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Buck converter at 100%
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cv007:
I have a circuit with a couple pairs of p-channel mosfets that control output from a couple inputs (see attached picture). The inputs are 5v on one side which also powers a buck converter, and the other input is the buck converter output of 3v3. The mosfet 'outputs' are tied together to get either 5v or 3v3, or no output. The back-to-back mosfets prevent the 3v3 side from conducting via the body diode when the 5v side is enabled. The extra p-channel on the 5v side is not needed, but these are 2-mosfets in one package so just added it.

A micro controls the output selection via a couple pins in open drain mode. The buck converter enable pin is also controlled by the micro. The micro runs from 5v.

None of this is actually tested, only simulated somewhat in my (parts limited) simulation software.


Now, looking closer at the buck converter datasheet I noticed it has 100% duty cycle capability.
Maybe I misunderstand, but it seems I can eliminate my mosfet switching circuit-

buck converter EN low- internal mosfets off = no Vin->LX = 0V out
buck converter EN high- FB = R1,R2 (to get 3V3 out) = 3V3 out
buck converter EN high- FB = >0.3v (prevent over current protection I think), <0.6v (keep duty at 100%) = Vin->LX = 5V (less pch mosfet loss)
(dac pin could drive fb pin to something less than 0.6v to get 5v/max output, or just use the pin to provide a path to ground for another resistor in fb)

these are my current buck converter choices (just need up to 1A, easy to use, low parts count, and larger than a grain of sand)
Diodes Inc. AP3924A or AP3417C
https://www.mouser.com/datasheet/2/115/AP3429_A-766309.pdf
https://www.mouser.com/datasheet/2/115/AP3417C-271484.pdf

Am I missing something, or can the buck converter take over my switching duties? And am I correct in assuming just keeping FB below the reference voltage of 0.6v gets me 100% duty cycle (in these parts)?

Thanks.
MrAl:

--- Quote from: cv007 on January 12, 2019, 06:50:15 pm ---I have a circuit with a couple pairs of p-channel mosfets that control output from a couple inputs (see attached picture). The inputs are 5v on one side which also powers a buck converter, and the other input is the buck converter output of 3v3. The mosfet 'outputs' are tied together to get either 5v or 3v3, or no output. The back-to-back mosfets prevent the 3v3 side from conducting via the body diode when the 5v side is enabled. The extra p-channel on the 5v side is not needed, but these are 2-mosfets in one package so just added it.

A micro controls the output selection via a couple pins in open drain mode. The buck converter enable pin is also controlled by the micro. The micro runs from 5v.

None of this is actually tested, only simulated somewhat in my (parts limited) simulation software.


Now, looking closer at the buck converter datasheet I noticed it has 100% duty cycle capability.
Maybe I misunderstand, but it seems I can eliminate my mosfet switching circuit-

buck converter EN low- internal mosfets off = no Vin->LX = 0V out
buck converter EN high- FB = R1,R2 (to get 3V3 out) = 3V3 out
buck converter EN high- FB = >0.3v (prevent over current protection I think), <0.6v (keep duty at 100%) = Vin->LX = 5V (less pch mosfet loss)
(dac pin could drive fb pin to something less than 0.6v to get 5v/max output, or just use the pin to provide a path to ground for another resistor in fb)

these are my current buck converter choices (just need up to 1A, easy to use, low parts count, and larger than a grain of sand)
Diodes Inc. AP3924A or AP3417C
https://www.mouser.com/datasheet/2/115/AP3429_A-766309.pdf
https://www.mouser.com/datasheet/2/115/AP3417C-271484.pdf

Am I missing something, or can the buck converter take over my switching duties? And am I correct in assuming just keeping FB below the reference voltage of 0.6v gets me 100% duty cycle (in these parts)?

Thanks.

--- End quote ---

Hello,

100 percent usually means it can connect the input directly to the output for a 1:1 ratio input to output.
So that usually does not mean 0 to 100 percent.
The data sheet for that first one says the output is 0.6v min, so that's not completely off.
You could look to see if they have a shutdown pin though.
cv007:

--- Quote ---The data sheet for that first one says the output is 0.6v min, so that's not completely off.
--- End quote ---
When enabled. I would assume the minimum output is simply due to the internal reference at 0.6v.

It would seem odd to me that they have an enable pin, have control over the output mosfets so the high side (p-ch) could be turned off (via a built-in pullup), list the shutdown current as <1ua, and still let 0.6v out of the output pin when not enabled.

the 3429 lists 'Output Voltage: 0.6V to VIN', '100% Duty Cycle Operation'
the 3417 lists 'Adjustable Output Voltage: 0.6V to 0.9×VIN' , 'Low Dropout Operation at 100% Duty Cycle'

so it seems the 3417 has more loss when at 100% duty cycle, or maybe its just a little newer where they have more a accurate description.
the 'Low Dropout' and 0.9xVin does not inspire confidence

May have to just do trial and error.
MrAl:

--- Quote from: cv007 on January 13, 2019, 12:34:34 am ---
--- Quote ---The data sheet for that first one says the output is 0.6v min, so that's not completely off.
--- End quote ---
When enabled. I would assume the minimum output is simply due to the internal reference at 0.6v.

It would seem odd to me that they have an enable pin, have control over the output mosfets so the high side (p-ch) could be turned off (via a built-in pullup), list the shutdown current as <1ua, and still let 0.6v out of the output pin when not enabled.

the 3429 lists 'Output Voltage: 0.6V to VIN', '100% Duty Cycle Operation'
the 3417 lists 'Adjustable Output Voltage: 0.6V to 0.9×VIN' , 'Low Dropout Operation at 100% Duty Cycle'

so it seems the 3417 has more loss when at 100% duty cycle, or maybe its just a little newer where they have more a accurate description.
the 'Low Dropout' and 0.9xVin does not inspire confidence

May have to just do trial and error.

--- End quote ---

I dont think those two have an enable pin.
NiHaoMike:
They both do, listed as EN on the datasheets.
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