Cannot switch off LM2576 regulator completely

Thread Starter

Happy Harry

Joined Apr 18, 2018
21
I am using an LM2576-Adj regulator to eventually provide a constant current supply at 2.16amps. The LM2576 is capable of running at 3 A and I am able to adjust the current through a 6 ohm load to the required 2.16 A. I need to be able to turn the regulator off and there is a pin (pin No 5) to accomplish this. With pin 5 grounded it runs quite happily at 2.16 A but if I take pin 5 high then it does not cut off the regulator completely but leaves a residual 0.27 A current.

Does anyone have any ideas why this should be happening?

Below is an image of the circuit I am using.

upload_2018-4-29_18-23-7.png

I forgot to say there are some alterations to the above circuit.
1) input is 24V regulated
2) D1 is a 5 A Schottky diode as is an additional 5 A diode placed between the positive side of the 1000uF capacitor and the top end of R2
3) I have used a 150uH torroidal inductance.
4) The output voltage measures 13.09 V
5) R1 is still 1.8K but R2 consists of a 10K fixed resistor in series with a 20K trimmer pot which was adjusted to provide the 2.16 A output current.
 

Thread Starter

Happy Harry

Joined Apr 18, 2018
21
No, I connected to Vout which is at 13V. I will try connecting to Vin+ tomorrow morning although I don't think that will help me when my final circuit takes shape as ultimately I wish to use a 6V output from an op-amp in a separate controller circuit.
 

Dodgydave

Joined Jun 22, 2012
9,923
I am using an LM2576-Adj regulator to eventually provide a constant current supply at 2.16amps. The LM2576 is capable of running at 3 A and I am able to adjust the current through a 6 ohm load to the required 2.16 A. I need to be able to turn the regulator off and there is a pin (pin No 5) to accomplish this. With pin 5 grounded it runs quite happily at 2.16 A but if I take pin 5 high then it does not cut off the regulator completely but leaves a residual 0.27 A current.

Does anyone have any ideas why this should be happening?

Below is an image of the circuit I am using.

View attachment 151554

I forgot to say there are some alterations to the above circuit.
1) input is 24V regulated
2) D1 is a 5 A Schottky diode as is an additional 5 A diode placed between the positive side of the 1000uF capacitor and the top end of R2
3) I have used a 150uH torroidal inductance.
4) The output voltage measures 13.09 V
5) R1 is still 1.8K but R2 consists of a 10K fixed resistor in series with a 20K trimmer pot which was adjusted to provide the 2.16 A output current.
Pin 5 needs 5V to shut it down, you can't use it's own output , you have to use the input voltage.
 

ebp

Joined Feb 8, 2018
2,332
The shutdown pin requires only about 1.4 volts to disable the switcher. With the pin connected to the output and the load in place, probably what is happening is that the switcher is just constantly cycling. When the output voltage is above 1.4 volts it is turned off. As soon as the output voltage decays below 1.4 volts, the switcher would be enabled. The output voltage rises, the switcher turns off, the output voltage falls and this just keeps repeating.

The shutdown pin current at HIGH is small, so you could easily pull it up with a resistor from the input supply and down with something powered by the output, being careful to protect the device that pulls it down from excessive voltage (which may actually be done inside the switcher chip, but if not, a small zener would suffice.)
 

Thread Starter

Happy Harry

Joined Apr 18, 2018
21
Thanks for your help everyone, It's all sorted now. Connecting to Vin worked eT and as you said Dave you cannot use it's own output. Your remarks were interesting ebp, There are so many weird things that can happen, some of which are covered in data sheets and some are not. In this case the data sheets made no mention of cycling on and off in peculiar circumstances and should this happen I expect that you will just see a lower meter reading as I did.
This regulated supply will be running a commonly grounded controller circuit to switch on and off the load on the LM2576 and will have it's own 6v regulated supply. I have partially constructed that and used it's 6v to activate the ON/OFF. That worked fine and is exactly what I need. Thanks again to everyone.
 
Top