LMR33630: Output Voltage vs. Output Current

Onawa-san,

Check out this app note. It goes into detail on how the LMR33630 operates including current limit and hiccup. Summary:

1. Current limit starts when the valley of the inductor current tries to go above the low-side current limit. The inductor current must ramp down to the low-side current limit before SW goes high again. This means the frequency will fold back but the output will still regulate to the desired voltage.

2. If the output current ramps more, it will start to operate in hysteretic mode, ramping up to the high-side current limit, then down to the low-side current limit. This gives an output current of ~4A (average of the high-side and low-side current limits). If the load tries to draw more current, the output voltage will droop and the output current will stay at ~4A.

This means a resistive load will cause the output voltage to drop as the load resistance decreases.

This also means a constant-current load will immediately cause VOUT to collapse.

This is why we don't have this curve. The VOUT behavior depends on the type of load.

One more note, the part will enter hiccup mode if the output voltage falls to below 40% of the set output voltage.

-Sam