[SOLVED] 4094 shift register current consumption for driving LEDs

[blakes7]

Hi,

I am developing a PIC application to light upto 60 LEDs by using the 74HC4094 shift register from NXP.

I would like to know the following –

Can all 8 pins of a 4094 shift register supply 25mA at the same time ?

If so, how long can the 4094 supply 8x25mA of current on its output pins without
being destroyed by heat ?

Is it necessary to operate the 4094 outputs in pulsed mode ?

Any help in this matter is appreciated.

Thanks

blakes7

 

[FvM]

Can all 8 pins of a 4094 shift register supply 25mA at the same time ?

Did you read the data sheet? The specification is 50 mA for VCC respectively GND pin. So the limit per pin is about 6 mA. And I don't see, how "pulsed mode" would increase the overall LED intensity.

 

[blakes7]

Hi FvM

Yes I have read the data sheet. The Texas Instruments datasheet says it can source 25mA per pin amd the VDD to VSS current is 50mA. But I am confused here. When the datasheet says 25mA per pin does that mean that all the other output pins can source 25mA per pin as well at the same time ?

Let me clarify about the "pulsed" mode. The OE pin can be toggled to reduced the current comsumption if the 4094's start to get hot. Will I need to do this if I am driving LEDs ?

I have built a 16 LED output board with x2 4094's. The current limiting resistors are 180R. All 16 LEDs can be lit up by the PIC and the 4094's do not get hot.

blakes7

 

[FvM]

When the datasheet says 25mA per pin does that mean that all the other output pins can source 25mA per pin as well at the same time ?

You also have to keep the 50 mA specification at the same time. This means, if the outputs are driving unipolar current (as it's obviously the case with LEDs), only two outputs can source 25 mA at the same time. That's simple and clear, I don't understand what you're further guessing about this theme.

So far the specification. If you experienced that the HC4094 can manage 8 LEDs with 16 or 20 mA each without a problem, that's operation beyond the specification. It's a zone, where you can't expect answers from the datasheet.

Let me clarify about the "pulsed" mode. The OE pin can be toggled to reduced the current comsumption if the 4094's start to get hot.

That doesn't clarify anything. How do you know that the "4094's start to get hot"? Will you mount a sensor? Of course a duty cycle reduces the thermal load. But the maximum current spec doesn't refer to duty cycles.

Will I need to do this if I am driving LEDs ?

The datasheet won't tell. (See above).

 

[blakes7]

Hi FvM

Thanks for your insights. The 4094 is now more clearer to me. I have measured the current sourced by the 4094 on my LED output board. I measured the voltage across the 180R current limiting resistors when all the 16 LEDs were lit up. See below.

Vo (from 4094) = 4.06V
V (across LED) = 2.07V (Vf=2.3-2.6V, If=15mA)
therefore current out of 1 pin of 4094 = 1.99/180 = 0.011A = 11mA

Also on my benchtop power supply the current reading is 0.21A. So each 4094 shift register must be getting 0.105A.
Each output pin on the 4094 must then be sourcing 0.105A/8 = 0.013A or about 13mA. Which is close to the above value.

I also noticed that when 1 LED was lit up the Vo(4094)=4.3V and this drops down progressively as all LEDs are lit up to 4.07V. The 4094 must be current limiting as it takes on more load. Still there is a discrepency since each 4094 is giving out a bit more current than 50mA which as you mentioned above is beyond the manufactures specification.

Thanks

 

[FvM]

I also noticed that when 1 LED was lit up the Vo(4094)=4.3V and this drops down progressively as all LEDs are lit up to 4.07V. The 4094 must be current limiting as it takes on more load.

The chip doesn't strictly current limiting. There are either voltage drops in the common supply node (bond wires, chip metallization) or the output driver resistance increases due to temperature rise.

Still there is a discrepency since each 4094 is giving out a bit more current than 50mA which as you mentioned above is beyond the manufactures specification.

I can't see a discrepancy. At worst case, you must expect permanent device failure when you disregard the maximum ratings. Fortunately, there seems to be a considerable safety margin. If you are designing a commercial product, you won't ignore the maximum ratings, because you have to fear increased failure rate and reduced lifetime. (Obviously some designers of cheap products from certain countries are continuously ignoring similar specifications...).

 

[blakes7]

This thread is solved.

The question was - Can all 8 pins of a 4094 shift register supply 25mA at the same time ?

A 4094 shift register can output a maximum of 50mA for all of its 8 outputs. This 50mA has to be shared equally among all 8 outputs. A suitable current limiting resistor has to be used to prevent the 4094 shift register from burning out. The value of 50mA was obtained from the datasheet for the 4094. It is prudent to stay within these limits so as to prevent device failure. Current limiting resistors must be used when driving LEDs from a 4094 shift register.

I recalulated all resistor values to drive LEDs from a 4094 shift register with 50mA total being sourced by all 8 outputs.

regards

blakes7