BQ24735_17 Datasheet, PDF (29/46 Pages) Texas Instruments – 1- to 4-Cell Li+ Battery SMBus Charge Controller for Supporting Turbo Boost Mode With N-Channel Power MOSFET Selector

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bq24735
SLUSAK9B – SEPTEMBER 2011 – REVISED APRIL 2015
10.2.2.4 Inductor Selection
The bq24735 has three selectable fixed switching frequencies. Higher switching frequency allows the use of
smaller 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
(4)
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)
(5)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging
voltage range is from 9 V to 12.6 V for a 3-cell battery pack. For 20-V adapter voltage, a 10-V battery voltage
gives the maximum inductor ripple current. Another example is a 4-cell battery. The battery voltage range is from
12 V to 16.8 V, and 12-V battery voltage gives the maximum inductor ripple current.
Usually, inductor ripple is designed in the range of (20% to 40%) maximum charging current as a trade-off
between inductor size and efficiency for a practical design.
The bq24735 has charge undercurrent protection (UCP) by monitoring charging current-sensing resistor cycle-
by-cycle. The typical cycle-by-cycle UCP threshold is 5-mV falling edge corresponding to 0.5-A falling edge for a
10-mΩ charging current sensing resistor. When the average charging current is less than 125 mA for a 10-mΩ
charging current-sensing resistor, the low-side MOSFET is off until BTST capacitor voltage must refresh the
charge. As a result, the converter relies on low-side MOSFET body diode for the inductor freewheeling current.
10.2.2.5 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 occurs where the duty cycle is closest to 50% and
can be estimated by Equation 6:
I_CIN = I_CHG ´ D × (1 - D)
(6)
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 19- to 20-V input voltage. 10- to 20-μF capacitance is suggested for typical of 3- to 4-A charging current.
Ceramic capacitors show a DC-bias effect. This effect reduces the effective capacitance when a DC-bias voltage
is applied across a ceramic capacitor, as on the input capacitor of a charger. The effect may lead to a significant
capacitance drop, especially for high input voltages and small capacitor packages. See the manufacturer's data
sheet about the performance with a DC-bias voltage applied. It may be necessary to choose a higher voltage
rating or nominal capacitance value in order to get the required value at the operating point.
10.2.2.6 Output Capacitor
Output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current is given:
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(7)
The bq24735 has internal loop compensator. To get good loop stability, the resonant frequency of the output
inductor and output capacitor should be designed between 10 kHz and 20 kHz. The preferred ceramic capacitor
is a 25-V X7R or X5R for output capacitor. A capacitance of 10 to 20 µF is suggested for a typical of 3- to 4-A
charging current. Place the capacitors after charging current-sensing resistor to get the best charge current
regulation accuracy.
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