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LMZ31707: LMZ31707 R_rt selection and INH pin

aleca101
Prodigy 50 points
Part Number: LMZ31707

Hello there,

Two questions:

1 - For testing purposes on my application, I will have the LMZ31707 supplying 1.7, 1.8 and 1.9 V. I am looking at Table 2, 'Standard R_set Resistor Values', on page 11 on the datasheet and the R_rt recommended for 1.7, 1.8 and 1.9 Vout are 487, 169 and 169 kΩ, respectively. I was wondering what would be the consequences if I were to choose 90.9 kΩ for R_rt (setting the switching frequency @ 1 MHz) for all three Vout? 

2 - I want to control the INH pin to turn on/off the LMZ31707 and I am planning to connect this pin directly to one of the pins to a microcontroller (more specifically, PIC18F46J50). Would that be an issue? 

Thank you in advance for the attention and I look forward to hearing your feedback.

AAB

  • 0
    TI__Mastermind 28020 points

    Hi AAB,

    1- If you are operating from 5V input voltage, then setting the switching frequency to 1MHz is fine. However, if you are operating from 12V input, the highest I would recommend setting the switching frequency is 600kHz. See table 7 of the datasheet for allowable switching frequency based on Vin and Vout.

    2 - Yes, the INH feature can be controlled using a logic signal from your microcontroller.

  • 0
    TI__Guru 50120 points

    Hi AAB,

    1.) It depends on your input voltage. Please refer to Table 7 for a table of valid operating frequencies given an input voltage and output voltage range. If your PVIN is 12V then you may not be able to have a 1MHz switching frequency because of minimum on-time requirements. For PVIN of 5V, you can have an output voltage between 1.1V to 4.3V. Picking a higher switching frequency will result in more switching losses and lower efficiency. Because this is a power module with a fixed inductor, it is always recommended to pick the lowest available switching frequency for optimized performance.

    2.) I think this should be okay. Just make sure the microcontroller doesn't output more than 6V since this is abs max of the INH/UVLO pin and the pin is open drain.

    Regards,

    Jimmy

  • 0 in reply to J Arrigo
    Prodigy 50 points

    Thank you!