I am designing a DC-DC boost converter for Vin = 5V to Vout = 15V. To generate gate pulses, I will be using a NE555P Timer by TI - http://pdf1.alldatasheet.com/datasheet-pdf/view/161277/TI/NE555P.html. For astable operation, from the data sheet I realised the following data [Pg-7]:
Frequency (f) = 10 KHz, Capacitance (C) = 0.1uF, Ra + 2Rb = 1K, Duty Cycle (D) = 1 - (Vin/Vout) = 0.66 or 66.6%,
Priod = th + tl = 0.1ms,
Now, how do I calculate the resistance values of Ra and Rb for the desired duty cycle?. And what is the difference between Output driver duty cycle and Output waveform duty cycle?. Which one of these formulae can I use to find Ra and Rb satisfying Ra + 2Rb = 1K?
//Some Calculations I did until now to find Ra and Rb//
D = Rb/(Ra+2Rb)), 0.66 = Rb/1K, Rb = 660 ohms,
Ra + 2Rb = 1K, Ra + 2(660) = 1K, Ra = 1000 - 1320, Ra = -0.32K????-----------> Getting a Wrong value since it should not come negative!!,
//My next try to calculate Ra and Rb//
D = 1-(Rb/(Ra+2Rb)), 0.66 = 1-(Rb/1K), Rb = 340 ohms,
Ra+2Rb = 1k, Ra+2(340) = 1K, Ra = 320 ohms,
So, 320 +2(340) = 1000, satisfies Ra + 2Rb = 1K( But not getting 66% duty cycle).
So which one is correct? or my approach and understanding itself to finding the values of Ra and Rb is wrong?. Please help.