LM5112: synchronous buck converter

Charles Haab

Part Number: LM5112

Hello,

I have the following doubt:

Is it possible to use the LM5112 as a gate driver for a synchronous buck converter using a microcontroller?
If it is possible, how should I make the connections between power, microcontroller and LM5112?

Regards,

Charles Haab

over 5 years ago

0 Jeffrey Mueller over 5 years ago

TI__Genius 12990 points

Hi Charles,

Thanks for you question about lm5112.

This device can be used in sync buck, do you have any major system concerns?. You can follow the datasheet section 10 on layout, when connecting everything, uC can be directly connected to input of LM5112 and the outputs directly to the gate. Small Gate resistance can be used to limit switch node ringing and gate driver power dissipation.

Let me know if you have any questions.
Thanks,

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff,

Thanks!

However, my doubt refers more specifically to the use of the LM5112 to power the high-side mosfet of the synchronous buck.

Regards,
Charles Haab

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Hi Charles,

Everything in the datasheet and from last post is consistent with driving LM5112 on the high side of a sync buck.

Here is an app note that may clear up any concerns with driving the high side with a low side driver. Let me know if you have any more questions.

Thanks,

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff,

This schematic is correct, since my buck converter is not isolated?
The voltage VCC_LM5112 is supplied by an auxiliary circuit connected to the same reference of the converter as of the microcontroller.

Regards,

Charles Haab

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Hi Charles,

Thanks for your update.
Since the switch node changes voltage the highside LM5112 SW reference needs to be isolated from the uC reference. Isolation happens with the bootstrap circuit for the highside output but will need signal isolation or a level shifter for the input. Since the input ground is referenced to SW then the return current that comes from the uC would see the SW voltage and some current if not isolated. So we need a level shifter or signal isolation like ISO77xx to isolate the uC signal ground from the SW node power ground. This can be seen in Figure 3 in the app note above.

Does this make sense?
Thanks,

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff,

Thanks for you help!

I was again studying the application note you passed me and made the schematic attached.

In this schematic I have:

1 isolated 3.3V voltage source referenced to the microcontroller

1 isolated 5V voltage source referenced to the SW node

1 non-insulated 5 V voltage source referenced to the GND of the buck converter

1 non-insulated 10 V voltage source referenced to the GND of the buck converter

I took as reference figure 3 of the application note.

Thanks.

Charles Haab

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Hi Charles,

Are these back to back drain N-FETs?

Since the gates are the same but the sources are not theres no way to turn both FETs on at the same time with only one gate driver.

A more common approach is the back to back source in which both gates and sources are the same so they would turn on and off at the same time. If these FETs are actually back to back source N-FETs then you have the correct connections.

I am curious, Why do you need back to back FET (reverse current protection?) on this sync buck? whats your application?

Thanks,

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff,

Thanks,

My application is a charge controller for PV system. In this project I have low voltage and high current in the input and output and I need a reverse current protection (2 MOSFETS).

The MOSFET I am using is the N-channel IPT015N10N5.

I missed the connections in the schematic, the correct form follows below, where the two sources then connected to each other.

In this case, the driver is correct?

Thanks!

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Hi Charles,

Thanks for letting me know. Everything looks goods with your highside reference connections. Please let me know if you have any more questions.

Thanks,

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff,

As soon as possible I will test the circuit and update the post confirming or not the gate driver solution.

Thanks,

Charles Haab

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Sounds good, Thanks!

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Hi Charles,

any update on this circuit connection confirmation?

Thanks,

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff, how are you?

Not yet, I'm waiting for the printed circuit board to be milled in the laboratory of the Federal University of Santa Maria, Brazil.

Thanks Jeff!

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Hi Charles,

Thanks for letting me know, and thanks for asking!

Im in Dallas, but Brazil sounds good too, the weather there looks amazing on google.

Thanks,

0 Jeffrey Mueller over 5 years ago in reply to Jeffrey Mueller

TI__Genius 12990 points

Hi Charles,

hope everything is going good, do you have an update for me?
I will think everything is resolved otherwise.

Thanks,

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff,

I'm still waiting for the lab ... :(

Thanks!!

Regards,

Charles Haab

0 Charles Haab over 5 years ago in reply to Jeffrey Mueller

Prodigy 110 points

Hi Jeff!

I tested the circuit and it did not work. I made minor changes and it worked.
Note: I still tested the converter with no load and only 10 V at the input.

As soon as possible I will update the schematic circuit and send together some oscilloscope waveforms.

Thank you for your help!

Charles Haab

0 Jeffrey Mueller over 5 years ago in reply to Charles Haab

TI__Genius 12990 points

Hi Charles!

Thanks for the update sounds good,

Im excited to see what changes you made!

Thanks,

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LM5112: synchronous buck converter