Synchronous Buck fet driver ICs dont have UVLO on the bootstrap cap - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 15, 2020, 02:20:05 pm
content: I’ve just looked through loads of Synchronous Buck FET driver ICs.I am unable to find a single one which shuts down when the bootstrap cap for the top gate goes undervoltage. As you know, in light load “burst mode” , the bootstrap cap doesn’t get refreshed very often, and so its voltage can fall seriously….then the top fet doesn’t get enough drive voltage and so operates in the linear region, wasting loads of power.Do you know of any of these ICs that protect against this? (ie, have a UVLO on the bootstrap cap voltage)Here is one of the great many that doesn’t have it…NCP5901Bhttps://www.onsemi.com/pub/Collateral/NCP5901B-D.PDF

author: Siwastaja
author_rank: Super Contributor
timestamp: November 15, 2020, 02:38:23 pm
content: I thought they all have it! I agree it's pretty fundamental feature (also why you want to use integrated bootstrap driver to begin with), otherwise it's just waiting for catastrophic failure. Have you tested NCP5901B doesn't actually have an undocumented high-side UVLO?I quickly checked datasheets of DGD0506A and MP1907 which I have used a few times, they both have it.

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 15, 2020, 06:22:23 pm
content: Thanks, but no i havent tested the NP5901B. Also, i noted your two kindly supplied examples have the feature, but are not specifically synchronous Buck fet drivers.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 15, 2020, 06:32:18 pm
content: Oh, now I see, you need the "diode emulation" mode for DCM operation, while the "typical" half-bridge bootstrap drivers like the ones I linked don't have this feature but you would need to add your own logic to do that (by driving both INL and INH low when inductor current drops to zero).

author: langwadt
author_rank: Super Contributor
timestamp: November 15, 2020, 06:55:31 pm
content: Quote from: opampsmoker on November 15, 2020, 02:20:05 pmis it really a big issue?, even when not switching the boost cap is charged to ~(Vin - Vout)and the spec is for 5V gatedrive

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 15, 2020, 08:06:13 pm
content: Thanks, actually, the attached sync buck, uses a sync buck driver IC....and it switches even when the load goes to no_load....so i wonder if this is wht the vendors expect people to do?As discussed, its when the bottom switch simply doesnt switch on that the bootstrap cap discharges, and then there is no voltage for top fet drive..

author: f4eru
author_rank: Super Contributor
timestamp: November 16, 2020, 12:05:37 pm
content: Hello,It depends on the design of the PWM IC.- Perhaps some do switch on the lower mosfet first, even when restarting after a "sleep" in "sparse mode"- Perhaps the minimal load of the output measurement resistor divider is enough to switch a small amount but often enough in "sparse mode" to just keep alive the bootstrap cap- Perhaps a typical regulator starts with a low enough Ton on the first PWM cycle (the problematic one) so that even a little bit of potential linear mode of the mosfet doesn't break itAttention with the NCP5901B, I used it once for a motor driver (because it is cheap, we made sure by SW to avoid the problematic condition of the bootstrap), and the "pre-OV" function, which is explained as "Power−On Reset feature" in the text does in fact short circuit the outut of the half bridge if ever it starts with a voltage already present there.This feature permits to guarantee to charge the bootstrap cap when powering up. Make sure this is desired before selecting the NCP5901Also a hint, in the DS :"The NCP5901B isdesigned to work with ON Semiconductor’s NCP6131multi−phase controller." -> this PWM controller probably (hopefuly) ensures that the bootstrap cap is correctly charged at all times, if the 5901 does in fact have no undocumnted UVLO on ht high side.Related to simulations : the supplied sim models of ICs often don't reproduce the behavior of ICs in transient switch ON/OFF conditions, don't trust the sim for anything out of the normal use conditions.