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Original question:

LMZM33603: Cff for variable output; also output caps

LMZM33603: Operating frequency vs. efficiency and ripple

Stephen Fenwick
Intellectual 260 points
Part Number: LMZM33603

We are using the LMZM33603 with a nominal 1.4A (2A peak) load, 24V VIN to 12V VOUT. The input caps are one 150uF AlPoly (Panasonic EEH-ZC1V151P) and two 4.7uF 50V X7R 1210 (Murata GRM32ER71H475KA88L), and the output caps are three 22uF 25V X7R 1210 (Samsung CL32B226KAJNFNE).

We are currently running at 900MHz with a 44.2K pulldown on RT.

The MechEs are complaining about the heat from the regulators. I'm wondering if we can try running at a lower switching frequency in order to try to improve efficiency. There is a downstream regulator (over a short cable, maybe 6") that can operate down as low as about 11V. Ripple needs to be kept to a few hundred mV.

Assuming we can tolerate additional ripple and load transients, how much would the efficiency improve if we reduced the switching frequency to 600kHz or 400kHz? Would CISPR compliance be affected?

Thanks!

  • 0
    TI__Guru 50120 points

    Hi Stephen,

    Table 3 in the datasheet will help provide some guidance on the recommended Vout range for a given switching frequency. For your application of 24Vin/12Vout, it is recommended to run at least 500Hz and above since the Vout_max at 500kHz is 12.2V. This part meets EN55011 Class B Radiation EMI standards so I don't expect it to change with changing switching frequency. I've generated a design using TI WEBENCH Designer tool that can help you better understand the device.

    WEBENCH Designer Link: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=1BAABAB40B8B6987

    Regards,

    Jimmy