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TPS81256EVM-121: Output voltage looks noisy

PATRICK OHARA
Genius 3185 points
Part Number: TPS81256EVM-121
Other Parts Discussed in Thread: TPS81256, BQ24075, TPS61027, TPS61022

Hello,

The input of the TPS81256-EVM is provided by a BQ24070X-EVM. When powered from either the battery or USB, the output of the TPS81256 is 5.0V as expected. The TPS81256 will drive a laser diode whos performance (low optical noise) is important to the proper function or our instrument. We're concerned about the electrical noise on the output of the TPS81256 shown in a DC couple state (fig 1) and AC coupled state (fig 2). Do you have suggestions to improve the output noise performance of the TPS81256?

Fig 1

fig 2

  • +1
    TI__Genius 9046 points

    Hi Patrick,

    This is a fixed feature of TPS81256 and is not a noise when it is operating in PFM. If you expect to reduce the amplitude of the output ripple, maybe you could try to increase output capacitor to reduce the amplitude. If you would like to make sure the ripple frequency does not change regarding different loading, you could use device which has FPWM mode. I am checking if there is any FPWM devices which meet you requirements. Could you please share me with your input voltage range, output voltage and output current? Thank you.

    -Wenhao

  • 0 in reply to Wenhao Wu
    Genius 3185 points

    Hello Wenhao,

    Thank you for your quick attention to this. I'm thinking of filtering the output to reduce the ripple. We use a MOSFET to turn on/off the laser which draws about 15mA. The gate is driven by I/O from a CC2640r2 BLE part at 1.0KHz. The input voltage is from the Bq24075. The voltage is provided by a single cell Li-ion battery. This can range from 3.3V to 4.2V.

    Perhaps you can suggest a suitable filter circuit or alternative to the PTS81256.

    Thank you,

    Patrick

  • 0 in reply to PATRICK OHARA
    TI__Genius 9046 points

    Hi Patrick,

    Maybe you could look at TPS61027 or TPS61022, which integrate FPWM mode and has low output ripple regardless of load. 

    -Wenhao