Hi,

I'm trying to design a small switching power supply. I found LM2575 in a nearby shop, and it seemed nice and easy.

However, I found 2 datasheets, one of them (THIS ONE) contains thorough step by step instructions on how to choose the components, and the other one (THIS ONE) has fixed values for the components.

Which datasheet should I use? Do those two work for the same chip, or the LM2575's from different manufacturers are different?

I'd like to reduce ripple as much as possible, so I'd rather use the first one if it works, so if someone knows...

 

Usually, ICs with the same designation perform the same functions, although there will be differences in the characteristics from different manufacturers (such as leak current). For many ICs, you can use the datasheet from any manufacturer to design your circuit, but I would use the one from the manufacturer of the IC I have to any limits I need to observe.

TI has given you the most information to select your supporting components. Whereas Semtech has provided the information to show how the device is connected. It also states:

The above component selections will be adequate for most applications... Applications with Vout below 5V or above 24V may require component adjustment for maximum performance; please contact factory for application assistance.

It appears that Semtech printed the datasheet before they were able to determine the best components for a specific application.

 

So I use the first one and it'll most likely work ok?

 

The circuit will work.

Remember that there are slight deviations in the values of the components that you select. A 1kΩ resister with a tolerance of 1% can have a resistance of 990Ω to 1.01kΩ. So if you are relaying on a specific outcome from using the specified values, you probably will be off. Maybe by as much as 5%. Most circuit will work even with this deviation, but it is a good idea to use trim pots if a precise output is required.

 

Great idea! Thank you!

So, if If I want to make 3V3/5V regulator for microcontrollers, little trim pot + voltmeter is trustworthy enough to get the desired values?

 

Yes, but you don't need to be that accurate in this case. Most MCUs are designed to run off batteries. The coin cells may start with a voltage above 3.4V and drop over time. The MCU may still run with a battery at 3.0V. A supply of 3.2V ± 5% will be fine.

 

So, I could use both resistors and trim pots... Decisions, decisions....

What would you do?

 

Like I stated, since what you want to power is not sensitive to the actual value, I would just use 1% resistors and 5% capacitors so I'm not spending a lot. So before you decide on your actually voltage output, look at the requirements of the components you are powering. Select the voltage for the typical amount to power the device. Then select the resisters for that voltage output.

 

Hi,

I'm trying to design a small switching power supply. I found LM2575 in a nearby shop, and it seemed nice and easy.

However, I found 2 datasheets, one of them (THIS ONE) contains thorough step by step instructions on how to choose the components, and the other one (THIS ONE) has fixed values for the components.

Which datasheet should I use? Do those two work for the same chip, or the LM2575's from different manufacturers are different?

I'd like to reduce ripple as much as possible, so I'd rather use the first one if it works, so if someone knows...

National Semiconductor invented that part and they have a free design software tool for it. They were bought by TI and I assume they still offer the software.

 

Pfffff... Could someone help me, I've been following the datasheet and I cannot find the required value of the inductor...

I'll be feeding my regulator from 12V 18W wall AC/DC adapter, and I want my max output to be 12V 1A, so when I follow the formula for finding the inductor, I get that the E*T constant is not to be found in the graph for determining inductor value...

 

The software tool will give you a finished schematic with all component values. Spend the five minutes.

http://www.pcbheaven.com/opendir/index.php?show=276lp1543bl595b3c54

 

Thank you, will try that, couldn't find it myself.

How about minimum load, does this SMPS require it?

 

As far as I know, no minimum load required.