# Maximum Voltage at CBOOT resistor and capacitor

I am using this Buck Converter to generate 3.3V output at 0.45A with Switching frequency of 2.1MHz.

Input voltage range is 6V - 16V. LM63625 2.5A Part is selected.

My questions :

Question 1:

In the datasheet, table 9.2 gives the value of the inductor and capacitors to be used. But there's this TI App Note for Buck Converter which helps to calculate the inductor value in section 3.

The Buck converter datasheet recommends to use 2.2uH inductor, whereas if I use the formula from the TI App Note, (considering 20% ripple current), I get an inductor value between 11uH to 13uH.

My question is why is there a difference between 10uH between the said value in the datasheet and the formula from the TI App Note? Which one should I follow?

Question 2:

Can someone tell me what would the maximum voltage across the Cboot capacitor 220nF? And how is it calculated?

• Edited my question. Since, my questions are related to each other, I asked them under a single title
– user220456
Commented May 16, 2021 at 17:57

16V in 3V3 out gives D=0.2

F=2.1MHz so Ton=95ns Toff=381ns

e=L di/dt so di=e*dt/L

With L=12µH, e=Vout, dt=Toff: di=100mA

So the 12µA value is correct for an output current of 450mA if you want 20% ripple current. You could use 10µH.

I guess the difference in inductance values comes from the fact the 2.2µH inductor was calculated for a much higher current of 2.5A. Designing for a target value of current ripple like 30%, with a higher current, a higher current ripple is allowed, which gives a lower inductor value. Notice in the datasheet they also increase the output cap, to compensate for the extra current (and therefore voltage) ripple.

But at 400mA, with a 2.2µH inductor, it would work in discontinuous mode. 12µH means it will work in continuous mode, which gives less output voltage ripple.

• Thank you for the answer. In that case, I'll use the 10uH inductor. Is it OK, if I take the same recommendations of the Input and Output capacitor according to the values mentioned in the Table 9.2 of the datasheet?
– user220456
Commented May 17, 2021 at 6:06
• Sure, also read top of page 28. If you increase the output cap, within reasonable bounds, you 'll get better transient response and less ripple. Also check price: one value is cheaper than several values due to qty discount, so two 10µF caps may be cheaper than a 22µF cap (also less inductance), then when you got 10µF in your BOM you can replace the input 4.7µF with 10µF.... Commented May 17, 2021 at 8:47
• Thank you for the clarification. I am taking 2x 10uF capacitors at the output and 1x 100nF capacitor at the Input. Is it OK?
– user220456
Commented May 17, 2021 at 9:56
• Datasheet says minimum Cin 4.7 µF + 220 nF, so 100nF is way too low. If you're buying 10µF 25V caps for the output, just put two on the input. If you're buying 10µF 10V caps for the output, that won't do on the input, so you need an extra BOM item for 4.7-10µF 25V input cap. If this is a personal project, get 10x 25V caps, it'll be cheaper. Commented May 17, 2021 at 12:06
• 100nF at the input sounds risky. It has to absorb the switching current. It's a 5 cents cap, don't spend hours thinking about whether or not you should put it in... Commented May 17, 2021 at 20:53

The maximum voltage on the BOOT pin, with respect to SW (which is exactly the voltage across the bootstrap capacitor) is given quite clearly on page 6 of the datasheet you linked: