Help me to get 3V,1.5A output from LM2576HV-ADJ

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samson.bunty1986

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Dear all,

I tried one design,to get 3.3V,1.5A from variable input voltage 20V-28V through LM2576HV-ADJ. The thing is the output Voltage is taking time to reach 3.3V at no load condition. I have calculated Inductor and capacitor as per the datasheet of LM2576HV-ADJ. Kindly suggest me why the delay am getting for 3.3V???

Thanks in Advance.
 

Try using a dummy load. Load the output with a resistor to allow 25 to 50mA current load. See if that helps.

You can start with a 100-220 ohm resistor.

Hope this helps.
Tahmid.
 
Hi Tahmid,

Thanks for your reply.

I have some doubt, Does the output takes time in open condition??
 

Switching regulators are not supposed to have no load at the outputs. There is to be a minimum current draw at all times (very small though). I am not sure about the specific value for LM2576, but you can experiment to find out the required current.
 
Hi tahmid,

I have checked by connecting 100 Ohm at output side. Now am getting only 1.4V output.

Kinldly help me out from here.
 

samson.bunty1986 said:
Hi tahmid,

I have checked by connecting 100 Ohm at output side. Now am getting only 1.4V output.

Kinldly help me out from here.
Click to expand...

You probably did not construct the circuit properly. You should post your schematic. 100 ohm resistor should draw about 33mA. That should not cause the output voltage to drop.

What is the inductance of the inductor you used? Also, did you use a large enough output bulk capacitor?
 
Last edited:

Check following...
-unregulated supply impedance is low enough to give needed current to regulator
-Cin is 100µ or bigger.
-Cout is big enough.
-Inductance L1 has right value ( Not critical) .
-Schottky diode, or any of the suggested fast-recovery diodes
-R1 between 1k and 5k

As in any switching regulator, layout is very important. Rapidly
switching currents associated with wiring inductance
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.
Single-point grounding (as indicated) or ground plane construction
should be used for best results. When using the
Adjustable version, physically locate the programming resistors
near the regulator, to keep the sensitive feedback wiring
short. ( page 10)

Read note below pages 11...17

LM2576/LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator
https://www.ti.com/lit/ds/symlink/lm2576.pdf
 
Also make sure to use a good quality low ESR capacitor

Alex
 

Dear all,

Here is my circuit. Kindly let me know is there any changes to do. As per the datasheet i have selected the inductor(based on required output current).

Thanks in advance.
 

Attachments

  • 3.3 Conversion.pdf
    6.9 KB · Views: 99

The schematic seems okay. Did you measure the inductance of the inductor? Are you sure it's 150uH?
 

Hi Tahmid,

In the datasheet of LM2576 they have given the selection for Inductors. As i require 1.5A output at input max 50V, i decided to go with 150uH. Is am correct?If not please clarify.

Thanks in advance
 

That's okay. I meant that, when you used the 150uH inductor, did you measure it to make sure that it's 150uH?
 

is it possible to vary the output using micro controller 7 to 20V?
 

Do u mean output voltage???

---------- Post added at 15:14 ---------- Previous post was at 15:13 ----------

Sorry,the output is fixed to 3.3V
 

Try using different values of resistance and load and tell us what voltage you get at the output. Try with 47Ω, 220Ω, 330Ω and 1 kΩ. Post the corresponding voltages. Do you have any other inductor at hand? If so, try with that and let us know the output voltage for each of the above resistances used as loads.

Hope this helps.
Tahmid.
 
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