LM5069 + CSD19534Q5A problem

Hi Shlomi,

We have a LM5069 Design Calculator tool which can be found on our website at www.ti.com/hotswap -> "Tools and Software" tab -> "Design Calculator" download link.

I also attached the tool below and filled it out based on the schematic.

The IRF1310N is much larger and has higher SOA. At room temperature, at 60V, it can handle 4A for 10ms.

In comparison, the CSD19534Q5A under the same conditions (room temp, 60V) can only handle 0.4A for 10ms (this is 1/10th the power capability).

This is the reason the CSD19534Q5A will fail if being used with only current limiting, as it cannot handle the 1A current limit for the 22ms fault time.

However, the LM5069 features a power limiting function to help enable the use of FETs with lower SOA. Thus for this low current application, a solution would be to use a resistor at the PWR pin (Rpwr) in the range of 38 - 50kohm (38.3kohm would be recommended). This would limit the power to 6W, and thus at 60V (max) it will limit current to 0.1A, thus keeping the FET within its SOA (including temperature derating).

LM5069_Design_Calculator_CSD19534Q5A.xlsx

Thanks!

Alex

Hi Shlomi,

It is more relating to the power limit than the timer.

To explain it differently, the CSD19534Q5A will fail the output short test without power limiting.

This is because its SOA curve (Figure 10) is being violated:

When the output is shorted, Vds = Vin. So with Vin = 48-60V, then it can handle 0.5A for 10ms (at room temperature). Since current limit is set to 1A, this FET cannot be used solely with current limit. The previous FET (IRF1310) can handle current limiting only, due to its higher SOA.

However, the LM5069 features power limiting. This protects the CSD19534Q5A against even the most stressful test (output short circuit). Thus if a resistor is used (recommend 38.3kohm) between the PWR pin and GND, then I suspect the CSD19534Q5A will work fine in the design. The timer can be kept at 470nF.

Another concern would be input capacitance. Is there any input capacitance for the LM5069? If not, then a few additional components would be recommended.

In summary, if the IRF1310N performed well for their system, the CSD19534Q5A should perform equally well so long as Rpwr = 38.3kohm and all other parameters are the same.

Thanks!

Alex