TPS74801: TPS74801DRCR Current Limit Waveform

Hi Jay,

If you use an e-load you cannot use it in constant current mode because the e-load and the LDO will both be attempting to regulate the load current to different values, and I suspect what you're seeing in the waveform is that the load current is oscillating due to this. Try using the e-load is constant resistance mode and see if the waveform looks cleaner. The load current in current limit should be a constant regulated value until the device heats enough to enter thermal shutdown, so you should not see the load current jumping up and down like you see in the waveform you provided. If using the constant resistance mode does not look cleaner, try using a power resistor to get the load current above the current limit to see how the device responds and limits the current. 

Jay Lo said:

Q3: We see the PG(CH4) drop when Vout equal to 0.9V, is the waveform we can consider it is current limit waveform?

When you see a drop in the output voltage when the current is increased by a relatively small amount (let's say if the load current increment is less than 10% of the rated current), the device is likely entering into current limit. The reason I say "if the current is increased by a relatively small amount..." is that the output voltage is expected to droop slightly over the load current range due to load regulation. So, in the waveform, it looks like the device enters current limit either after the step I labeled "1" below, or after step "2. Step 1 may be where the device just barely gets to current limit, and for sure in step 2 the device is in current limit. 

Regards,

Nick