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I am using SN74HCT245 Buffer operating on VCC=3.3V. So, I am getting 3.3V at each output pin of buffer (B1 to B8).

But if I want to drive a load with 5V input, what options are available to me by keeping VCC=3.3V?

Is 5V pullup on output pin of buffer is valid option?

Please refer below image where I have connected pull up resistor R1 (10K) to 5V.

Buffer

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2 Answers 2

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You Have a few options here. I'm assuming you are driving a logic input since if you simply need to switch a higher voltage load (e.g. LED) then you should probably use a transistor instead of an octal buffer (OR - octal buffer with OC outputs, see below).

The first is the simplest

It is safe to drive 5V inputs with 3V3 logic levels

3V3 Output will drive a 5V logic level correctly on most logic families.

Most IC's today will output rail to rail to handle the less sensitive 5V inputs. You must make sure that the buffer or the higher logic family can only be configured as input.

You can use an "Open Collector" device

There are open collector octal buffers that will work as you want them to, by having a pullup to 5V on the output side will allow you to use a basic IC as a level shifting tranceiver

The device to look at is the SN74621A, but there are many other families

You can use a dedicated level shifting buffer chip

This is pretty easy if you only need one direction. Multiple manufacturers offer similar parts, searching "ic level shifter" or similiar should give you a lot of options

There is a "3V3 to 5V" octal tranceiver in the same "74xx" family - the SN744245

Addendum

All 5V systems will tolerate a 3V3 input safely, however certain logic families will have higher input thresholds for 5V logic. The last letter of the middle numbers being - *T for TTL typically refers to TTL compatible CMOS subfamilies. LVTTL (3V3) and compatible CMOS families (including LVCMOS types) will work with TTL (5V), but not with vanila CMOS logic.

from wikipedia

Note: there are many other subfamilies, (the middle letters), so care must be taken when mixing jelly beans, for a somewhat comprehensive list you can checkout this list from Texas Instruments

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  • \$\begingroup\$ Please mention which families (*T) are 3.3V-safe. \$\endgroup\$
    – CL.
    Feb 2, 2016 at 8:13
  • \$\begingroup\$ @Crasic Thanks. But, if I connect 5V pull up on output pin, Can this damage Buffer IC (since VCC=3.3V and output pin will see 5V which is > VCC ) \$\endgroup\$ Feb 2, 2016 at 9:16
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5 Volts on a pull up is still going to be 3.3V at the line. At best it isn't going to pull up anything. By the way this chip is designed to operate between 4.5V and 5.5V, so you are outside of the specification, however... the input high level logic is 2 (minimum), the output will be Vcc, so if you were putting 5.5 V into it, then its output levels would be 5.5V hint hint. If you are driving this with a microcontroller be careful not to reverse the bus direction you will burn out your pins.

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