Please ask about the problem in TI official application manual - boost principle

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Please ask about the problem in TI official application manual - boost principle [Copy link]

Does TI have calculation documents for step-down DC chips and LDO principles?

There are doubts about the two formulas

1. Which formula for calculating duty cycle is correct?

Which formula is correct, D=(Vo-Vi)/Vo or D=1-Vi*n/Vo (where n is efficiency)?

2. Does the switching peak formula include step-down and step-up?

In buck, we see Isw=Iload+IL/2 (where IL is the ripple current), but in boost, we see the formula Isw=Iload/(1-D)+IL/2 (where IL is the ripple current), so is there a distinction between buck and boost?

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answer:

The calculation with efficiency is more accurate, but we usually follow the previous calculation, which is more concise. Generally, the efficiency is about 90%, so the difference between the two calculations is not much.

The calculations for boost and buck are of course different, the topy is different, and the difference is huge.

1. Not all of them are accurate. For example, if it is a buck circuit, working in CCM mode, the duty cycle formula is based on volt-second balance and has nothing to do with efficiency.

2. This is related to the topology. For different topologies, the relationship between peak current and output current is different.

alan000345

Thank you for sharing. Both duty cycle calculation formulas are used. The answer you shared is still very classic. The first formula is the most basic in the textbook.