The device includes a programmable fuse protection for each channel, that is based on a defined time-current curve and is commonly referred to as I2t protection in melting fuse data sheets. The intent is to match the switch turnoff behavior of a melting fuse. The NOM_CUR_CHx [2:0] bits and I2T_TRIP_CHx [3:0] bits set the time-current curve but the device also uses a fixed delay shutdown (I_SWCL) and immediate shutdown protection (I_OCP) to create the full I2T protection for the device. The I2T protection of the TPS2HCS08-Q1 consists of four regions:

1. Nominal current
2. Fuse shutdown
3. Fixed delay shutdown
4. Immediate shutdown protection (I_OCP)

The nominal current region (1) defines the region where the device can supply current indefinitely without turning off. This is roughly equivalent to the fuse current rating of a melting fuse. This region is set by the NOM_CUR_CHx [2:0] bits and if the output current is less than the NOM_CUR_CHx setting then the device can supply current indefinitely as previously stated and it will not start the I2T accumulation. If the output current is greater than or equal to the NOM_CUR_CHx setting then the device will enter the I2T accumulation loop and will start to accumulate until the I2T_TRIP_CHx [3:0] threshold is met. If the output current falls back below the NOM_CUR_CHx before the I2T_TRIP_CHx value is reached then the device will stop the I2T accumulation but will still keep track of the accumulated energy as long as power is provided to the device.

Above the nominal current region, is the fuse shutdown region (2) which is set by the I2T_TRIP_CHx [3:0] bits. This region defines the curvature of time-current curve and the region where the I2T accumulation of the device is active. Based on the output current level and the NOM_CUR_CHx setting the device will trip at different time intervals based on the I2T_TRIP value that is set. The time-current curve of the device is defined by Equation 1.

If the accumulation does not exceed the I2T_TRIP value and the current falls below NOM_CUR_CH then equation 1 is used to decrement the accumulated energy based on the ISNS value until the accumulated energy reaches zero. While the device continues to decrement down to zero, the I2T_MOD bit will remain 1 until accumulated energy returns back to zero and then the I2T_MOD bit will be set back to zero. If any conversions were disabled due to a channel entering the I2T loop then they will be re-enabled when I2T_MOD = 0.

Above the fuse shutdown region, is the fixed delay shutdown region (3) where the device provides a fixed delayed shutdown that is set by the ISWCL_CHx [1:0] and SWCL_DLY_TMR_CHx [1:0] bits. The ISWCL_CHx [1:0] sets the output current value and the SWCL_DLY_TMR_CHx sets the time. If the output current exceeds the ISWCL_CHx level continously for SWCL_DLY_TMR_CHx then the channel will immediately turn off.

If a shutdown occurs either due to the I2T_TRIP_CHx value being exceeded or due to the ISWCL_CHx function, the device will remain off for a period set by the TCLDN_CHx [1:0]. If the TCLDN_CHx [1:0] = 00 then the device will remain off and will not retry. To retry in this setting, the TCLDN_CHx [1:0] bits need to be changed to another setting. Once the setting has been changed, the device will retry after the defined cool down time of the new setting. Note, when the channel enters the I2T shutdown state the accumulator value will be reset to 0 so the retry time should be adjusted to make sure enough time has elasped for the wire harness to cool down. Also note, when the channel enters the I2T shutdown state, the values for NOM_CUR_CHx, I2T_TRIP_CHx, and ISWCL_CHx can not be changed.

Above the fixed delay shutdown region, is the immediate shutdown overcurrent protection (I_OCP) region (4). This region is set by the ILIMIT_SET_CHx [3:0] bits. If the output current exceeds the I_OCP level then the device will turn off immediately. The retry or latched off behavior for the I_OCP is set by the LATCH_CHx bit and discussed in the next section.

These operational regions for the I2T protection are show in Figure 8-23.

The values for the NOM_CUR_CHx, I2T_TRIP_CHx, and ISWCL_CHx in the register map are based on an R_SNS value of 700?. The device offers the flexibility for these values to be scaled based on different R_SNS values. The equations for scaling the NOM_CUR_CHx, I2T_TRIP_CHx, and the ISWCL_CHx are defined below.

where,