Resistor Value

[Gregory]

I am building a Voltage multiplyer and I have downloaded the circuit from www.national.com.
I wish to change the Input voltage to 12 Volts DC and a output voltage of 32 volts DC.
I do not understand about the ratio of R1 & R2 and R5 & R6.
to change the voltage to the LM5002.
Can you explain this to me as how to work out the resistor ratio for this voltage and Current.

 

[mvs sarma]

I am building a Voltage multiplyer and I have downloaded the circuit from www.national.com.
I wish to change the Input voltage to 12 Volts DC and a output voltage of 32 volts DC.
I do not understand about the ratio of R1 & R2 and R5 & R6.
to change the voltage to the LM5002.
Can you explain this to me as how to work out the resistor ratio for this voltage and Current.

If you go thro' the datasheet, you will find on Page 11, the method of calculation for R1 and R2. The said value Idivider can be pre-assumed to be small and depends on Input voltage /(R1=R2)
Now the formula is quoted as below.
" R1=(Vpwr-1.26V)/Idivider & R2=1.26V/(Idivider+6uA)"


For the calculation of R5 & R6, we have to manage like this

"Calculate R5 and R6 such a way that (R6/(R5+R6))* Vout = 1.25"

Perhaps you may have to use a scientific calculator or Excel sheet for ease of computing.

Hope this serves your needs.

 

[k7elp60]

The ratio of R5 and R6 sets the output voltage. The voltage on the FB pin needs to be 1.26V from the voltage divider. As shown in the example the voltage across R5 = 45-1.26 or 43.74V To change the output to 32V I would use ohms law to recalculate the value of R5.
R1 & R2 are used for LV shut down. IF that isn't going to be a problem I would leave the values as stated.

 

[Gregory]

In the voltage devider selsctionR! & R2 and is arbitary current.
What current would I use in this formular.
The current in the R5 & R6 what current would I use in the formular.
I know ohm's law and how to use a scientific caculator. but I do not know what values to use to obtain the Current in both formular's

 

[k7elp60]

Well the way I read the data sheet, if you are worried about low voltage dropout then change the value of R1 and R2, if you are not leave the values the same.
If you divide the voltage across R5(43.74V) by the value of R5 you will have the current thru it. Then divide the new voltage (32) by this current it will give you the new value of R5, and leave R6 it's current value.