Power MUX Priority Switch (LM74800-Q1) - Page 1
Forum section: electronics / projects

author: OwenH
author_rank: Contributor
timestamp: August 25, 2024, 02:19:21 pm
content: Hi everyone,I’m working on a power mux setup using the LM74800-Q1 in a common drain configuration, based on the Texas Instruments Application Brief "Priority Power MUX using Ideal Diodes in Automotive Zonal Modules." The example in the application brief doesn’t include a physical switch for selecting power supply priority, but I need to add one to my design. Specifically, I want to let the user manually control which power supply takes priority between the primary and auxiliary sources.The priority switch is really small and only rated for 5v so it can t handle the 36v max input voltage directly.The main thing is to ensure the system remains always-on, regardless of the switch position. The switch would simply allow manual control over the priority selection.I feel like there might be a simple way to implement this, but I'm unsure of the best approach. Has anyone dealt with something similar or have suggestions on how to add this manual priority switching feature while keeping the system always powered?Thanks in advance for your help!OwenLM74800-Q1 Data sheet:https://www.ti.com/lit/ds/symlink/lm7480-q1.pdf?ts=1724560615922&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM7480-Q1

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 25, 2024, 04:03:42 pm
content: It will be a bit complicated if you want each sources to cut-in and cut-out at specific voltages, but easiest to do is to use DPDT switch to connect the OV pin (thru resistor) you want to be secondary one to primary input. Forcing the secondary one into OV disable when primary exist.Of course size the resistor so it allows secondary to get out of OV disable nice when primary voltage get too low so it can take over. Preferably before primary guess into UVLO.The problem is that when primary goes over voltage then both supplies are disabled regardless of secondary availability. Idk if that's OK.Even better method is to use LM749x instead, and use switch to select which FLT signal is connected to disable the other supply.

author: Marco
author_rank: Super Contributor
timestamp: August 25, 2024, 06:28:15 pm
content: Is the switch SPST or SPDT?The switch just has to determine priority, but both supplies can always provide power when the other supply drops enough regardless of switch position?PS. I suspect the image from the priority paper you included is subtly wrong, the one they have for LM74900-Q1 shows a different resistor network which makes more sense.

author: OwenH
author_rank: Contributor
timestamp: August 25, 2024, 06:47:25 pm
content: I will look into the LM749x as this might simplify the problem.

author: OwenH
author_rank: Contributor
timestamp: August 25, 2024, 06:51:43 pm
content: The switch that's picked out is only available in spst. I might be able to find a different switch but mechanically it may get a bit complicated. This switch is also only rated for 5v max. The switch also needs to be tiny like no bigger than 4mm x 4mm to fit mechanically.

author: Marco
author_rank: Super Contributor
timestamp: August 25, 2024, 08:48:59 pm
content: Any way, looking at the internal block diagram, H-gate is essentially a Power-Good output.The following is the logic you want, only question is how to implement it.if (priority==U1 & HGATE1==high) then EN2=lowif (priority==U2 & HGATE2==high) then EN1=lowPS. unfortunately HGATE rides on the output voltage of course, so it's awkward to use. Not hard to ground reference it, but that takes an extra transistor.