How to adjust a Buck Regulator???

P

PhilipWarence

New Member

• Mar 20, 2015

• #1

Good day.

I have this Circuit using A8447. My output voltage is 5V.
My problem is I dont know where is(are) the divider resistor(s) to vary my output into 7.5V. Can somebody here help me how?
Also, do i need to remove the connection of Vbias and Vout if im going to target an output voltage higher than 5V?
TIA.

 

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P

PhilipWarence

New Member

• Mar 20, 2015

• #2

up

 

Ian Rogers

User Extraordinaire

Forum Supporter

Most Helpful Member

• Mar 21, 2015

• #3

I should imagine Header 3 is for an external pot to vary the output voltage... Without the pot it is a fixed output..
But I don't know why there are three pins.. Maybe you can have three different ranges???

 

Joe G

Member

• Mar 21, 2015

• #4

the data sheet says ;

VOUT. The output voltage is adjustable from 0.8 to 24 V, set by an
external resistor divider. The voltage can be calculated with the
following formula:
VOUT = VFB × (1 + R1/R2)

 

k7elp60

Active Member

• Mar 21, 2015

• #5

According to the data sheet the FB or feedback voltage is very close to 0.8V. And the calculation for the R1 and R2 is expressed by Vout=VFB(1+R1/R2). Using this formula I think there is a jumper connecting pins 1 and 2 of header 3. To change the output to 7.5 volts you need to change the values of R1,R2,and R3. The formula Vout=VFB(1+R1/R2) and be transposed to:
R1=(Vout/VFB-1)R2

 

Last edited: Mar 21, 2015

F

Flyback

Well-Known Member

• Mar 21, 2015

• #6

Yes its the FB pin resistors...but you are better off to bring the error amplifier outside the chip , and vary the reference to the error amp than switch resistors in and out.

 

E

eTech

Well-Known Member

• Mar 21, 2015

• #7

Hi

I don't think the schematic is correct. Here's a schematic that shows the way I think you want to configure the headers.
Also, VBIAS has a max input of 7v, so...only connect it if target output is between 3.3v-5v (as stated on the datasheet)

eT

 

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Last edited: Mar 22, 2015