# What is a best way to get a 25% duty cycle square wave (5 V, 1 MHz) on a board?

I am trying to get two off-phase 25% duty cycle square waves(1MHz) to control a full bridge circuit.

Update:Sorry for the confusion here, what I mean was to have 2 square wave signals, and introduce 180 phase difference in between, off-phase is not the correct way to descirbe it. Please see picture below, green and red line represent these 2 signals, respectively:

The idea is to have everything on the board, instead of pluging in the external square wave signals.

I am not very experienced with this type of design so I have done some research on how to get a square wave circuit on board:

1. Use a function generator IC such as AD9833, and program it to output the square wave
2. Use a 1Mhz oscillator, a dc voltage source from battery and a comparator to generator the square wave: in my simulation program I did it as followed(not exactly 25% duty cycle but just something near):

3. Combine op amps to generator the square wave, an example from TI is shown below:

4. Use Micocontroller + PWM controller.

It seems to me the op amp option is very intuitive but it takes some time to be fully oscillated, thus I haven't seen any example that it is used for above 1MHz application. I need a sharp square wave(I have been simulating it with 10n rise/fall time) with defined duty cycle.

My question is, what is the commonly accepted way to do it? It will be really appericated if you can provide me with some examples or reference so I can study about it.

Kind regards, Suns

• Please explain exactly what you mean by "off phase". It would help if you could draw a timing diagram for the waveforms you need. Apr 23, 2020 at 13:40
• You could use a higher frequency crystal, at least 4MHz, and count for example the rising edges and create a 25% offset that way. No need for microcontrollers Apr 23, 2020 at 13:43
• Imo this is a task which is just begging for a cheap & simple micro like a PIC12F or ATtiny with an on-board oscillator. One IC with not much more than a power decoupling cap. Apr 23, 2020 at 13:53
• I have use controllers which have specific PWM for bridges. They have a programmable 'dead' time which makes steering a bridge easier. *(You have not mentioned dead time by the way, You might need it) Apr 23, 2020 at 14:54
• 4MHz oscillator and dividers. Apr 24, 2020 at 2:50

If you're considering this: -

Combine op amps to generator the square wave

Then, the LTC6992 from here appears to do what you want.

I am trying to get two off-phase 25% duty cycle square waves(1MHz) to control a full bridge circuit.

You might be interested in adding this circuit to the output of the LTC6992 then: -

Picture from here and original from here.

I mention the above because if you need to avoid MOSFET shoot-through in your H bridge, these could be useful additions.

• I've always kind of wondered how to manually make dead time, this is such a fantastic post for that little circuit note. Apr 23, 2020 at 15:43
• Thanks a lot Andy! super helpful, yes dead time will be needed for my case, I will definitely try this out Apr 24, 2020 at 6:45

This was not my own idea. I got it from @Swedgin's comment.

simulate this circuit – Schematic created using CircuitLab

Also, Thanks @StainlessSteelRat for suggesting an improvement.

Output Waveforms:

• @StainlessSteelRat, OP said they want "two off-phase...waves." Still hasn't said what "off-phase" meant. I interpreted that to mean two signals that are 180 degrees out of phase with each other. Apr 23, 2020 at 14:45
• Then take the $\bar Q$ output and save a NOT gate! Apr 23, 2020 at 14:48
• D'Oh! Yes. Thank you. Apr 23, 2020 at 14:49
• Thank you! you are intepreting my strange expression of "off-phase" correctly haha. I have updated my question for that part.... This solution looks good! I will look into it! Apr 24, 2020 at 6:42