LT1936 Step Down Regulator Voltage out Calculation

[koba30]

Hi,

I made the calculation for the Output voltage of the LT1936 ic, but the voltage obtained is different of the voltage on the real circuit output.


This is the typical application, when I use the equation on this works fine the result is 3,288.


This is the equation for calculate R1, I solve for Vout.


Block diagram, the same configuration of the real circuit.


This is the real circuit, R1 is 33kohm, R2 is 27,05kohm the parallel of 150kohm and 33kohm resistor.
The Output Voltage that I measured is 5,407v but the calculated with the ecuation is 2,66v. 
The Output Voltage required must be 5v.

Can someone please help me to understand what is happening.

Thanks in advance

[Zero999]

That sounds right.

V_OUT = V_REF*(1+R1/R2)
V_REF = 1.2V
R1 = 33k
R2 = 27.05k
V_OUT = 1.2*(1+33/27.05) = 2.66V

The data sheet recommends R2 < 20k to minimise the effect of bias currents, but even with the worst case bias current of 300nA, you'd only get 8.1mV of error with R2 = 27k05.

Are you sure the resistors are the values you believe they are? Try measuring them.

[Ian.M]

Reading the markings on the resistors,, there's a 10K off on the left, then just above the chip, two 33K with a 15K in between them.  There is no 150K visible.

If R1 is the top 33K and R2 is the bottm two: 15K||33K, that would give a nominal output voltage of 5.04V

[koba30]

Hi,

I made a mistake, thank you very much Ian.M and Hero999 for your help, the correct value for the resistor is 15K 

I made calculations for many hours