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LV/5V level shifting using 74HC gates

Started by Bruce Varley August 27, 2012
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. 


On Aug 27, 9:56=A0am, "Bruce Varley" <b...@NoSpam.com> wrote:
> I need to convert LV (3.3v rail) signals to TTL levels (5V rail). My pres=
ent
> 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 b=
y
> 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.
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.
On Aug 27, 3:56=A0pm, "Bruce Varley" <b...@NoSpam.com> wrote:
> I need to convert LV (3.3v rail) signals to TTL levels (5V rail). My pres=
ent
> 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 b=
y
> 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.
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
Bruce Varley wrote:
> > 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.
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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
On Mon, 27 Aug 2012 10:18:57 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Bruce Varley wrote: >> >> 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. > >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
Right. With Vcc=5 and Vin=3.3, expect shoot-through currents in the sub-mA range for an HCT04. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators

Bruce Varley schrieb:

> 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.
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
On Mon, 27 Aug 2012 21:56:12 +0800, "Bruce Varley" <bv@NoSpam.com>
wrote:

>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. >
74LVC4245A -OR- Page 2 of this: http://www.analog-innovations.com/SED/LogicLevelTranslator.pdf -OR- http://www.analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf -OR Simply- http://www.analog-innovations.com/SED/MAGIC-BLOCK.pdf ...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.
On 8/27/2012 9:27 AM, Uwe Hercksen wrote:

> 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 >
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: http://www.nxp.com/documents/data_sheet/74HC_HCT04.pdf "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
On Mon, 27 Aug 2012 21:56:12 +0800, Bruce Varley wrote:

> I need to convert LV (3.3v rail) signals to TTL levels (5V rail).
My inner nerd says: Actually, TTL output high levels are significantly below 5V -- I can't remember whether it's 2.7V or 3V or what, but it's a stretch below 5. So for really-o truly-o _TTL_ levels, you're already fine. If you want 5V CMOS, of course, you're screwed.
> 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.
(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
On Monday, August 27, 2012 6:56:12 AM UTC-7, Bruce Varley wrote:
> I need to convert LV (3.3v rail) signals to TTL levels (5V rail).... > rise/fall times are < 1uS, load is approx 1 LSTTL load. I'd really prefer to > avoid SMD devices if possible.
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.