LMZM23601: IC Power Dissipation when the current limit is activated

Hello Shinichi,

During a short circuit event on the output, the LMZM23601 will initially try to supply the maximum amount of current possible by folding back the frequency and switching between the HS current limit and the LS current limit. This will result in the IC temperature rising initially. After some cycles of switching between HS and LS current limits, Hiccup mode will activate to prevent the maximum junction temperature from being exceeded. Hiccup mode will remain active as long as the short is present at the output. 

To estimate the power dissipation of the IC during short circuit, first use Figure 13 in the datasheet to find the input power when the input voltage is 24V, output current is 1A. Power dissipation is 1.1W according to the figure.

Pin (W) = Pout (W) + Ploss (W) = (3.3V*1A) + 1.1W = 4.4W input power.

During a short circuit event, the maximum output current the device can supply is around 1.2A. 

Assume that the input power is the same when the output is shorted and that there is 100mOhm of resistance from Vout to GND.

Pout = 1.2A*1.2A * 0.1Ohm = 144mW

Ploss = 4.4W - 144mW = ~4.3W power dissipated by IC while switching in current limit. 

When hiccup mode is activated, this power dissipation will only occur for a small period of time. Assuming the horizontal divisions in Figure 34 are 1ms then the average power dissipation during hiccup would be:

P = (0.5/6) * 4.3W = ~0.3W.

Note that I made a lot of assumptions in the above estimation. The customer should verify using an EVM if possible.

Regards,

Harrison Overturf