To solve the problem stated in Section 3.2, TI eFuses implement ACS, which is triggered when the current crosses a certain threshold. The active current sharing loop works by regulating the Rdson of the FETs. The eFuse carrying higher current increases the Rdson slightly to reduce current and allow other devices in the chain to take higher current, thus achieving redistribution or balancing of the current.

Setting ACS threshold low can increase the overall resistance in the path, which increases the power loss or self-heating unnecessarily at lower currents. At the same time, this does not yield any benefit in terms of long-term reliability.

The active current sharing threshold needs to be set at the maximum rated DC current for each device. The best choice is to set this close to the overcurrent protection threshold of each device. Note, that this is the overcurrent threshold which is not affected by the mismatches. This makes sure that once the system load current start approaching the max DC current, the active current sharing kicks in. This is particularly helpful from a long-term reliability perspective if the system is operating in this region for extended periods of time. Once the load current increases further beyond this point (during load transients or faults), the active current sharing loop is deactivated and the device relies on the overcurrent protection circuit (with blanking timer) instead.

Current threshold where the device goes into active current sharing depends upon VREF and RILIM value. TPS1685 design calculator can be used to determine R_IREF , V_IREF and R_ILIM value.

Equation 3 is used to determine RILIM resistance based on ACS threshold.

Figure 3-4 eFuse Devices Connected in Parallel with Common Over Current Limit and ACS to Support 80A Load Current

To evaluate the performance, load current is increased in steps of 2A as shown in Figure 3-5. Due to mismatch in path resistance, eFuses can be seen carrying unequal current distribution initially during the current step. Current through individual eFuse can be seen converging together with ACS.

This waveform is taken with system parameters as discussed in Table 3-1.

Figure 3-5 Current Sharing Among Four TPS1685 eFuses with ACS

The TPS1685x implements a proprietary current balancing mechanism during start-up, which allows multiple TPS1685x devices connected in parallel to share the inrush current and distribute the thermal stress across all the devices. This feature helps to complete a successful start-up with all the devices and avoid a scenario where some of the eFuses hit thermal shutdown prematurely. This increases the inrush current capability of the parallel chain. The improved inrush performance helps to support very large load capacitors on high current platforms without compromising the inrush time or system reliability.

Figure 3-6 Current Distribution Among Four TPS1685 eFuses During Start-up