BQ24171_15 Datasheet, PDF (26/39 Pages) Texas Instruments – bq24171 JEITA-Compliant Stand-Alone, Switched-Mode Li-Ion and Li-Polymer Battery Charger With Integrated MOSFETs and Power Path Selector

bq24171
SLUSAF2C – FEBRUARY 2011 – REVISED APRIL 2015
Typical Application (continued)
10.2.1 Design Requirements
For this design example, use the parameters listed in Table 4 as the input parameters.
PARAMETER
Input Voltage Range
Input Current DPM Limit
Battery Voltage
Charge Current
Table 4. Design Parameters
EXAMPLE VALUE
4.5 V - 17 V
600 mA min
13.5 V max
4 A max
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10.2.2 Detailed Design Procedure
10.2.2.1 Inductor Selection
The bq24171 has a 1600-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
(12)
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
(13)
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.
10.2.2.2 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)
(14)
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 25-V rating or higher
capacitor is preferred for a 15-V input voltage. A 20-μF capacitance is suggested for a typical 3-A to 4-A charging
current.
10.2.2.3 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
(15)
The output capacitor voltage ripple can be calculated as follows:
DV_O
=
V_OUT
8LCfs²
æ
ççè1-
V_OUT
V_IN
ö
÷÷ø
(16)
At certain input/output voltages and switching frequencies, the voltage ripple can be reduced by increasing the
output filter LC.
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