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LV/5V level shifting using 74HC gates

I need to convert LV (3.3v rail) signals to TTL levels (5V rail). My present arrangement is to feed the LV to 74HC04 inputs, with a 5v supply on the

74HC04. That looks like it should be OK from the device datasheet, but I've experienced one runaway, the chip got smoking hot. Could this be caused by the fact that the inputs are not getting close enough to the rail voltage when they're high? Or should I be looking for some other cause?

Is there another recommended approach for this level conversion? Required rise/fall times are < 1uS, load is approx 1 LSTTL load. I'd really prefer to avoid SMD devices if possible.

Reply to
Bruce Varley
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A grounded emitter npn? several hundred ohms in the base and a few k ohms on the collector. Take the output from the collector... this inverts things. You may need a schottky diode from base to collector. (Baker clamp)

George H.

Reply to
George Herold

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Vihmin for a 74hc04 at 5V is ~3.5V so ..

you can get voltage trnaslators s with separate supplys for each side e.g. 74LVX3245

or you can use a 74hct04, Vihmin is 2.0V

-Lasse

Reply to
langwadt

HCT devices are good for that--TTL went away, so 3.3->5V level shifting is the only reason they still exist in any quantity. Alternatively, you can put a diode in series with the HC's VDD pin to drop its supply to

4.3V and get a bit more margin.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Right. With Vcc=5 and Vin=3.3, expect shoot-through currents in the sub-mA range for an HCT04.

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John Larkin                  Highland Technology Inc
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Reply to
John Larkin

Bruce Varley schrieb:

Hello,

it is not a good idea to feed a 74HC04 with a high signal of 3.3 V or even less. If the HC04 has a 5 V supply, the high level on input should be close to 5 V. A 74LS04 should be compatible with the logic levels of the LV signals, a 74HCT04 should be possible too. Just compare the output levels of the LV signal with the input specification.

Bye

Reply to
Uwe Hercksen

74LVC4245A

-OR-

Page 2 of this:

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...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

I follow the opposite levels.

Vcc on the 74HCT04 gets 3.3 volts and the outputs go to the lower voltage side.

The first line of this data sheet states:

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"The inputs include clamp diodes that enable the use of current limiting resistors to interface inputs to voltages in excess of VCC."

A simple series resistor between the +5V output from a microcontroller is all that's needed.

On the other side 3.3V output into the input of a microcontroller is more then high enough.

hamilton

Reply to
hamilton

So for really-o truly-o _TTL_ levels, you're already fine.

If you want 5V CMOS, of course, you're screwed.

(Jumping on the bandwagon)

74HCT. Or AHCT. Or just about anything with a "CT" in the name. 
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Reply to
Tim Wescott

Avoiding SMD isn't always possible, of course; one path that hasn't been mentioned yet, since you can tolerate relatively slow rise/fall times, is a biased transistor. These are one-input/one-output devices, and will always invert the logic. UNR4211 from DigiKey is typical.

There's still leaded parts available; buy 'em quick, though! Three terminals is typical (input, output, ground); you just add a pullup resistor on the output.

Reply to
whit3rd

The OP used mixed metaphors... "1 LSTTL load" and "feed the LV to

74HC04 inputs".

I posted several possible ways to address that. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

___ 5 VOLT o-----o|___|-o----------o Pull Up | | 5 V logic | |\ o o-----o-----o-| >O- Vdd 3.3 | 1200 |/ |/ o--o|___|-o----o--o| NPN | |>

|-----o----| o | | | | o---------------o | | IO | | | uC | | | | | | | | | | | | | | | |----------- -

Something like that?

Jamie

Reply to
Jamie

Cascodes are an excellent way to level shift; they work for signals and power (extending the voltage range of an already sufficiently high current switch), and they tend to go fast. If the base is biased inbetween rails, the logic level also provides turn-off current. PNPs work great for below-ground work, etc.

Tim

-- Deep Friar: a very philosophical monk. Website:

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"Jamie" wrote >

Reply to
Tim Williams

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It works, but for a simple 3.3-5 translation it would use more power and be slower than a HCT gate, NXP spec max 590uA per pin at Vin =3D Vcc -

2.1V

-Lasse

Reply to
langwadt

CMOS 3.3V LVC can drive 1 U.L. LSTTL directly, no translator required. Is that simplified enough for you?

Reply to
bloggs.fredbloggs.fred

simplified enough for you?

Fred, You missed...

"...arrangement is to feed the LV to 74HC04 inputs, with a 5v supply."

OP has very loose understanding of LSTTL vs HC and HCT.

3.3V, even into HCT, may not be adequate to quench overlap current.

In...

Date: Mon, 27 Aug 2012 09:52:23 -0700 Message-ID:

I posted multiple ways to attack the problem. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

that simplified enough for you?

NXP specs HCT, max 590uA per pin @ Vin = Vcc - 2.1V

first lots of parts

last two will use more current than HCT at wc

-Lasse

Reply to
langwadt

present

to

that simplified enough for you?

Sounded to me like the 74HC04 was his cut at level translating to drive an ultimate load of LSTTL. There's no way a HC anything looks like LSTTL loading.

Reply to
bloggs.fredbloggs.fred

present

I've

to

that simplified enough for you?

[snip]

ultimate load of LSTTL. There's no way a HC anything looks like LSTTL loading.

I'm not sure _what_ the OP really has ;-)

In my ASIC world I've been doing the device-level equivalent of...

Page 1 of this:

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Dating back to my (patented) USB work with Intel... ~18 years ago. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

The HC wont work but the HCT will as long as he can get his rise/fall times in the In...

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Chisolm
Republic of Texas
Reply to
Joe Chisolm

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