9.2.2.1 ACP-ACN Input Filter

The bq25703A 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 47 nsec to 200 nsec, 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 47. ACN-ACP Input Filter