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Relationship of VDD and VCC

 

Looking at the schematics, I thought VDD was 3.3V and VCC was 5V but they seem to be interconnected like on the 74CBT3253 schematic power pins.

73 Kees K5BCQ
 

Vcc and Vdd have been mixed up since the start of commercial MOSFET acceptance.? SO DON'T QUOTE ME!!!?

And don't make fun of me, either.? If you know the real answer, share it. ?

As far as I have been able to figure out, they're the same -- voltage from one of the three transistor poles to circuit common,?but not to ground..? Vcc is the collector voltage; Vdd is the drain voltage. ? For our purposes they're the same, but with different names.? On datasheets, consider this to be the highest DC voltage that the pole can see,?bias not included.? Clearly, if you heavily bias a transistor, your Vcc / Vdd failure point can be significantly higher or lower than the datasheet's?"maximum voltage."?

This confusion is the second-biggest reason I retired from being a tech writer.? COVID-19, contracted?at work in April 2020, was the biggest reason. ?

If someone asks, I'll tell you the third biggest reason.? The statute of limitations on a violation of my nondisclosure agreement has long since passed.

73
Jim N6OTQ



On Wed, Aug 23, 2023 at 1:14?PM Kees T <windy10605@...> wrote:
Looking at the schematics, I thought VDD was 3.3V and VCC was 5V but they seem to be interconnected like on the 74CBT3253 schematic power pins.

73 Kees K5BCQ

 

Hi Kees

The component library names for things are sometimes different from the schematic net names. This is a case of that. The 74CBT3253 calls it's supply pins Vdd. They are on the +5V rail in QMX which happens to be called Vcc.

It IS all confusing...

The way I think of it - which I believe (perhaps incorrectly) to be the most common usage is:

Vcc is for bipolar transistors, C = Collector; going back half a century to 74-series TTL (transistor transistor logic) they ran off +5V so in my schematics Vcc is +5V.

Vdd is for CMOS/MOSFETs where D = Drain. And commonly or at least, often, ok, sometimes, this means a +3.3V supply. So in my schematics Vdd is +3.3V.

73 Hans G0UPL


On Wed, Aug 23, 2023, 9:14 PM Kees T <windy10605@...> wrote:
Looking at the schematics, I thought VDD was 3.3V and VCC was 5V but they seem to be interconnected like on the 74CBT3253 schematic power pins.

73 Kees K5BCQ

 

There is a lot of history to this with different technologies adopting Vcc Vdd etc as the 'high voltage' despite confusion in negative power supplies etc.

?

Many engineers prefer other simpler ways of identifying the key supply lines?
 

Hans,?

Yes, VDD vs VCC can get a little messy especially since the two appear to be connected together on the little IC402P/IC403P power pin schematic. My point was why is there not a 1N4148 blocking diode (or is that just a fixed voltage drop for the LDO) on both VDD power sources (the 3.3V LDO output D103 and the 3.3V output from the switcher Q111).?

Maybe I misread the schematic and VCC is not connected to VDD anywhere.

73 Kees K5BCQ
 

I found some info on this article which explains all V?? cases pretty good:


73?

Barb WB2CBA
 

Hi Kees

The 1N4148 diode in the 3.3V power supply is to isolate the 78M33 when the SMPS is being used and the 78M33 is off.?

Vdd and Vcc rails are NOT connected anywhere. The "Vdd" on the IC402P/IC403P names is just a remnant of the library component! I will change it in future to avoid confusion.??

73 Hans G0UPL



On Thu, Aug 24, 2023 at 2:23?AM Kees T <windy10605@...> wrote:
Hans,?

Yes, VDD vs VCC can get a little messy especially since the two appear to be connected together on the little IC402P/IC403P power pin schematic. My point was why is there not a 1N4148 blocking diode (or is that just a fixed voltage drop for the LDO) on both VDD power sources (the 3.3V LDO output D103 and the 3.3V output from the switcher Q111).?

Maybe I misread the schematic and VCC is not connected to VDD anywhere.

73 Kees K5BCQ

 

The way I deal with this is to name my supplies with the actual voltages, e.g., +5V, +3.3V, etc. If there are analog and digital supplies at the same voltage, I use +3.3V_A, +3.3V_D. There’s absolutely no confusion with this naming convention.?

Tony - AC9QY

On Wed, Aug 23, 2023 at 1:54 PM Hans Summers <hans.summers@...> wrote:
Hi Kees

The component library names for things are sometimes different from the schematic net names. This is a case of that. The 74CBT3253 calls it's supply pins Vdd. They are on the +5V rail in QMX which happens to be called Vcc.

It IS all confusing...

The way I think of it - which I believe (perhaps incorrectly) to be the most common usage is:

Vcc is for bipolar transistors, C = Collector; going back half a century to 74-series TTL (transistor transistor logic) they ran off +5V so in my schematics Vcc is +5V.

Vdd is for CMOS/MOSFETs where D = Drain. And commonly or at least, often, ok, sometimes, this means a +3.3V supply. So in my schematics Vdd is +3.3V.

73 Hans G0UPL


On Wed, Aug 23, 2023, 9:14 PM Kees T <windy10605@...> wrote:
Looking at the schematics, I thought VDD was 3.3V and VCC was 5V but they seem to be interconnected like on the 74CBT3253 schematic power pins.

73 Kees K5BCQ

 

On Wed, Aug 23, 2023 at 05:50 PM, <wb2cba@...> wrote:

I found some info on this article which explains all V?? cases pretty good:


73?

Barb WB2CBA


Link appears to be dead or broken

Bob
KG5GTE

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