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Old 12th Oct 2018, 3:13 pm   #1
astral highway
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Question Quick query: IC with 2 ENA pins and at least 2 input / outputs

Pretty much as it says up there: operating frequency is around 300KHz.

Obvs I can ignore spare gates.. It doesn’t need to source or sink current. I’m away of motor drive control chips with ENA pin but I doubt these are fast enough.

I have a couple of capable chips here but these would be way too expensive and elaborate this simple job.

A uk -available part ideal to avoid China supply delays .

Cheers folks
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Old 12th Oct 2018, 3:43 pm   #2
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

7400?
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Old 12th Oct 2018, 4:26 pm   #3
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

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Originally Posted by merlinmaxwell View Post
7400?
Thats a stalwart of a chip! Something great about using such a work-horse

So ENA is just by using one input per gate?

A=1, ENA (B)=1, Q=1?
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Old 12th Oct 2018, 4:45 pm   #4
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Almost, 7400 is a quad nand, A=1, B=1 so Q=0, use another of the four to invert the output.

It was the easiest gate to fabricate on silicon and all you need is nand to implement any logic function.
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Last edited by merlinmaxwell; 12th Oct 2018 at 4:47 pm. Reason: Added another bit
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Old 12th Oct 2018, 7:31 pm   #5
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Thank you; that’s a great observation , too!
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Old 21st Nov 2018, 3:57 pm   #6
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

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Originally Posted by merlinmaxwell View Post
Almost, 7400 is a quad nand, A=1, B=1 so Q=0, use another of the four to invert the output.
This is going to be tricky to test as a sub-assembly. In the original circuit, a flip-flop receives square pulses at the drive frequency from one source and an enabling pulse on its CLR pin from another. Complimentary gate drivers meant it was possible to drive them (the gate drivers) without needing to invert the signal to one of them.

The 7400 is an additional complication. If I test drive that circuit with two drive signals at different frequencies (I have two to hand) then Y will only be 0 when the two signals are HiGH simutaneously. It will keep changing state when they are not, and that will still happen when I invert the original output state with another gate.

It sounds like I'm talking myself into realising that I can only test the 7400 circuit with the flip-flop in situ.

Alternatively, I could ask the question, what's the easiest way to test the 7400 sub-assembly for proof that it's working as designed?

I promised to check each sub-circuit as I build, in order to avoid headaches later. This is really important for the logic board, even if it isn't that complex.
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Old 21st Nov 2018, 4:55 pm   #7
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Quote:
It will keep changing state when they are not
It shouldn't, both high = output low any other state = output high.
Quote:
and that will still happen when I invert the original output state with another gate
Yes, but the other way up.

Are you using a 'scope with line alternate multiplexing?
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Old 21st Nov 2018, 5:08 pm   #8
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Line alternate multiplexing:

Ah, no, that doesnt feature on my scope. Sounds intriguing !

Also I think I need TTL level signals into the 7400?
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Old 21st Nov 2018, 7:19 pm   #9
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Quote:
Also I think I need TTL level signals into the 7400?
TTL is fine with 0..5V inputs on a 5V supply, the output of a classic TTL (not CMOS) won't go all the way up (or down).
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Old 21st Nov 2018, 9:22 pm   #10
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

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Originally Posted by merlinmaxwell View Post
Almost, 7400 is a quad nand, A=1, B=1 so Q=0, use another of the four to invert the output.
So, just checking...

for example with reference to the diagram - we're dropping the UCC37321 and 37322 and replacing them with gate drivers with no ENA pin.

We're doing this with a 7400.

...pins 1A and 1B of 1/8 of a 7400 output to pin 1Y. 1Y is input to pins 2A and 2B connected together. Their output, 2Y is the inverted output we need for the first gate driver.

This is repeated for the second gate driver, using the other two gates of the 7400, and this is how we have our 'virtual ENA' pin on the complimentary pair of gate drivers.

Segment of original circuit showing J-K flip-flop connection to the gate drivers that I'm not using.

(Not relevant to this post, but for context: CLR senses a one-shot pulse in overcurrent conditions. It resets the flip-flop on the next peak of the drive signal if the fault condition has cleared. If there is no fault condition the drive signal continues to be PWM modulated by an ‘interrupter.’ This merely pulses on and off at a variable low frequency to reduce the duty cycle of the power devices.
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Last edited by astral highway; 21st Nov 2018 at 9:26 pm. Reason: Added schematic
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Old 22nd Nov 2018, 12:25 am   #11
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Al,

I'm not sure exactly what you are trying to do but I assume you are looking for a simpler way to replace the UCC37321/2 driver IC's ?

74 or 74S, 74LS etc has an unsuitable output stage as they only sink current well and cannot source it well and the output voltage when high is only around 3.6V on a 5V supply rail. If you were driving an actual fet gate your driver has to be able to sink & source current well to be able to charge and discharge the power Mosfet's gate capacitance, if used as a gate driver. If you went to 74HC series IC's you get the rail to rail output swing and it would help, and if you are driving that from TTL you should have a pullup resistor on the inputs, or use the 74HCT series IC's.

Looking at those UCC37322 IC's they have a transistor assisted mosfet output stage so clearly they were designed for a very low Z output and handle high peak currents and a high capacitive load on their outputs. The circuit you have shows them driving a coil/transformer, but I see from diagrams they often feed heavy capacitive loads in power supplies and they could drive mosfet gates well directly. Perhaps this was a circuit where some high voltage isolation of the drive was required.

To make a circuit you can enable you can use the Nand as suggested, perhaps 74HC00 then it might or might not be better to invert that with a 74HC14 schmitt trigger inverting buffer to be sure there are sharp rises and falls on the outputs. But whether the output stage of a 74HC series IC is up to your task, I cannot be sure.

Hugo

Last edited by Argus25; 22nd Nov 2018 at 12:33 am.
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Old 22nd Nov 2018, 10:19 am   #12
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Hi Hugo,

Sorry it if wasn’t clear but absolutely not intending to use the little 7400 as a gate driver. I’m dropping the gate drivers in the circuit shown and instead using beefier devices, TC4451 and TC4452. These can source 23A pulses to quickly charge a large gate capacitance - 20 or 30nF - typical of the larger IGBT half-bridge bricks (Qg=Ig x t).

Some people parallel the driver chips shown but this isn’t very satisfactory at all, and one of a parallel pIr can easily end up doing more work or see more noise. They ‘only’ source around 6-9A each, ideal for directly driving largish MOSFETs but a bit under-powered for physically larger devices with 2-3 times more gate capacitance.

So the additional circuitry I’m describing isn’t to provide a substitute for a gate driver, it’s to provide a signal stage with a virtual enable pin to these two burly gate drivers. A gate driver -driver! The TC4451 and TC4452 don’t have an enable pin.

I have already built the gate driver power supply capable of providing big gulps of power to the IGBT drivers, , designed and built the huge gate drive transformer, and am now working on the logic circuit . Outsmarting the missing ENA pin is mandatory!

Thank you for you alternative solution with the hex Schmitt trigger inverting buffer; I like this chip a lot. I’ll look again at the input waveforms that a TC4451/2 can cope with.

Cheers
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Old 22nd Nov 2018, 2:24 pm   #13
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Default Re: Quick query: IC with 2 ENA pins and at least 2 input / outputs

Quote:
Originally Posted by merlinmaxwell View Post
Almost, 7400 is a quad nand, A=1, B=1 so Q=0, use another of the four to invert the output.

Ta-da! I've found this, which will simplify things a bit, quand AND, from favourite supplier TI: CD74HC08, an indirect descendant of the original 7400.

I know it's a tiny extra fiddle removed but I think I'll go with it. http://www.ti.com/lit/ds/symlink/cd74hct08.pdf
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