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Recently I've read a book that says it's absolutely necessary to use a linear voltage regulator like LM7805 in circuits that use any of 74HCxx chips (CMOS logical gates). I thought that this advice is kind of odd (LM7805 requires 9V instead of 5V I most frequently use, 4V are used just to heat an air) and decided to re-check.

I didn't find this requirement in any of 74HCxx specs, at least in specs from Texas Instruments which made chips I use. This requirement is also not mentioned in Wikipedia or any other source. All circuits I tried work just fine without LM7805, with only 100 uF capacitor as a filter. So I'm thinking, maybe this advice is outdated (the book was published in 2010) or maybe author just needed an excuse to give an example of linear voltage generator usage?

In your opinion is LM7805 required to use 74HCxx? Do you use it in your circuits?

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    \$\begingroup\$ It does not NEED a LM7805, but it does need to be regulated within the specified tolerance (2~6V) and should have <2% ripple if you want good immunity from conducted noise \$\endgroup\$
    – Tony Stewart EE75
    Commented Dec 20, 2016 at 7:43
  • \$\begingroup\$ @TonyStewart.EEsince'75 I'm sorry, but could you please give a reference to some reliable source that supports this statement regarding 2% ripple? \$\endgroup\$
    – Aleksander Alekseev
    Commented Dec 20, 2016 at 8:29
  • \$\begingroup\$ Does that book also mention CD40xx devices? \$\endgroup\$
    – CL.
    Commented Dec 20, 2016 at 9:00
  • \$\begingroup\$ @CL. the book is "Make: Electronics" (1st edition). I intentionally didn't mentioned it since it's quite a famous book and I didn't wanted author's authority to affect the answers. This book mentions CD40xx chips as well. \$\endgroup\$
    – Aleksander Alekseev
    Commented Dec 20, 2016 at 9:36
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    \$\begingroup\$ Does he say "linear, not switching", or does he just say you need a regulator because HC does not work with voltages as high as those for CD? \$\endgroup\$
    – CL.
    Commented Dec 20, 2016 at 10:15

5 Answers 5

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There is no requirement that would require a linear regulator for 74HCx. You should read the family spec to assure yourself.

Read section 5 on the voltage ratings.

In particular look at Figure 16 which shows the family VCC relationship.

If you are using 74HC devices then they are rated for operation from 2 - 6 V ....but if you are using 74HCT then the device is only rated for 5 V +/-10% ....many do confuse this. The 74HCT devices can be mixed with 74LS TTL devices, while the 74HC are not guaranteed to interoperate with the same fanouts when mixed.

If you are mixing 74LS and 74HCT devices, you may have difficulties meeting the +/-5% VCC tolerance of the 74LS devices (ripple from switching regulators) but should have no problems for 74HC devices.

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  • \$\begingroup\$ 5% on a 5 volt line is +/- .25 volts, or 0.5 volts pk-pk. If you're actually using a switching regulator with that much ripple - well, words fail me. \$\endgroup\$
    – WhatRoughBeast
    Commented Dec 20, 2016 at 14:26
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    \$\begingroup\$ @WhatRoughBeast. I tend to agree that ripple values will tend to be low, but it does depend on circumstances. I suggest you join TI's Webench and compare some solutions: ti.com/lsds/ti/analog/webench/overview.page While the higher frequency Buck solutions are low ripple there is a cost, and some ripple values approach 500 mV. ....so words may actually fail you. \$\endgroup\$
    – Jack Creasey
    Commented Dec 20, 2016 at 16:25
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A regulator of some kind is advisable. It doesn't have to be the ancient 7805. Anything that provides a clean 5V should be OK. Provide good decoupling and make sure the regulator can provide enough current for the circuit.

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  • \$\begingroup\$ What is a good decoupling cap value? \$\endgroup\$
    – user2497
    Commented Jul 13, 2017 at 7:29
  • \$\begingroup\$ @user2497: Depends on the regulator and the circuit you are powering. Check the regulator datasheet, and use what it says and any notes for special cases. Use decoupling caps in your circuit very close the the power connections on each chip/critical circuit section. \$\endgroup\$
    – JRE
    Commented Jul 13, 2017 at 7:34
  • \$\begingroup\$ Yes, I have the datasheet for the 7805 right here. Was wondering if a 1uF would be acceptable to catch the +1 to +2 volt spike on the 7805 when it is powered up? \$\endgroup\$
    – user2497
    Commented Jul 13, 2017 at 7:36
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Do 74HCxx chips require a linear voltage generator?

No you don't.

The operating supply voltage range for the 74HC series is quite large, see Mario's answer: 2 to 6 V

The 74HCT series is more critical as it is TTL compatible, I found here that the operating supply voltage range is 4.5 to 5.5 V.

You can meet these supply voltage requirements without using an LM7805. There are plenty of other solutions (linear and switching supplies) which will be able to generate the required supply voltage.

Using an LM7805 is just one possible solution to get the required supply voltage. In a world without the LM7805 I can still use the 74HC or even 74HCT series logic.

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All the answers here are correct. I still wanted to add mine because I am pretty sure this is written in the context of a comparison of 74xx families versus CMOS 4000 (and I guess this book must be rather old, too).

Indeed, the ancient CD4000 family could work with higher voltages (up to 15V). So you could directly power them with a 9V battery, for example. Whereas the 74xx families, in a similar application, absolutely need a regulator to drop the voltage due to the 5V rating. And at that time, regulators other than linear were less commonly found.

So this explains why it could have been formulated this way.

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Most likely you don't need a voltage regulator if you can ensure that the voltage doesn't get too high.

From the datasheet:

enter image description here

enter image description here

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