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Help with TPS60403--affecting other circuitry

TI Lady
Genius 4025 points
Other Parts Discussed in Thread: TPS60403

Need some tech assistance on the pretty simple part TPS60403.

Using it for at most 30ma supply, to be the negative rail for three op amps and the 250khtz is leaking into the laser diode optical output.

Have 4.5v going in, get negative 3.8v out loaded, which is fine.

If I leave the circuit on the board and just pull the output pin, and use an external negative linear supply, laser is clean.

Have the exact LC post filter shown in fig 25. Have since tried varying Lp and Cp without much change.

 Also the filtered output goes to a Murata 3 terminal 1uf cap for additional filtering.

 Haven’t seen the noise on the op amp output but added some filtering there just in case…..nothing.

 Data sheet didn’t say anything about ground plane, the component does share that.

 

Any suggestions ?

  • 0
    TI__Guru 71526 points

    Could very well be related to layout; we will need to see schematic and pcb layout/artwork.  If necessary, send to me directly.

    -Leonard

      

  • 0
    TI__Guru**** 150870 points

    Thanks for sending your design to Leonard.  I'm a little confused with what your issue is.  It seems like you see 250 kHz noise in your load.  And your load is somehow powered by the negative output.  Is this correct?

    If so, then we need to determine what sort of noise your load and application can accept.  Do you have a spec for what noise level you need to meet?

    Many customers follow up the inverting switching supply with a negative LDO to provide a cleaner supply for the load.  We have a few negative LDOs to choose from.  You can use the power quick search at power.ti.com to find them or visit the linear regulator forum.

  • 0 in reply to Chris Glaser
    Genius 4025 points

    The laser diode, which is NOT powered by the TPS part, but a regulated 7v supply is picking up the 250khtz switching frequency.

    The TPS is used for the negative rail of three op amps, the last of which has its output driving the voltage to current circuit for the laser diode.

    Was able to bypass the op amp output, and go direct to a 16bit DAC, which should have eliminated the pickup, but was still there at the same amplitude.

    Removing L1 inductor and thus not powering the op amps removed the noise pickup. (TPS part still on board, just no load)

    Our spec calls for < 0.2% RMS noise from 0 to 2Mhtz. So would need to reduce pickup by a factor of four at least.

    Having tried different values in the LC filter with no change, would infer noise is actually on the ground.

    The higher voltage laser diodes (>5v) show the pickup, but not as bad as the low voltage ones (2v).

    Received the eval board and other frequency parts to see if any other shows similar pickup.

    Thanks!!