Electronic – IC Enable Pin not working as expected

digital-logick-buspic

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

Best Answer

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.