8.3.18 Over-Current Detection

Several mechanisms are provided to sense output current fault conditions. This allows for the design of power systems with multiple layers of protection.

1. Integrated gate drivers such as the UCD72xx family can be used to generate the FLT signal. The driver monitors the voltage drop across the high side FET and if it exceeds a resistor/voltage programmed threshold, the driver activates its fault output. A logic high signal on the FLT input causes a hardware interrupt to the internal CPU, which then disables the DPWM output. This process takes about 14 microseconds.
2. Inputs CS1A, CS2A, CS3A, and CS4A each drive an internal analog comparator. These comparators can be used to detect the voltage output of a current sense circuit. Each comparator has a separate threshold that can be set by the FAST_OC_FAULT_LIMIT PMBus command. Though the command is specified in amperes, the hardware threshold is programmed with a value between 31mV and 2V in 64 steps. The relationship between amperes to sensed volts is configured by the IOUT_CAL_GAIN command. When the current sense voltage exceeds the threshold, the corresponding DPWM output is driven low on the voltage rail with the fault.
3. Each Current Sense input to the UCD9244 is also monitored by the 12-bit ADC. Each measured value is scaled using the IOUT_CAL_GAIN and IOUT_CAL_OFFSET commands and then passed through a digital smoothing filter. The smoothed current measurements are compared to fault and warning limits set by the IOUT_OC_FAULT_LIMIT and IOUT_OC_WARN_LIMIT commands. The action taken when an OC fault is detected is defined by the IOUT_OC_FAULT_RESPONSE command.

Because the current measurement is averaged with a smoothing filter, the response time to an over-current condition depends on a combination of the time constant (τ) from Table 5, the recent measurement history, and how much the measured value exceeds the over-current limit. When the current steps from a current (I₁) that is less than the limit to a higher current (I₂) that is greater than the limit, the output of the smoothing filter is

Equation 12.

At the point when I_smoothed exceeds the limit, the smoothing filter lags time, t_lag is

Equation 13.

The worst case response time to an over-current condition is the sum of the sampling interval (Table 5) and the smoothing filter lag, t_lag from Equation 13.