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Working with the TL2575..

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si2030

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Hi there,

I am looking at using the TL2575-12 to generate 12 volts from 15volts.

IT seems relatively easy to set up except for the inductor. There are thousands of inductors on the market and in the datasheet they give three manufacturers SCHOTT, PULSE ENGINEERING and RENCO ELECTRONICS. I live in Australia and want to use Element14 to purchase parts. This supplier doesnt stock these lines but does stock thousands of inductors...

I want the freedom to pick from a range of inductors based on the datasheet.

This component operates at 52KHz so I need and inductor that will handle that frequency.

How can I choose the right inductor for this part based on (apart from the inductance which is part of the design) filters they have:

Inductance
Inductance Tolerance
DC Resistance Max
Q Factor
DC Current Rating
Self Resonant Frequency
Q Test Frequency

I am in the dark about what to focus on in order to pick an inductor that will work for the TL2575..
Put another way given the inductance is 220μF how do I confidently select a switching inductor?

Kind Regards

Simon
 
220uF is a capacitance not an inductance, dont you mean 220uH.

Selection is a complicated process, manufacturers spend a lot of effort on selecting the right choke.

Important considerations are that the resonant frequency of the inductor is preferably a lot higher than what your switching freq is, and the saturation current is important the peak current from the switcher needs to be less than the sat current of the inductor, sometimes you have to measure this to see what it is.

If its a one off get a few types and try them.
 
Last edited:
Hi there,

I am looking at using the TL2575-12 to generate 12 volts from 15volts.

IT seems relatively easy to set up except for the inductor. There are thousands of inductors on the market and in the datasheet they give three manufacturers SCHOTT, PULSE ENGINEERING and RENCO ELECTRONICS. I live in Australia and want to use Element14 to purchase parts. This supplier doesnt stock these lines but does stock thousands of inductors...

I want the freedom to pick from a range of inductors based on the datasheet.

This component operates at 52KHz so I need and inductor that will handle that frequency.

How can I choose the right inductor for this part based on (apart from the inductance which is part of the design) filters they have:

Inductance
Inductance Tolerance
DC Resistance Max
Q Factor
DC Current Rating
Self Resonant Frequency
Q Test Frequency

I am in the dark about what to focus on in order to pick an inductor that will work for the TL2575..
Put another way given the inductance is 220μF how do I confidently select a switching inductor?

Kind Regards

Simon

Hi,


The inductance tolerance reflects the accuracy of the inductance value.
A 200uH, 10 percent inductor can be 180uH to 220uH.

The DC current rating must be higher than the peak current your design
will encounter. The design procedure should include an estimation of
this value. Sometimes the max current causes a decrease of inductance
to some degree so this has to be taken into account along with the
tolerance. An inductor of 200uH might change to 180uH at full current,
and with a tolerance of 10 percent that means the minimum inductance
for this 200uH unit could be as low as (about) 160uH. This of course
means that sometimes you'll have to get an inductor that is of slightly
higher value than you are looking for.

The DC resistance represents the loss of the coil. Higher DC resistance
means lower efficiency. It partly depends on the output current you are
looking for here though. 0.1 ohm is probably the highest you want to
go unless it is a very low current or very high current switcher. For
a lower current switcher 1 to 10 ohms might work, while for a higher
current switcher you might want to look for lower than 0.1 ohm. For
a 1 amp output for example 0.1 ohms is going to be ok.

The rest doesnt matter as much.
The Q factor for most inductors should be high enough for a switcher. The
test frequency is just the frequency that the Q was tested at.
The self resonant frequency should be higher than your operating frequency,
which is only 52kHz, so this shouldnt matter as much for this design either.

So the more important aspects are:
1. The inductance (must be the correct value)
2. The change in inductance with peak current (must always meet min inductance requirement)
3. The DC resistance (not too high)
 
Hi MrAl

Hey many thanks again for your input... getting there. I have the high side NCP5181 working now as I built a demo board a couple of days ago (For others this is to do with the post on Grounding a power supply and filtering). This post related to constructing part of a power board for the various DC power rails the Pre-reg and low voltage require... will post an update when I have something meaningful :)

Cheers

Simon
 
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