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LMZ31707 UVLO Question

Shareef
Expert 1985 points
Other Parts Discussed in Thread: LMZ31707

Hi,

I am using LMZ31707 for generating 3.3V from 12V. I would like 3.3V to start after another supply of 5V is stable. The Switcher which generates 5V does not have PGOOD signal. Hence in order to sequence 3.3V after 5V has reached say 4.5V, can I connect the UVLO circuit through R1/R2 to 5V of LMZ31707. The PVIN and VIN in this case is 12V. Table-11 in LMZ datasheet provides resistor values for RUVLO1 = 68.1K and RUVLO2 = 24.3

It is mentioned in Fig 40 and 41 of datasheet that UVLO Resistor divider is always connected to VIN or PVIN or both but there is no mention of a supply other than VIN/PVIN. Please let me know if this is fine.

Regards,

Shareef

  • 0
    TI__Mastermind 28020 points

    Yes, the UVLO divider can be connected to the 5V supply. The LMZ31707 will remain off until the UVLO threshold is exceeded.

  • 0 in reply to J Arrigo
    Expert 1985 points

    Thanks Arrigo. There are two possible approaches now -

    Approach-1:

    In this case, I am setting the UVLO resistor values corresponding to 4.5V externally derived from Table 11 (RUVLO1 = 68.1K and RUVLO2 = 24.3K) pulled up to 5V eventhough both VIN and PVIN are set to 12V. Is this still fine?

    Approach-2:

    In the 2nd approach, I can connect 5V to VIN of LMZ31707 whereas PVIN is still 12V. Now, my device does not need external UVLO because the device has internal UVLO of 4.5V (max). And sequencing is ensured because even if 12V PVIN is available before VIN the device will not start as a result of internal UVLO.

    Please suggest which approach could be the best one. I am thinking of going ahead of 2nd approach, request your suggestion.

    regards,

    Shareef

  • 0 in reply to Shareef
    TI__Mastermind 28020 points
    Shareef, Both approaches will work. I like approach #2 as well.
  • 0 in reply to J Arrigo
    Expert 1985 points

    Thanks Arrigo. Should it be fine to have PVIN being already stable much before VIN supply in case of split rail operation of LMZ31707?

    Thanks,

    Shareef

  • 0 in reply to Shareef
    Expert 1985 points
    Hi Arrigo,

    Please confirm.

    Thanks,
    Shareef
  • 0 in reply to Shareef
    TI__Mastermind 28020 points

    Yes, PVIN can be present before VIN. The device will remain off until a valid VIN in present.