
-
D)
(13)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging
voltage range is from 9V to 12.6V for 3-cell battery pack. For 20V adapter voltage, 10V battery voltage gives the
maximum inductor ripple current. Another example is 4-cell battery, the battery voltage range is from 12V to
16.8V, and 12V battery voltage gives the maximum inductor ripple current.
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.
The bq24610/7 has cycle-by-cycle charge under current protection (UCP) by monitoring charging current sensing
resistor to prevent negative inductor current. The Typical UCP threshold is 5mV falling edge corresponding to
0.5A falling edge for a 10mΩ charging current sensing resistor.
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)
(14)
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 20V input voltage. 10-20µF 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 I_COUT is given:
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(15)
The output capacitor voltage ripple can be calculated as follows:
DV_o
=
1
8LCf_s2
æ
ççè1-
V_OUT ²
V_IN
ö
÷÷ø
(16)
At certain input/output voltage and switching frequenccy, the voltage ripple can be reduced by increasing the
output filter LC.
The bq24610/7 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 25V or higher rating, X7R or X5R for 4-cell application.
Copyright © 2009, Texas Instruments Incorporated
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