8.3.17 Current Sense Input Filtering

Each power stage current is monitored by the device at the CS terminals. The device monitors the current with a 12-bit ADC and also monitors the current with a digitally programmable analog comparator. The comparator can be disabled by writing a zero to the FAST_OC_FAULT_LIMIT.

Because the current sense signal is both digitally sampled and compared to the programmable over-current threshold, it should be conditioned with an RC network acting as an anti-alias filter. If the comparator is disabled, the CS input should be filtered at 35% of the sampling rate. An RC network with this characteristic can be calculated as

Equation 9.

where N_rails is the number of rails configured and T_Iout is the sample period for the current sense inputs. Therefore, when the comparator is not used, the recommended component values for the RC network are C = 10 nF and R = 35.7 kΩ.

When the fast over-current comparator is used, the filter corner frequency based on the ADC sample rate may be too slow and a corner frequency that is a compromise between the requirements of fast over-current detection and attenuating aliased content in the sampled current must be sought. In this case, the filter corner frequency can be calculated based on the time to cross the over-current threshold.

Equation 10.

where V_{OC_thres} is the programmed OC comparator threshold, V_{CS_nom} is the nominal CS voltage, ΔV_Imon is the change in CS voltage due to an over-current fault and τ is the filter time constant. Using the equation for the comparator voltage above, the RC network values can be calculated as

Equation 11.

where T_det is the time to cross the over-current comparator threshold. For T_det = 10 µs, ΔV_Imon = 1.5V, V_{OC_thres} = 2.0V and V_{CS_nom} = 1.5V, the corner frequency is 6.4 kHz and the recommended RC network component values are C = 10 nF and R = 2.49 kΩ.