Buck converter (Step Down) circuit help

AlbertHall

Joined Jun 4, 2014
12,345
The equation itself is fine. If you are using it to calculate the value of R2 then you put in the known values which does not include R2.
Vout = vref * ((R1 + R2) / R2)
Substitute the known values:
7.5V = 1.25V * ((10k +R2) / R2)
And then solve that equation to get the value of R2.
 
The equation itself is fine. If you are using it to calculate the value of R2 then you put in the known values which does not include R2.
Vout = vref * ((R1 + R2) / R2)
Substitute the known values:
7.5V = 1.25V * ((10k +R2) / R2)
And then solve that equation to get the value of R2.
Thanks Albert so the question states to find the nearest preferred value from the E24 series so the closest being 10.
 

AlbertHall

Joined Jun 4, 2014
12,345
R2 isn't 2 Ohms.
The numbers on the E24 series can be multiplied by any power of 10. Thus 4.7 MOhms, 47 kOhms, or 470 Ohms all use the same entry in the E24 list.
 

AlbertHall

Joined Jun 4, 2014
12,345
There is another value missing to be able to calculate Vin. Either the value of RL or the load current is needed and is not given (at least in the part of the question that we have in here).
 
I think I've got figured it out. Duty Cycle, Vin and Vout are related. D=Vout/Vin and Vout=DVin. So I have 2 knowns which are D=30% and Vout=7.5V. So if Vout is 30% of Vin then Vin=25. To check 7.5/25=0.3333 which confirms the Duty cycle. To verify 7.5=0.3333×25. Vin=25V
 

AlbertHall

Joined Jun 4, 2014
12,345
I think I've got figured it out. Duty Cycle, Vin and Vout are related. D=Vout/Vin and Vout=DVin. So I have 2 knowns which are D=30% and Vout=7.5V. So if Vout is 30% of Vin then Vin=25. To check 7.5/25=0.3333 which confirms the Duty cycle. To verify 7.5=0.3333×25. Vin=25V
That's what I make it :)
 
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