1117 LDO and reverse polarity protection - Page 1
Forum section: electronics / projects

author: panoss
author_rank: Frequent Contributor
timestamp: April 02, 2019, 01:55:54 pm
content: I 've built a device which is powerεd by a 5V PSU.The voltage firstly goes through an FQP27P06 (P-Channel MOSFET) for reverse polarity protection.Then it goes to the 1117.So my question is: do I need the MOSFET for reverse polarity protection?Or the 1117 has it's own protection against reverse polarity?

author: balage
author_rank: Regular Contributor
timestamp: April 02, 2019, 02:37:36 pm
content: Hi,Here is the simplified internal schematic. I would say it is not reverse polarity protected. However I cannot see any path to the reverse current to flow as all of the transistors are blocking that way.I use a schottky diode for revesre polarity protection. I don't know which one is the better... I always see the least in TI's reference designs. They must have a reason.

author: panoss
author_rank: Frequent Contributor
timestamp: April 02, 2019, 02:41:59 pm
content: If a reverse polarity PSU gets connected the current will stop at the transistors at the bottom of the schematic?

author: balage
author_rank: Regular Contributor
timestamp: April 02, 2019, 02:47:40 pm
content: I think yes, they block the way. But the question is the voltage that those transistors can carry on their E-C without blowing.When I get home I will try.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 02, 2019, 02:58:25 pm
content: My take on this is, unless the datasheetspecifically indicatesthat it is reverse-polarity protected, and the reverse voltage withstanding level, I would assume that it is not protected.For LDOs, I also prefer a PMOS rather than a schotky where extremely low dropouts are required.

author: Gyro
author_rank: Super Contributor
timestamp: April 02, 2019, 07:30:44 pm
content: Agreed, it's an essential rule of product design - If it isn't in the datasheet then it doesn't exist. No amount of peering at simplified schematics changes that. Even if you get away with it on a specific sample, it can't be assumed that it will work for other production batches or second-sources.

author: David Hess
author_rank: Super Contributor
timestamp: April 03, 2019, 12:26:32 am
content: I have heard that 1117s from different manufacturers have significant differences.As shown the reverse voltage cannot exceed 1 Vbe + 1 reverse Vbe so about 6 volts. I think all of the junction isolation substrate diodes go to the adjustment pin so the series resistance at that pin will limit current.

author: panoss
author_rank: Frequent Contributor
timestamp: April 03, 2019, 10:56:59 am
content: Ok, thank you all. I 'll keep the MOSFET.

author: Peabody
author_rank: Super Contributor
timestamp: April 03, 2019, 01:35:09 pm
content: There's a possibility the MOSFET wouldn't be needed at 5V. When I've tested this with a PNP transistor, I found that applying reverse polarity produced no current - up to 8V. So it acted like a Zener. You could test this with your meter. Apply reverse voltage through an ammeter, and see at what voltage current begins to flow.Also, is there actually a risk of reverse polarity here? The risk usually comes with batteries, but you said 5V, which doesn't match most battery types.

author: Psi
author_rank: Super Contributor
timestamp: April 03, 2019, 01:48:53 pm
content: LM2937IMP-5.0 have reverse polarity protection and 60V transient protection.Downside is they're not cheap

author: panoss
author_rank: Frequent Contributor
timestamp: April 03, 2019, 01:56:32 pm
content: The risk for reverse polarity is, indeed, very small because the 5V comes (from a PSU) from a standard connector like this one:But for this 1 in a million case...