CD4081B: Max output current with 5V power supply

Gabriele Pellegrinelli1

Prodigy 90 points

Part Number: CD4081B
Other Parts Discussed in Thread: CD4018B, SN74LVC1G08, SN74AHC08

Hi guys.

In my project I used the CMOS AND Gate CD4081B IC.

As you can see in the image below I place two NPN BJT (BC817-40) ad the outputs of two AND gate of CD4081B.

With 1kOhm base resistors, the current flow from pin 10 to the Q3 base is (4.6-0.7)/1k=3.9mA

I attend that the current flow from pin 4 is major than the above current, because that current supply Q3 base and the U5C inputs.

The datasheet of CD4018B say "Output High (Source) Current IOH min" is -1mA(typ) @25°C when the VDD is 5V (that is my case).

The problem is: how many current (max) can I sourced from the two outputs (pin 4 and pin 10)? I think the CD4081B can't give the current that I need.

BR,

Gabriele.

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 257000 points

The 1 mA value is typical, so it is not guaranteed. Depending on the temperature, you have to design for 0.5 or 0.4 mA.

CD4000 devices are optimized for higher voltages. Use something like the SN74AHC08, or a single gate that can drive both transistors like the SN74LVC1G08.

0 Gabriele Pellegrinelli1 over 2 years ago in reply to Clemens Ladisch

Prodigy 90 points

Thank you for your reply.

I can't change the IC, because the PCB is already made and the other ICs (for example TTL AND GATE) have a different pinout from CD4081B (TSSOP14).

So I think the best choice is to modify the base resistor value, from 1kOhm to 10kOhm. With 10kOhm I have (4.6-0.7)/10k=0.39mA. Furthermore with 10kOhm value I can drive correctly the NPN transistor. The load applied to the collector is a LED with 20mA current.

What do you think?

BR,

Gabriele.

0 Clemens Ladisch over 2 years ago in reply to Gabriele Pellegrinelli1

Guru 257000 points

Increasing the base resistor just reduces the current that you get out of it.

For a 20 mA LED, you want about 1 mA base current. Just use 2.2 kΩ or so; together with the CD4081B's output impedance (about 500 … 1000 Ω), this will come out right.

0 Gabriele Pellegrinelli1 over 2 years ago in reply to Clemens Ladisch

Prodigy 90 points

Hi Clemens.

But 1mA is above that the current I can sourced from CD4081B. We talk about 0.5 or 0.4 mA MAX at each channel.

So, I thought the correct base resistance value is 10kOhm. Isn't it so?

0 Clemens Ladisch over 2 years ago in reply to Gabriele Pellegrinelli1

Guru 257000 points

The specified current is guaranteed for the specified voltage drop. It is possible to draw higher currents with a higher voltage drop (see figures 8 and 10).

0 Gabriele Pellegrinelli1 over 2 years ago in reply to Clemens Ladisch

Prodigy 90 points

This is clear to me.

Since I want to use all the headroom of the CD4081B I think I will choose a higher value of the base resistor.

I did the test with 10kOhm resistance and it works correctly.

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CD4081B: Max output current with 5V power supply