BQ25713 datasheet(89/111 Pages) TI2

89 / 111 page

switching frequency is employed, 10 nF is recommended for CDIFF; if 800-kHz switching frequency is chosen,

then CDIFF can be left open.

RAC

ACP

ACN

RACP

4.99ohm

RACN

4.99ohm

CACP

33nF

CACN

33nF

CDIFF

10nF for 400kHz

Open for 800kHz

6x10uF

1nF(0402)

HIDRV1

10nF(0402)

Figure 9-3. ACN-ACP Input Filter

9.2.2.3 Inductor Selection

The BQ25730 has two selectable fixed switching frequency. Higher switching frequency allows the use of

smaller inductor and capacitor values. Inductor saturation current should be higher than the charging current

(ICHG) plus half the ripple current (IRIPPLE):

SAT

CHG

RIPPLE

+ (1/2) I

(2)

The inductor ripple current in buck operation depends on input voltage (VIN), duty cycle (DBUCK = VOUT/VIN),

switching frequency (fS) and inductance (L):

IRIPPLE_BUCK = VIN × DBUCK× (1-DBUCK) / (fS × L)

(3)

During boost operation, the duty cycle is:

DBOOST = 1 – (VIN/VBAT)

and the ripple current is:

IRIPPLE_BOOST = (VIN × DBOOST) / (fS × L)

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 3-cell battery pack. For 20-V adapter voltage, 10-V battery voltage gives

the maximum inductor ripple current. Another example is 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 – 40%) maximum charging current as a trade-off between

inductor size and efficiency for a practical design.

9.2.2.4 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 (plus system current there is any system load) when duty cycle

is 0.5 in buck mode. 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 4:

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BQ25730

SLUSE65A – FEBRUARY 2021 – REVISED JANUARY 2024

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