Today I learned something new that I thought I’d share here. I’ve always taken linear regulators pretty much for granted - you pick one that can handle your current, power, noise, regulation, etc. requirements and after that they’re pretty interchangeable. I’ve seen “active discharge” mentioned on a few data sheets recently but never paid it much attention, assuming it wasn’t relevant to my fairly mundane requirements, but it turns out that it’s a very important feature in some applications, particularly those like my present case, which involves fairly low (≈2mA) baseline current with large (≈160mA), brief (600µs) spikes. I’m using an ON Semi NCP718B LDO, which is inexpensive and has seemingly good specs: you read “10mV load regulation at 300mA” and figure that it’s going to be just fine. I was mightily surprised, therefore, to see 1V p-p ripple on my 3.3V rail whenever my BT radio sends out an advertising packet
Big shout-out to ON Semi’s outstanding tech support, who got back to me in a couple of days with this detailed explanation:
"The problem is that the NCP718B has no active discharge. When first current spike appears, regulation loop is trying to open internal power P-MOS as much as possible to cover increased load demand. When current spike disappears, regulation loop needs some time to fully close open P-MOS. During that time, output voltage increases above nominal 3.3V. This is caused by charging output capacitor 10uF through fully open P-MOS and light load condition.
When regulation loop realizes that output voltage is “too high”, it fully close internal P-MOS and output capacitor is discharged by light load only (~1.8mA).
If next current peak appears BEFORE output voltage drops down to nominal 3.3V, internal control loop can’t react fast as internal P-MOS is fully closed. It takes some time to charge gate capacity to be able to control this P-MOS and during this time is output capacitor the only one source of charge for load. This cause that output voltage drops significantly lower than during first transient.
When control loop takes over the situation, output capacitor is charged back by fully open P-MOS and procedure repeats.
In fact, NCP718B Is a bad choice for this type of application. Please use NCP718A instead (A means Active discharge, which helps to return output voltage back to nominal 3.3V much faster after first current spike."
Yellow trace is 12V input rail with 330mV of ripple; blue trace is LDO 3.3V output with 1V(!) of ripple:
Here’s the current drain (yay, Joulescope!):