• State TI Thinks Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 49 subscribers
  • Views 785 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMH2832: The device draws way high currents (>450mA)

Youngho Suh
Intellectual 490 points
Part Number: LMH2832

Hi,

I have recently designed a board with multiple 5V devices such as LNA, switch and LHM2832. Total current draw expected is ~450mA @ 5V for all three devices. I use LT3045EDD for voltage regulator, which can produce up to 500mA. I found with LNA and switch only, it draws ~260mA @ 5V which is as expected. But with LMH2832 installed at the board, current soared to hit current limit (700mA) of tghe power supply and voltage dropped to ~2.6V.  There is no RF input applied, DC only testing. I wonder if IF amp oscillates. Would you please check schematic and layout as attached? 

Also, in what case the amp draws way high currents than expected 220mA? 

Thanks,

  • 0
    Guru 140200 points

    Hi Youngho,

    your layout differs considerably from the layout example shown in figure 64 of datasheet. Especially the VCC decouplings is suboptimal. So, yes, the device could oscillate.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 490 points
    is it possible to oscillate without RF applied? I can put DC decoupling close to DC pins but just wondering if there is any source to cause this high current draw.

    Thanks
  • 0 in reply to kai klaas69
    Intellectual 490 points
    Also, can we leave PDA and PDB pin open if we are not powering down? Or do we need to pull it down?

    Thanks
  • 0 in reply to kai klaas69
    Intellectual 490 points

    I just found AC coupled only at input.

    I  have input / output filter as shown below which has filter output impedance to LMH2832 is 150ohm differential while input impedance is 200ohm to match up with mixer output. (Mixer (200ohm) - (200ohm) filter (150ohm) - (150ohm) LMH2832 (150ohm) - (150ohm) filter (200ohm))

    Filter is LC tank and no DC block was made at both in / output of LMH2832. 

    Question here is:

    1. Do I need to put DC block by putting series cap at differential RF input?

    2. Do I need to put DC block by putting series cap at differential RF output?

    3. Do I need to put series resistor at differential RF output even if I have filter input impedance set to 150ohm?

    Would you please let me know about it?

  • 0 in reply to Youngho Suh
    Guru 140200 points
    Hi Youngho,

    yes, the chip can oscillate without any HF-signal applied, especially if the inputs and outputs are not terminated properly. You should ground the inputs if no HF-signal shall be applied. But use proper termination.

    Please have a look again at section 12.1 (layout guidlines) of datasheet. Read carefully the first phrase:

    "With a small bandwidth greater than 1 GHz, layout for the LMH2832 is critical and nothing can be neglected."

    The layout is really critically here! Not even the slightest mistake is forgiven. Every millimeter counts. Not only a ground plane must be used but also a power plane.

    I would not let float the power down pins but would connect them low ohmically to the desired potential.

    I have no idea what you mean with the input/output filter. Can you show us a complete schematic of your circuit?

    Kai
  • 0 in reply to kai klaas69
    Intellectual 490 points

    Hi Kai,

    Yes, layout should be changed. Also, PD pins will be grounded even if it was floating at Eval board schematic.

    Here is the full schematic near LMH2832 area in pdf and jpeg format. I have input and put filters before and after LMH2832. Data sheet 9.3 says input should be AC coupled only. If you see my schematic, I don’t have DC block at both in and output side but only filters. FYI, the filter has input impedance 200ohm and output impedance of 150ohm to be matched to LMH2832 input. Output side the filter orientation was flipped to match output impedance of it.

     

    I wonder if this filters without DC block and series cap matters.

     

    Would you please advise?

     

    Thanks

    LMH2832 schematic.pdf

  • 0 in reply to Youngho Suh
    Guru 140200 points
    Hi Youngho,

    input and output lines work with a 2.5V bias. So, yes, you need DC blocking at output, when you use a center-tapped transformer. And you need a DC blocking at the input lines as well. But if you have already DC blocking in the source, then you don't need DC blocking at the LMH2832.

    Termination resistances are missing at the output filter. Figure 61 shows what I mean.

    By the way, do you want to connect cables to the inputs and outputs? I ask, because termination resistances at inputs and outputs must match the characteristic impedance of cables.

    Kai