0
\$\begingroup\$

I want to turn ON and OFF a LM2596 based buck converter with a microcontroller GPIO pin. 5th pin of the LM2596 can be used to for that purpose.

Please have a look on the schematic.

enter image description here

My question is whether using resistor is R1 is sufficient here?

LM2596 can take upto 45 V, but allowance on ON/OFF pin is only up to 25 V. So, won't 5th pin see exactly Vin when MOSFET is turned OFF?

In datasheet, regarding the ON/OFF pin it has (on 4th page):

Allows the switching regulator circuit to be shut down using logic signals thus dropping the
total input supply current to approximately 80 μA. Pulling this pin below a threshold 
voltage of approximately 1.3 V turns the regulator on, and pulling this pin above 1.3 V (up 
to a maximum of 25 V) shuts the regulator down. If this shutdown feature is not required, 
the ON/OFF pin can be wired to the ground pin or it can be left open. In either case, the 
regulator will be in the ON condition.

Update

(After seeing answers from @Justme and @Andy, telling we can connect MCU pin directory to the ON/OFF pin)

I missed to add this in my original question.

I also want to make sure the converter is turned OFF by default until MCU turn it ON. Default behavior of ON/OFF pin is making the converter ON until it get logical HIGH.

So, if we connect MCU pin directly then the converter will start working soon as it got powered, until the MCU pull it UP. Thus I guess I have to use the MOSFET based controlling as I showed above, right?

\$\endgroup\$
4
  • \$\begingroup\$ I have been for years using Rpi 3V3 logic GPIO pin to switch on/off LM2596/LM2941 modules without any problem. You might like to look at pictures how I rewire the on/off pin: (1) "Rpi freezes every now and then, how to fix it with a watchdog (LM2941/LM2956 On/Off Switching)? - RpiSE 2019jun14": raspberrypi.stackexchange.com/questions/99584/…. \$\endgroup\$
    – tlfong01
    Jan 9, 2021 at 11:49
  • \$\begingroup\$ @Junaid What is the min and max range of Vin the circuit must operate? \$\endgroup\$
    – Justme
    Jan 9, 2021 at 12:11
  • \$\begingroup\$ @Justme, it will be powered with a 24 V supply. But I have plans to use same set up with 12 V and 36 V PSUs. So range would be 12V - 36V. \$\endgroup\$
    – Junaid
    Jan 9, 2021 at 12:16
  • \$\begingroup\$ If the default states cause a turn on at power up put an inverter in the signal control chain. If the MCU being powered off completely is a problem then you need to state this. \$\endgroup\$
    – Andy aka
    Jan 9, 2021 at 13:16

2 Answers 2

Reset to default
3
\$\begingroup\$

The datasheet you quoted yourself has the answer for you.

The LM2956 ON/OFF pin allows to be controlled by logic levels. There is no need for any extra transistors or resistors.

\$\endgroup\$
8
  • \$\begingroup\$ Would it make any issue if microcontroller using a separate supply? \$\endgroup\$
    – Junaid
    Jan 9, 2021 at 11:21
  • 1
    \$\begingroup\$ That would depend on what issues you are worried about, or then you left important information out from your question which would have caused different answers. \$\endgroup\$
    – Justme
    Jan 9, 2021 at 11:25
  • \$\begingroup\$ You are right. I initially concerned about the converter turn ON before MCU dictates it as default behavior of the ON/OFF pin is to turn ON the conversion. That is why I used R1 to pull it up by default. I forgot to mention that in my question! \$\endgroup\$
    – Junaid
    Jan 9, 2021 at 11:35
  • \$\begingroup\$ @Junaid Yes, well you get answers based on what you ask and what info you give - we are not mind-readers and don't know what yout are doing and why. Yes, the converter will definitely turn on if MCU is directly connected and has no power supply. \$\endgroup\$
    – Justme
    Jan 9, 2021 at 11:43
  • 1
    \$\begingroup\$ @Junaid yes because your Vin will be 12V to 36V and ON/OFF pin can take up to 25 only. Please look at figure 21 of the datasheet you posted for a simplest possible solution. \$\endgroup\$
    – Justme
    Jan 9, 2021 at 12:43
2
\$\begingroup\$

Pin 5 (on/off control) can be directly controlled from a logic supply and you don't need to use a MOSFET. Drive (for instance) a logic voltage of 3.3 volts to deactivate the LM2596 or, drive 0 volts to activate the LM2596. You don't need a transistor. Pin 5 current is 15 μA maximum so it's not going to be a bother to any logic driver such as from an MCU.

enter image description here

The data sheet informs you that you need to exceed 2 volts to turn-off the LM2596 and you need to go below 0.6 volts to guarantee it switches on.

\$\endgroup\$
2
  • \$\begingroup\$ Would it make any issue if microcontroller using a separate supply? \$\endgroup\$
    – Junaid
    Jan 9, 2021 at 11:21
  • \$\begingroup\$ If the MCU shares the same 0 volts then it's no problem. \$\endgroup\$
    – Andy aka
    Jan 9, 2021 at 11:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.