BQ25708 datasheet(61/77 Pages) TI2

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RIPPLE_BUCK

SAT

CHG

RIPPLE

+ (1/2) I

RAC

ACP

ACN

RACP

10 :

RACN

10 :

CACP

15nF

CACN

15nF

bq25708

CDIFF

Open

4~6x10uF

(0805)

1nF+10nF

(0402)

HIDRV1

bq25708

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SLUSCU2 – NOVEMBER 2017

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Typical Application (continued)

DESIGN PARAMETER

EXAMPLE VALUE

Minimum System Voltage(2)

6144mV for 2s battery

9.2.2 Detailed Design Procedure

The parameters are configurable using the evaluation software. The simplified application circuit (see Figure 39,

as the application diagram) shows the minimum component requirements. Inductor, capacitor, and MOSFET

selection are explained in the rest of this section. Refer to the EVM user's guide (SLUUBG6) for the complete

application schematic.

9.2.2.1 ACP-ACN Input Filter

The bq25708 has average current mode control. The input current sensing through ACP/ACN is critical to

recover inductor current ripple. Parasitic inductance on board will generate high frequency ringing on ACP-ACN

which overwhelms converter sensed inductor current information, so it is difficult to manage parasitic inductance

created based on different PCB layout. Bigger parasitic inductance will generate bigger sense current ringing

which will cause the average current control loop to go into oscillation.

For real system board condition, we suggest to use below circuit design to get best result and filter noise induced

from different PCB parasitic factor. With time constant of filter from 47nsec to 200nsec, the filtering on ringing is

effective and in the meantime, the delay of on the sensed signal is small and therefore poses no concern for

average current mode control.

Figure 35. ACN-ACP Input Filter

9.2.2.2 Inductor Selection

The bq25708 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):

(3)

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

switching frequency (fS) and inductance (L):

(4)

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