Post Go back to editing

ADP5071RE-EVALZ board: Selection of RNR4

Category: Hardware
Product Number: ADP5071RE-EVALZ

Question about the ADP5071RE-EVALZ board:  Specifically, the rationale behind selecting the RNR4 noise reduction resistor for the ADP7182AYJZ (adjustable 1.2V version) -28V, -200mA Low Noise Linear Regulator.  

Reading the Data Sheet for the ADP7182 (p. 25), it states that for the adjustable 1.2V version of the regulator, the noise reduction resistor (RNR) should be chosen to be nearly equal to RFB2.  In Figure 87, RFB2 is 13 K-ohms and so RNR is chosen to also be 13 K-ohms as recommended.  

Looking at the schematic for the ADP7182 portion of the ADP5071RE-EVALZ board, the RNR4 resistor value is 619 ohms.  This is ~10.5% of the value of RFB4 which is 5.9 K-ohms.

I see in the Data Sheet for the ADP7142 (adjustable 1.2V version) 40V, 200 mA, Low Noise CMOS Linear Regulator (p. 16) which regulates the positive voltage on the eval board that the RNR should be selected as 1% to 10% of the value of R2.  This is illustrated in Figure 50.

This recommendation has been met as the RNR3 (noise reduction resistor for the positive regulator) with the 1 K-ohm value (exactly 10% of RFB3).

Before I start calculating and measuring, is there a reason (beyond what is stated in the Data Sheet) the value of RNR4 was chosen to be 619 ohms?

Thanks!



Correction: Product was quoted incorrectly (supposed to be ADP5071RE-EVALZ)
[edited by: sivads at 3:19 AM (GMT -5) on 27 Nov 2022]
  • I answered my own question after receiving the physical board:  The schematic for the ADP7182AUJZ LDO negative regulator portion of the ADP5071RE-EVALZ board has a wrong value component (also found in the BOM).

    In the ADP5070RE-EVALZ/ADP5071RE-EVALZ User Guide (UG-848, Rev.A), "Evaluating the ADP5070/ADP5071 DC-to-DC Switching Regulators/Converters" on p. 8, the RNR4 component should be 5.9 K-ohms (as measured on the physical board and recommended as the value selection in the Data Sheet for the ADP7182); not 619 ohms.  

    Also in Table 4 on p.12, "Evaluation Board Components—LDO Regulators and Miscellaneous Components", the RNR4 component is 619 ohms.  This should again be 5.9 K-ohms.

    I would assume that there are some folks like me that look at and evaluate these User Guide schematics before purchasing the physical evaluation boards.  This RNR4 value selection was confusing to me.

    I hope this helps someone planning to purchase this board.