BQ25708 datasheet(75/94 Pages) TI2

75 / 94 page

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.

10.2.2.3 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 5:

CIN

CHG

= I

D × (1

(5)

Low ESR ceramic capacitor such as X7R or X5R is preferred for input decoupling capacitor and should be

placed in front of RAC current sensing and as close as possible to the power stage half bridge MOSFETs.

Capacitance after RAC before power stage half bridge should be limited to 10 nF + 1 nF referring to Figure 10-2.

Because too large capacitance after RAC could filter out RAC current sensing ripple information. Voltage rating of

the capacitor must be higher than normal input voltage level, 25-V rating or higher capacitor is preferred for 19-V

to 20-V input voltage. The minimum input effective capacitance recommendation is shown in Table 10-1.

Ceramic capacitors (MLCC) 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 datasheet about the derating 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 effective capacitance

value at the operating point. Considering the 25 V 0603 package MLCC capacitance derating under 19-V to 20-V

input voltage, the recommended practical capacitors configuration can also be found in Table 10-1. Tantalum

capacitors (POSCAP) can avoid dc-bias effect and temperature variation effect which is recommended for 90 W

to 130 W higher power application.

Table 10-1. Minimum Input Capacitance Requirement

INPUT CAPACITORS VS TOTAL INPUT

POWER

65W

90W

130W

Minimum effective input capacitance

4 μF

6 μF

13 μF

Minimum practical input capacitors

configuration

4*10 μF (0603 25 V MLCC)

6*10 μF (0603 25 V MLCC)

3*10 μF (0603 25 V MLCC)

1* 10 μF (25 V to 35 V

POSCAP)

10.2.2.4 Output Capacitor

Output capacitor also should have enough ripple current rating to absorb output switching ripple current. 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 25-V X7R or X5R for output capacitor. Minimum

7 pcs of 10-μF 0603 package capacitor is suggested to be placed as close as possible to Q3&Q4 half bridge

(between Q4 drain and Q3 source terminal). Total minimum output effective capacitance along VSYS distribution

line is 50 μF refers to Table 10-2. Recommend to place minimum 20-μF MLCC capacitors after the charge

current sense resistor for best stability.

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 output capacitor of a charger. The effect may lead

to a significant capacitance drop, especially for high output voltages and small capacitor packages. See the

manufacturer's data sheet about the derating performance with a dc bias voltage applied. It may be necessary

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BQ25713, BQ25713B

SLUSD83C – JUNE 2018 – REVISED MAY 2023

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