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December 06, 2016, 10:47:55 22:47


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Author Topic: L4970 switching mode power supply  (Read 2636 times)
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mris99
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« on: May 19, 2009, 11:49:22 11:49 »

Hello,

I just made a switching mode power supply with L4970.
I used the suggested circuit diagram from the datasheet:
http://pdf1.alldatasheet.co.kr/datasheet-pdf/view/22500/STMICROELECTRONICS/L4970.html
Figure 34, except the following:
- I use 80uH inductor
- and a voltage divider made from 10k and 5,1k connected from the output to the feedback pin (11)

The desired output voltage is 15V.

It works well if there is load on the output,
but without load the output voltage sinks to about 7.5V.

I tried to put some resistor to the output to achieve small constant load,
but it could not help well. At some resistor values I can hear also periodic soft clicking
from the circuit.

Do anybody have experience with this circuit?

Thank you in advance,
Mris99
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abcsantosh
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« Reply #1 on: July 09, 2009, 09:48:39 09:48 »

    The circuit you mentioned is continuous conduction mode buck converter type. That's why you can't leave the output unloaded. You have to calculate minimum load resistor and put it on output. otherwise your feedback loop is began to oscillate and your output also starts to oscillate. Make sure that any security feature on chip is not cause this condition. read data sheet, application notes carefully. 
    There are many reason's also to oscillate the feedback loop. Google for stabilizing feedback loop and read the documents available. Many good application notes are available on internet. Due to large size of this subject, it is not possible to cover all conditions in this topic. Go step by step.
    Have a Good Day       
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tAhm1D
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« Reply #2 on: July 11, 2009, 06:15:07 18:15 »

Hi,
 No smps circuit will  function properly if minimum load is not applied. If you follow the datasheet and make the circuit accordingly, there will not be any problem. I have used this Ic of ST Microelectronics and this IC is very reliable and the circuit constructed with this IC is functional with all aspect. But one has to be careful and should have some knowledge regarding circuit/parts applied/functionability of smps circuit.
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