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LMZ31503: Switching Frequency

Part Number: LMZ31503
Other Parts Discussed in Thread: LMZ31506, LMZ31710,

The LMZ31503 data sheet, as well as the LMZ31506 and LMZ31710 data sheets provide tables relating output voltage to required switching frequency (Table 2 on the 31503 data sheet).  We will have a number of ADCs on my board.  While the LMX31503 won't be powering the ADCs directly, I have never seen an instance where some of the switching noise didn't find its way into the output spectrum.  We would like to be able to tune the switching frequency to place the switching noise outside of the signal of interest.  The data sheet only provides specific switching frequencies for each output voltage, not a working range.  How can I know what range of switching frequencies can be used to produce each output voltage?

  • Hi Bruce,

    In the LMZ31503 data sheet, Table 7 specifies the range of attainable output voltages associated with each switching frequency. To maximize efficiency, you would want to select the lowest switching frequency that can provide your desired output voltage and that places the switching noise outside of your signal of interest.

    Best regards,
    Ryan Voogjarv
  • The table does not tell me what voltage I can maintain with a particular switching frequency that is not listed. For instance, can I get a 1.8V output with a switching frequency of 430 kHz? That frequency is just an example. We will want to experiment with a range of a couple 10's of kHz to see what out spectra look like. We need to know the limits on the frequency range for common voltages like 0.85, 1.2, 1.8, 2.5 and 3.3. I don't care that much about efficiency. I can't risk a supply dropping out and back powering a circuit from another supply, or worse, having a supply oscillate.
  • Hi Bruce,

    Table 7 shows provides the frequency range for different output voltage. The frequencies allowed are not limited to only what is on the table. For example if you PVIN = 12V, for all three output voltage 1.2V,1.8V and 2.5V, the synchronization frequency can be anywhere between 480kHz and 630kHz.

    If you don't care about efficiency, then you can simply provide PVIN=5V. This way, you can use a synchronization frequency between 330kHz to 780kHz because the table shows that Voutmin = 0.8V and Voutmax = 4.3V for that PVIN application. If you want to also provide Vout = 5V, PVIN=5V will not work otherwise.

    Regards,
    Jimmy