BQ24650 Datasheet, PDF (21/33 Pages) Texas Instruments – Synchronous Switch-Mode Battery Charge Controller for Solar Power With Maximum Power Point Tracking

bq24650
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APPLICATION INFORMATION
SLUSA75 – JULY 2010
INDUCTOR SELECTION
The bq24650 has a 600-kHz 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
(11)
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)
fs × L
(12)
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% to 40% of the maximum charging current as a trade-off between inductor size and efficiency for
a practical design.
INPUT CAPACITOR
The 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)
(13)
A low ESR ceramic capacitor such as X7R or X5R is preferred for the input decoupling capacitor and should be
placed as close as possible to the drain of the high-side MOSFET and source of the low-side MOSFET. The
voltage rating of the capacitor must be higher than the normal input voltage level. A 25V rating or higher
capacitor is preferred for a 20V input voltage. A 20mF capacitance is suggested for a typical 3A to 4A charging
current.
OUTPUT CAPACITOR
The output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current I_COUT is given as:
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(14)
The output capacitor voltage ripple can be calculated as follows:
DV_O
=
V_OUT
8LCfs²
æ
ççè1-
V_OUT
V_IN
ö
÷÷ø
(15)
At certain input/output voltages and switching frequencies, the voltage ripple can be reduced by increasing the
output filter inductor and capacitor values.
The bq24650 has an internal loop compensator. To achieve 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 has a 35V or higher rating, X7R or X5R.
Ceramic capacitors show a de-bias effect. This effect reduces the effective capacitance when a dc-bias voltage
is applied across a ceramic capacitor, as on the output capacitor of a charger. The effect may lead to a
significant capacitance drop, especially for high voltages and small capacitor packages. See the manufacturer’s
datasheet about performance with a dc bias voltage applied. It may be necessary to choose a higher voltage
rating or nominal capacitance value in order to achieve the required value at the operating point.
POWER MOSFETS SELECTION
Two external N-channel MOSFETs are used for a synchronous switching battery charger. The gate drivers are
internally integrated into the IC with 6V of gate drive voltage. 30V or higher voltage rating MOSFETs are
preferred for 20V input voltage, and 40V or higher rating MOSFETs are preferred for 20V to 28V input voltage.
Copyright © 2010, Texas Instruments Incorporated
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