LM4051-N: Equation wrong in datasheet.

May

Genius 9880 points

Part Number: LM4051-N

Hi Team,

We received a query from customer and noticed that the R1 and R2 are reversed in equation (7) and (8) of the datasheet.

LM4051-N

https://www.ti.com/lit/ds/symlink/lm4051-n.pdf#page=13

Kindly confirm if the equation is correct or wrong. If it is correct, it is much appreciated also if you can provide explanation why it came up in that formula.

Thank you and looking forward for your kind response.

Regards,

Maynard

over 4 years ago

0 Ben C over 4 years ago

TI__Guru 56345 points

Hi Maynard,

Equation 7 and 8 are correct. To obtain equation 7, the current through R2 is (VREF/R2). The current through R1 is (Vo-VREF)/R1. If you set both of those equations equal to each other, you will get equation 7.

Equation 8 represents a more accurate equation that includes the current that flows into the LM4051.

Ben

0 Joe Norris over 4 years ago in reply to Ben C

Prodigy 70 points

Hi Ben,

You need to take a second look at this.

The Vref is measured from the + side of the regulator to the FB pin, not from the GND side to the FB pin. The voltage divider is thus R1/(R1 + R2) = (1 / (1 + R2/R1)).

I happen to build a board with R1 = 5.76K and R2 = 2.94K and measured Vo = 1.85 volts. I swapped the resistors and got the expected Vo = 3.6V which is what I planned for this circuit (Vi = 5V, 140 ohm Rl).

Using your methodology, the current through R1 is Vref/R1, not Vref/R2. The voltage across R2 is thus R2 * Vref/R1. Total Vo is then Vref + R2 * Vref/R1 = Vref * (1 + R2/R1)........adjust for the leakage into FB accordingly.

Notice that Figure in section 8.2 "Functional Block Diagram" shows Vref measured from the + side to the FB pin, and the orientation of the PNP transistor supports that. Clearly, Vref is the voltage from Vo to FB.

Thank You,

Joe Norris