Simplified Circuit

Buck Design Formulas

    The internal reference voltage is V_ref =1.245 volts.

    Peak inductor current is twice the maximum output current.

    The value for the current sensing resistor.    

    t_on and t_off > 10 ms    and    t_on + t_off < 50 ms

    Inductor size.

    Timing capacitor.

    Minimum output filter capacitor

    Converter efficency.

Buck Design Example (AN711)

 

V_in = 25 volts V_out = 10 volts

I_{out (max)} = 500 ma V_ripple < 1%

 

Step 1 Calculate the peak current

-= 1 amp

Step 2 Calculate the current sense resistance

Step 3 Calculate the ton/toff ratio

Step 4 Calculate the values for t_on and t_off

Since and

Let then

Step 5 Calculate the timing capacitor C_T

Step 6 Determine the inductor size

Step 7 Determine the minimum storage capacitor size

Step 8 Determine the feedback resistor sizes

(select 1.3k)

(use a 10k potentiometer)

Final Circuit

 

 

Waveforms