# Calculation for the Boot Strap capacitor

For design of a synchronous buck converter (which would be used to charge the battery) which would be driven by Micro-controller's PWM output, i selected the IRLB8748 NMOS Power transistors (for both high and low side switches) which would be driven by the gate IC NCP5351. As its my first time, while going through the data sheet of the driver IC NCP5351 (page 7, Figure 3) I found the concept of the BOOST capacitor. Searching more for it I found the purpose of its use and also the method to calculate its value.

So for the capacitor's value calculation purpose I followed this design document from Silabs and calculated the value of capacitor and the resistor which would be needed in my case.

But i have a few confusions, which i needed to clear:

(My application specifications are : Vin = 12V , Vout = 4.5 , DutyCycle(max) = 0.375 , Switching Freq = 62.5kHz).

1) In the Design document (at Page 4, 2.2 Design Example) the value of Ib (supply bias current) is needed for the calculation of Eq. 1. I found this value for my case to be 50uA, given in NCP5351's datasheet page 5 (given as VBST Quiescent Current, Operating). So is this the right value to be chosen for my case for Equation 1's calculation ?

2) For equation 2, the value of Maximum allowable Ripple was taken to be 5% of VDD. What significance does this value has ? I also did the calculations in my case using 5% for this value. I hope it works fine.

3) If we look at the NCP5351 's Data sheet (page 7, Figure 3), the connection of the resistor and the boot capacitor is not the same as it is in the Design document in Figure 1 and Figure 2. The resistor in the design document is placed in series with the Diode and the Capacitor, however in the NCP5351's datasheet page 7, the connection is a bit different. So can i calculate the values for my Resistor and the Bootstrap capacitor (which turned out to be 21.5 Ohms and 93.2nF respectively), and place them like they are in NCP5351's datasheet page 7 ?

4) In NCP5351's datasheet page 3, its written that In conjunction with a Schottky diode to VS, a 0.1 uF to 1.0 uF ceramic capacitor connected between BST and DRN develops supply voltage for the high−side driver (TG).. However the value for the Resistor is not given. Does that mean that i will select the value of the resistor which i calculated by the above mentioned Design Document ? (As the value of the capacitor that i caluculated came out to be 93.2nF which is approx equal to 0.1uF).

5) Also the main question, does This design document that i followed, provides the right way to calculate the Bootstrap Capacitor and Resistor values which would be needed in my case ?