LMX2594EVM: Voltage Drop between Supply Voltage and VCC

Lawrence Trevor1

Part Number: LMX2594EVM

Hi,

I am working with the LMX2594EVM and am having trouble getting it to produce an output. I used a multimeter and measured the VCC voltage between VCC_TP and GND_TP on the board, and found that when I supply 3.3V at the VCC_SMA connector input, I am only getting 2.85V VCC. This is presumably below the minimum voltage required to drive the chip, so could explain why I am not getting any function from it. Upon start-up, with 3.3 V supply, I am drawing 0.14 A from my power supply.

If I apply small changes to the input voltage, I get a proportional linear change in VCC, which seems indicative of some voltage division, but inspection of the schematic doesn't show me where this voltage division may be. I'm not current limited in any way - it is set at 0.6 A per the user manual.

I am supplying my power from a DC power supply, using banana plugs, then into the DC input of a Bias-T. The RF input of the bias-T is open, and then the RF + DC output is connected to a coax-SMA cable to the VCC input port of the EVM.

Can anyone indicate whether this may be a problem with the board, or perhaps my method of supplying power? (I could not find much data online about providing power over coax - I'm unsure if there may be matching issues.)

Also, I'm quite tempted to just bump up my supply voltage to 3.8 V, expecting ~3.23 Vcc on the board, but I thought I should figure out a proper explanation first.

Regards.

Lawrence

over 5 years ago

0 Lawrence Trevor1 over 5 years ago

Prodigy 90 points

Edit: I should say as well, that whilst I cannot get an RF output, I appear to be able to program the chip registers via the Reference Pro Board. After I apply some register settings, my current boosts to 0.27 A. I can trigger the POWERDOWN bit and I get 0 A draw, which I can then turn back on via the same bit.

If this is the case, and my original issue is the lack of RF output, could then my register settings be the issue? Or could this low voltage affect some functions of the chip but not others?

0 Noel Fung over 5 years ago in reply to Lawrence Trevor1

TI__Guru** 109000 points

Hi Lawrence,

Did the lock detect LED turn ON after programming all registers? Can you get RF output from channel B?

0 Lawrence Trevor1 over 5 years ago in reply to Noel Fung

Prodigy 90 points

Hey Noel,

No to both. I have the Lock Indicator switch in the ON position, plus set the MUXOUT_LD_SEL bit to 1. Not seeing output on any channel, nor the LED.

However, I will admit that I am not fully across the setting of the registers to get a valid output. I simply observed that low voltage on the board and wanted to rule that out before I wondered why my register settings were not working.

0 Noel Fung over 5 years ago in reply to Lawrence Trevor1

TI__Guru** 109000 points

Hi Lawrence,

maybe you should consider to use a short sma cable instead of bias-T to provide power supply to the board.

Suggest follow the instruction in the EVM user's guide in order to get a good start.

0 Lawrence Trevor1 over 5 years ago in reply to Noel Fung

Prodigy 90 points

Hey Noel,

I swapped out the Bias Tee for a direct Banana Plug - BNC Converter and that voltage drop disappeared (I had to turn the lab upside down to find it!). Thanks for the tip.

Regards,

Lawrence

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LMX2594EVM: Voltage Drop between Supply Voltage and VCC