BQ25600 Datasheet, PDF (49/66 Pages) Texas Instruments – I2C Controlled 3.0-A, Single Cell Battery Charger With up-to 40-V Overvoltage Protection Controller for High-Input Voltage and Narrow Voltage DC (NVDC) Power Path Management

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bq25600, bq25600D
SLUSCJ4 – JUNE 2017
Typical Application Diagram (continued)
8.2.2.2 inductor Selection
The 1.5-MHz switching frequency allows the use of small inductor and capacitor values to maintain an inductor
saturation current higher than the charging current (I_CHG) plus half the ripple current (I_RIPPLE):
I_SAT ≥ I_CHG + (1/2) I_RIPPLE
(3)
The inductor ripple current depends on the input voltage (V_VBUS), the duty cycle (D = V_BAT/V_VBUS), the switching
frequency (f_S) and the inductance (L).
I_RIPPLE
=
V_IN
´ D ´ (1 -
fs ´ L
D)
(4)
The maximum inductor ripple current occurs when the duty cycle (D) is 0.5 or approximately 0.5. Usually inductor
ripple is designed in the range between 20% and 40% maximum charging current as a trade-off between
inductor size and efficiency for a practical design.
8.2.2.3 input Capacitor
Design input capacitance to provide 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 using Equation 5.
I_CIN = I_CHG ´ D ´ (1 - D)
(5)
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. A rating of 25-V or higher capacitor is
preferred for 15 V input voltage. Capacitance of 22-μF is suggested for typical of 3A charging current.
8.2.2.4 Output Capacitor
Ensure that the output capacitance has enough ripple current rating to absorb the output switching ripple current.
Equation 6 shows the output capacitor RMS current I_COUT calculation.
I_COUT
I_RIPPLE
=
2´ 3
» 0.29 ´ I_RIPPLE
(6)
The output capacitor voltage ripple can be calculated as follows:
DV_O
=
V_OUT
8LCfs²
æ
ç¹
-
è
V_OUT
V_IN
ö
÷
ø
(7)
At certain input and output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The charger device has internal loop compensation optimized for >20μF ceramic output capacitance. The
preferred ceramic capacitor is 10V rating, X7R or X5R.
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