I have one of the 5/3.3V 7A step down regulators, but I’m struggling with the minimum startup time limitation of a “few” milliseconds. Most of the time it doesn’t work, and I have to rapidly disconnect and reconnect the input in order to get it to start up, which isn’t good enough for my application.
Is there a workaround for this? Seems a pretty severe design flaw. For instance, can the enable input be used as an undervoltage lockout, so that the regulator won’t try to start until the input voltage is close to it’s nominal value? My input voltage is fixed at 24V.
Did you try connecting the Enable to the input? Otherwise, adding a capacitor from Enable to Ground would mean that it would only rise slowly from the current through the 100K connected to Vin. Something around 1uF will give you ~20ms of delay on startup, bigger and smaller will give you proportionally more/less.
Also, how slowly is your voltage rising? A noisy turn-on from a connector might be causing the trouble, whereas a switch would be faster.
Thanks for the good suggestions, but no luck with a cap or directly connecting the enable to VIN. I checked out the rise time (I’m using a OpenUPS from Minibox) on VIN, and it was quite horrendous at around 500ms.
What I’ve done to solve it is short the enable pin low for 500ms after connecting power. The regulator then starts up correctly. Luckily the OpenUPS has an output that can already do this (used to provide on/off pulses to motherboards) automatically, and so it was straightforward to implement in my case. Others might need some extra logic to achieve the same thing.
Glad you sorted that out. Seeing as your rise time is so long, a larger cap would have worked as well. Something around 100uF should give you ~0.75s before it charges to 2V.
Yes, I thought about that, but didn’t have any suitable caps on hand. The nice thing about the 1uF cap was that I could just scrap off some solder mask and solder an 0805 straight down, which was nice and neat.