How to calculate bootstrap Resistor in MC34163 Buck configuration

Question

I am designing a DC-DC buck converter with MC34163DWG. I try to understand what value of resistor RB must I use at the buck/step down configuration and I also have questions about the CB capacitor. Page 10:

The datasheet says I need to make sure the capacitor's ESR (or an external resistor) to be of a value that limits the 7V internal Zener's current to <100mA. And then it gives me a formula to calculate the minimum CB value. Page 9:

So I need to know what the voltage will be across CB to calculate the RB value to use to limit the current to <100mA. As I try to calculate it, I made some assumptions and thoughts. Let me know if what I thought of is right:

I started from their CB(min) calculation:

They assume \$(I=4mA, ΔV=4V)\$, where I is the current flowing from the capacitor CB in pin 16 and its split in two: to the Zener and the 2mA current source. So their resistor RB is limiting the current at 4mA. Now why is \$ΔV=4mA\$? I assume because their Vin is 12V, Vzener is 7V so the voltage across the CB should be \$(12Vmax-7V=) 5V\$ minus losses + Rsc resistor Vdrop + RBVdrop so they assumed/measured 4V. From that calculation I understand that they need a capacitor large enough to keep current flowing through it as long as the switch is on.

Now to calculate my RB, at pins 14,15, the voltage will be \$Vin=21V\$ in my case.
So the voltage across CB will be \$(21V-7Vzener=) 14V\$. So I need to limit the current @14V to be less than 100mA. I assume 4mA must be enough (as the example's calculations with CB). The RB should be \$R=V/I=14V/0.004A=3k5Ω\$.

If my assumption is correct, the output voltage is independent of the calculations regarding the RB. In that case they could add a formula to the datasheet to calculate the \$RB=(Vin-7V)/~4mA\$.

1. I want to know if my assumptions/calculations are correct for the RB and CB.
2. Regarding the capacitor CB, why do they show a polarized capacitor with its positive side at the pin 16? pin 16 goes from 0 to 7V max, while pin 15 (aka the other side of CB) goes from 0V to Vin (I assume). I would use either a ceramic cap there or a polarized cap but opposite polarization to what they show.
   
3. What is the "0.02" written above the CB, RB?
   
4. Last but not least, how do I know that my bootstrap CB, RB are correct once I have my PCB soldered? Where should I probe on the circuit and what should I see on both cases (if my bootstrap is wrong and if my bootstrap is correct)?

Answer 1

I would keep the resistor value towards the minimum, so more like 150 or 200 ohms.

The average voltage on the right of Cb is 5.05V and the voltage on the left is between Vcc and Vcc+7, so the polarity is correct. They used a somewhat unconventional symbol for the Vcc connection that can be mistaken for ground at a quick glance.

0.02 is the capacitance- 20nF or 0.02uF.

You can measure the peak voltage across the resistor and test the circuit at the corners of min/max input voltage and output current to ensure there is typical margin.

Answer 2

@Sphero is right, I checked the Pin 16 (Bootstrap input) with the oscilloscope:

The voltage goes from 19V (my Vin) to 28V (Vin + 7V).

But you see there's ringing due to the fast/violent switching. I needed to use a bootstrap resistor of higher value (1k) to reduce the ringing: