Part Number: LMG5200
I would like to use the LMG5200 for an application that switch the drain power to an RF amplifier. I need to be able to provide operation at DC.
I ran the simulations using a power supply rail of 28V connected to Vin and could see that this would be a great part for my application if I can operate it down to DC.
If anyone has an easy solution that would allow me to operate this part at DC I would appreciate any assistance you could provide.
I assume that when you are referring to DC operation, you mean 100% duty cycle , i.e. constant on for the high-side FET.
If that is the case the structure of the LMG5200 does not support this directly as the high-side power is derived from a bootstrap from the low side.
In order to enable 100% duty cycle you would have to opt for one of the following:
1. use an isolated power supply
2. use a charge pump (it would have to have same voltage and slew rates as the operation point of the LMG5200 in your circuit)
3. use an intermediate capacitor with a bootstrap circuit connected to the switched node, this would require several more discrete components.
Please feel free to reach out with any further questions.
In reply to Alberto Doronzo:
Thank you for your response. I think I like the idea of the isolated supply. Since we will have many of these circuits I think that I can afford a boost converter for this function.
I tried this in the simulation with Tina-TI simulator. The simulation worked properly when I connected the HB pin to the external supply (33V) through a a 2.5 K resistor.
I was using a 28V supply for VIN. What would you recommend as a means of optimizing the resistor value?
Thank you again,
In reply to David Bennett1:
Yes I created an external (non-isolated)33V supply for the simulation. The supply was connected to the HB pin through a 2.5K resistor to limit the maximum current.
Based on the datasheet description the bootstrap supply and capacitor are only used for the High Side Drive. Also shown in the Block diagram.
The datasheet says that the design includes a proprietary clamp circuit which limits the High Side Drive to protect the High Side FET.
Granted I know that simulation models are not necessarily identical to the part, everything in the simulation seemed to make sense.
The HB pin when observed went from 33V to 5V at the appropriate times. The current through the resistor went from 0 to about 12 mAmps. Which also made sense.
I went this way specifically because of the write-up in the datasheet that said the device included the clamp circuit. There is extensive information on the clamp.
The datasheet also indicates that the HB pin is rated to 86 Volts to Agnd.
Is my interpretation of the datasheet incorrect?
I am assuming that the isolated supply you mentioned would have to be referenced, floated on the 28V supply return?
Could the isolated supply be a step-up converter that is connected to the 28V supply output?
I do appreciate your assistance with this application.
In reply to Robert Brown2:
just a couple of points I want to clarify:
1. The minimum pulse width for LMG5200 is 10ns, and you will be able to drive it reliably down to that level.
2. If you are driving at very high duty cycles, you will have to ensure that the bootstrap capacitor is large enough to maintain an acceptable voltage (at least above UVLO), during the entire on time for the high side.
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