3.3 Volt to 5.5 Volt Octal TTL line Level Conversion - Electronic Circuits Projects Diagrams Free

jbbjshlws · 7th October 2008, 02:17 PM

Hello, I have a microcontroller that outputs IO lines at 3.3 volts, but the chips i want to drive operate at 5.5 volt. I'm sure this is probably a simple task and (hopefully) their is a single IC someone could direct me to. I was thinking a octal buffer IC, Obviously latency is an issue.

Any help is greatly appreciated.

Josh.

blueroomelectronics · 7th October 2008, 07:42 PM

The 74HCT125 is often used as a level converter.

speakerguy79 · 7th October 2008, 07:48 PM

Is this for an I2C bus or other?

jbbjshlws · 8th October 2008, 01:06 AM

it is to drive TLC5940's (LED Drivers) off of a Parallax Propeller micro controller.

if i was to use the 74HCT125 chips, i would just use a pair of them to get 8 lines of buffered 5.5 volt is this correct?

also the outputs enable inputs (1OE to 4OE) i would just tie all of those to ground for the signal to go straight through, or would i put those onto a clock line?

Also my local electronics supplier can only get in the 74HC125's not the 74HCT125's, Im told the difference is the 74HCT125's are TTL compatible. should this matter? or will i get some from overseas?

Thank you in advace,

Josh

speakerguy79 · 8th October 2008, 01:42 AM

The HC will be fine. Enable lines can just be tied high or low as necessary, they are not clocked. You'll need two chips for eight lines.

Ubergeek63 · 8th October 2008, 01:53 AM

the T is for TTL input. there are 20 pin octal buffers. the other thing you can do is use a micro that has 5V tolerant I/O and just use pull up resistors.

Latency? that depends on what you are doing how much you can tolerate. Hell you can put a 7400 series on the output of your micro but you wont like how much power it uses or how "fast" it is if you actually NEED low latency.

Besides you are better off running theTLC5490 chip at 3V and terminating the LEDs at either 5V or 3V depending on the forward drops.

jbbjshlws · 8th October 2008, 02:40 AM

thanks for the replies,

according to the datasheet on the 74HC125's they are quad buffers jaycar.com.au/products_uploaded/ZC4842.pdf so i would need two, the circuit is operating at around 10Mhz, only reason i thought the latency would have been an issue is if the lines became un-synchronized.

the LED's from the TLC5940's are driven off of a separate power supply and input pin into the TLC5940's so the buffer is only for the io lines to control the chip.

I tried running the TLC5940's with 3.3 volts, and they seem to be a little unpredictable, i figured this is probably because they are at an indecisive point between whether it is a 1 or a 0, so in increasing the voltage should increase the definition and should in turn fix that issue. (hopefully!).

blueroomelectronics · 8th October 2008, 02:45 AM

The 74HC125 can easily handle 10MHz

jbbjshlws · 8th October 2008, 04:01 AM

Cool, ill try it out later this week when i have some time,

Thankyou heaps!

Cheers,
Josh

Ubergeek63 · 8th October 2008, 12:03 PM

according to the spec sheet the TLC5940 has a rather high "1" voltage of 0.8VCC, most are 0.7VCC, that the outputs are good for 17V max and deliver up to 60mA at VCC less than 3.6V and 120mA if more than 3.6V.

If you are running 15 outputs at 60mA with reds on them from 5V you will easily dissipate more than the 2.4W package rating. Maximum intensity is only available from individual power rails for each color used to minimize the dissipation in the driver.

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7th October 2008, 02:17 PM	#1
jbbjshlws	3.3 Volt to 5.5 Volt Octal TTL line Level Conversion Hello, I have a microcontroller that outputs IO lines at 3.3 volts, but the chips i want to drive operate at 5.5 volt. I'm sure this is probably a simple task and (hopefully) their is a single IC someone could direct me to. I was thinking a octal buffer IC, Obviously latency is an issue. Any help is greatly appreciated. Josh.

7th October 2008, 07:42 PM	#2
blueroomelectronics	The 74HCT125 is often used as a level converter. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

7th October 2008, 07:48 PM	#3
speakerguy79	Is this for an I2C bus or other?

8th October 2008, 01:06 AM	#4
jbbjshlws	it is to drive TLC5940's (LED Drivers) off of a Parallax Propeller micro controller. if i was to use the 74HCT125 chips, i would just use a pair of them to get 8 lines of buffered 5.5 volt is this correct? also the outputs enable inputs (1OE to 4OE) i would just tie all of those to ground for the signal to go straight through, or would i put those onto a clock line? Also my local electronics supplier can only get in the 74HC125's not the 74HCT125's, Im told the difference is the 74HCT125's are TTL compatible. should this matter? or will i get some from overseas? Thank you in advace, Josh

8th October 2008, 01:42 AM	#5
speakerguy79	The HC will be fine. Enable lines can just be tied high or low as necessary, they are not clocked. You'll need two chips for eight lines. Last edited by speakerguy79; 8th October 2008 at 01:47 AM.

8th October 2008, 01:53 AM	#6
Ubergeek63	the T is for TTL input. there are 20 pin octal buffers. the other thing you can do is use a micro that has 5V tolerant I/O and just use pull up resistors. Latency? that depends on what you are doing how much you can tolerate. Hell you can put a 7400 series on the output of your micro but you wont like how much power it uses or how "fast" it is if you actually NEED low latency. Besides you are better off running theTLC5490 chip at 3V and terminating the LEDs at either 5V or 3V depending on the forward drops. Last edited by Ubergeek63; 8th October 2008 at 01:53 AM.

8th October 2008, 02:40 AM	#7
jbbjshlws	thanks for the replies, according to the datasheet on the 74HC125's they are quad buffers jaycar.com.au/products_uploaded/ZC4842.pdf so i would need two, the circuit is operating at around 10Mhz, only reason i thought the latency would have been an issue is if the lines became un-synchronized. the LED's from the TLC5940's are driven off of a separate power supply and input pin into the TLC5940's so the buffer is only for the io lines to control the chip. I tried running the TLC5940's with 3.3 volts, and they seem to be a little unpredictable, i figured this is probably because they are at an indecisive point between whether it is a 1 or a 0, so in increasing the voltage should increase the definition and should in turn fix that issue. (hopefully!).

8th October 2008, 02:45 AM	#8
blueroomelectronics	The 74HC125 can easily handle 10MHz __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

8th October 2008, 04:01 AM	#9
jbbjshlws	Cool, ill try it out later this week when i have some time, Thankyou heaps! Cheers, Josh

8th October 2008, 12:03 PM	#10
Ubergeek63	according to the spec sheet the TLC5940 has a rather high "1" voltage of 0.8VCC, most are 0.7VCC, that the outputs are good for 17V max and deliver up to 60mA at VCC less than 3.6V and 120mA if more than 3.6V. If you are running 15 outputs at 60mA with reds on them from 5V you will easily dissipate more than the 2.4W package rating. Maximum intensity is only available from individual power rails for each color used to minimize the dissipation in the driver.