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Can the internal LDO output be connected while supplying VCC_VCO with an external regulator?

Category: Hardware
Product Number: ADF4371


I am currently powering the VCC_VCO pin from the internal LDO (pins 6 and 7 connected), but I am supplying the VCC_LDO pin from a 5V regulator (LT3045).  I realized after the board was fabricated that I should have either supplied the output of the external regulator to both VCC_LDO and VCC_VCO (tying pin 7 to pin 8 and disabling the internal LDO) OR connected the VCC_LDO to the 6V unregulated supply. 

Can I just short pins 6, 7, and 8 together and power them from the external regulator?  Will this damage the device?  Should the LDO be enabled or disabled in that case?  What about the fact that the LDO is enabled on startup? 

Also, with 5.0Vdc on VCC_LDO, I am getting 4.46V from the LDO and to the VCC_VCO pin.  Is this normal?  The data sheet says that VCC_VCO should be between 4.75 and 5.25 volts. 

In any case, the output of my N-Divider/2 MUXOUT setting is around 13.5-13.7 MHz, and will not lock to my 10 MHz reference signal. 


Dr. Jim Aarestad

University of New Mexico

  • Hi Jim,

    Have you got the boards and any process for this issue? Because we did not test this scenario connecting pins 6,7, and 8 together is a bit risky. I need to review the internal structure with the designers for this scenario.


  • Hi Jim,

    I checked your config with the designers and It looks for the bench test, your configuration can be tried. but for long term use, there are some reliability concerns so ı can't say there would not be any trouble. using the suggested application configuration can be the right solution.

  • Bakcil, 

    Thank you for your response.  My interim solution was to remove the external regulator from the board, jumper the unregulated 6V to pin 8, and use the internal LDO (pin 6) to power the VCC_VCO (pin 7).  But, for other reasons as well, it appears likely that I will end up needing to do a respin of the board, so will correct my original mistake at that time. 

    But I appreciate the response.