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I'm working with a TH3122 I/K-Bus transceiver with a built in 5v/100mA regulator. I'm trying to use my PIC24FJ1024GA606 to pull the EN pin of the TH3122 up/down in order to enable and disable the regulator.

Per the TH3122 datasheet, the regulator works this way:

The regulator is active and can only switched off with a falling edge on EN. The regulator remain with EN=high in active mode and therefore also the VCC voltage is active. The input EN has an internal pull down resistor. If EN=high, the internal pull down current is switched off to minimize the quiescent current.

I have the EN pin pulled up to 5v through a 10k resistor and the regulator turns on as expected. However, when I pull the EN pin down (via the PIC's LATCH for the pin), the regulator drops to 2.75v. I have verified that there are only a couple mV on the EN pin when it's pulled down.

What am I missing? For what it's worth, if I wire EN directly to 5v, the regulator drops to zero voltage as soon as it starts floating (5v removed).

Here are the electrical characteristics for the EN pin (min/typical/max/unit for the last four columns): enter image description here

P.S. I'm not an EE, so I apologize if I have missed any details.

P.P.S Here's the schematic around the part (GND4 floating isn't an issue anymore as I solder bridged it to another ground):

enter image description here

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    \$\begingroup\$ Do you have any other connections such as data lines going to what the regulator powers? Those will need to be taken low, too, as you generally can't have I/Os outside a part's supply voltage. \$\endgroup\$ Jul 24, 2019 at 23:09
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    \$\begingroup\$ @t3ddftw - Hi, (a) "I have verified that there are only a couple mV on the EN pin when it's pulled down" How did you verify that? DMM or scope? If you have a scope, please confirm that the EN signal really is a solid low when it should be. (b) When VCC = 2.75V, what does that look like on a scope? (c) "the regulator drops to zero voltage as soon as it starts floating (5v removed)" That test suggests that EN floating = regulator "off" (as expected). Have you tried changing the code on your PIC24 to set that output to a "Hi-Z" state, instead of driving it low, and then see what happens? \$\endgroup\$
    – SamGibson
    Jul 25, 2019 at 0:05
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    \$\begingroup\$ @SamGibson - Thanks! I don't have access to a scope, unfortunately. The DMM is what is showing a few mV on the EN pin. I have not tried setting the pin to hi-z -- I'll give that a shot! \$\endgroup\$
    – t3ddftw
    Jul 25, 2019 at 0:14
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    \$\begingroup\$ @t3ddftw - Thanks for the reply. (d) "The DMM is what is showing a few mV on the EN pin." I understand. However the problem is a DMM won't show rapid changes. It is easy to be misled by a DMM reading and expect that things are in a steady state, when in fact something more complicated is happening. (e) "I don't have access to a scope, unfortunately." That may affect the ability to troubleshoot this efficiently :-( If budget allows, you may want to consider buying one (you can buy a cheap one for the same cost as renting for relatively few weeks, these days). (f) Can you add a schematic? \$\endgroup\$
    – SamGibson
    Jul 25, 2019 at 0:32
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    \$\begingroup\$ @SamGibson - Ah, thanks for clarification! Yeah, I've been looking on eBay for a good deal on a digital scope, because you're right, I need one. Schematic has been added to the OP. Thanks again for your help! \$\endgroup\$
    – t3ddftw
    Jul 25, 2019 at 0:40

1 Answer 1

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I was able to figure this out by breaking it out onto a breadboard. Essentially, the PIC24's UART module is keeping the TX pin pulled high, which for some reason is impacting the EN pin on the TH3122 and keeping the voltage at 2.7v. The fix here is to disable the UART module before pulling EN low, then the TH3122's vreg shuts down as expected.

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