I bought some 7-segment LED displays along with some BCD-7 segment decoders. The clerk said he didn't have the exact part I asked for and instead gave me a Texas Instruments CD4056BP. He said it was compatible but now that I've seen the datasheet, I'm not so sure as it describes the device family as a liquid-crystal display driver.

The datasheet makes no mention of use with LED displays, however the presence of the display frequency input suggests the IC can drive other display types

Quoting the datasheet, emphasis mine

The 7-segment outputs are controlled by the DISPLAY-FREQUENCY (DF) input which causes the selected segment outputs to be low, high, or a square-wave output (for liquid-crystal displays). When the DF input is low the output segments will be high when selected by the BCD inputs...

With that in mind, is it safe to directly drive the LED display from this device?

  • 1
    \$\begingroup\$ Due to low source current of those drivers, you shouldn't drive LED displays directly.Instead, you should control transistors with the chip and use them to drive the LED display. \$\endgroup\$ – AndrejaKo Nov 3 '12 at 9:03

Without additional driver ICs the 4056BPP is unsuited to driving LED displays.

At 25 degrees C it is capable of sourcing slightly under 1 mA with a 5V supply and capable of sinking about 0.5mA per segment. While this may produce a glimmer in your displays it is far below the drive level that is acceptable for something sold to drive LED displays - you would be entirely justified in returning them for a refund.

Logic wise, if they do what you want funtionally you could add output drivers. This could be by using ag a ULN2803 or ULN2003 type darlington transistor driver array ICs, or by using indifidual driver transistors - one per segment. The cost is not especially high, and you will learn worthwhile things along the way, but it should not be necessary.

What IC did you ask for?

Usually, suppliers like Mouser are easy to deal with, respond promptly, sell in small quantities and are reasonably priced if not always actually 'cheap'.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.