And that reverse protection might just save the day.=20 Do note that reverse diodes are normally only used for "ordinary" linear re= gulators with bipolar transistor pass elements. If you use one for your LDO= then the Vforward needs to be less than the Vf of the PMOS body diode, whi= ch is ??? Looking at a few P-channel 500mA 20V transistor datasheets, they = have Vf of about 0.7 to 1.2 (surprise! They really are "just" diodes). This= suggests Vf for the internal diode will be about the same so an external S= chottky may well do the job. Or just apply the same 3.3V supply to the in and the out, as you suggested = previously... Stephen -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of= James Cameron Sent: Tuesday, 3 May 2016 3:39 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] Can an LDO be ignored if input is open? Aye, thanks. Saw it recently; Horowitz & Hill, Art of Electronics, third edition, sectio= n 9.3 fully integrated linear regulators, application hints, page 606 says: > F: A linear regulator can be damaged by fault conditions in which=20 > bypass capacitors discharge suddenly through the regulator circuit,=20 > [...] Diode D2 prevents [this]. Their schematic shows diode connected backwards across the regulator, from = output to input pin. A later selection table and accompanying notes say so= me regulators have one. This SPX3819 might not have one; guess incompatible with how the datasheet = says it has reverse battery protection feature to -20V. On Mon, May 02, 2016 at 11:03:22PM -0600, David VanHorn wrote: > The workaround I have seen is to add a diode to charge the input cap. -- James Cameron http://quozl.netrek.org/ -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/chang= e your membership options at http://mailman.mit.edu/mailman/listinfo/piclis= t --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .