CMOS IC Current requirement?

Thread Starter

tvist21

Joined Jun 11, 2021
7
I am using a CD4060 as a timer and I need to know how much current it requires to properly size my voltage divider resistors. The datasheet does not specify a current requirement so I'm unsure. Does anyone know a general rule of thumb when it comes to powering different kinds of ICs? CMOS?What about TTL?
 

crutschow

Joined Mar 14, 2008
28,204
I am using a CD4060 as a timer and I need to know how much current it requires to properly size my voltage divider resistors. The datasheet does not specify a current requirement so I'm unsure. Does anyone know a general rule of thumb when it comes to powering different kinds of ICs? CMOS?What about TTL?
Below is the quiescent current it takes, which is the minimum it requires when not oscillating.
When oscillating, there will be a small added current required to charge and discharge the output pin load stray circuit capacitance, which is thus proportional to the frequency out of the pin.
At the typical low frequencies used for it being used as a timer, the total will be not much above the quiescent current.

But why are you using voltage divider resistors?
Do you intend to power the device from that divider?
What is the supply voltage feeding the divider?

The generate rule of thumb is that, at low frequencies, CMOS takes much lower current (several microamps) than standard TTL, which generally takes several mA to more than 10mA for each IC.

1624337790846.png
 

Ian0

Joined Aug 7, 2020
3,761
When oscillating, there will be a small added current required to charge and discharge the output pin load stray circuit capacitance, which is thus proportional to the frequency out of the pin.
There will also be current due to charging and discharging internal capacitances at all frequencies it produces, and some due to cross-conduction in the MOSFETs, which is more prevalent at higher voltages.
See the graph in figure 10 of this datasheet
https://www.ti.com/lit/ds/symlink/cd4060b.pdf

@crutschow 's point about voltage dividers is important. Why/how are you using a voltage divider?
 

Thread Starter

tvist21

Joined Jun 11, 2021
7
There will also be current due to charging and discharging internal capacitances at all frequencies it produces, and some due to cross-conduction in the MOSFETs, which is more prevalent at higher voltages.
See the graph in figure 10 of this datasheet
https://www.ti.com/lit/ds/symlink/cd4060b.pdf

@crutschow 's point about voltage dividers is important. Why/how are you using a voltage divider?
@crutschow & @Ian0
I am making a timer that turns a 24V light off after it is turned on. I am using a P channel mosfet as a high side switch for the 24V light. This timer and some BJT latching circuitry will also be powered as part of the load. When the timer outputs a high, it sinks the BJT latch so that the high side switch cuts off power to the light and the timer until it is manually turned on again. I don't think this timer IC can handle 24V so I was going to have a voltage divider branching off of the 24V light.
 

BobTPH

Joined Jun 5, 2013
4,049
Voltage dividers are not a good way ti supply power to a circuit. The voltage will change with varying current draw. Use a voltage regulator, which maintains a stable voltage. For the small current you need, they come in a TO92 package.

Also, why are you using a high side switch? There are multiple advantages to using a low side switch.

Bob
 

Thread Starter

tvist21

Joined Jun 11, 2021
7
Voltage dividers are not a good way ti supply power to a circuit. The voltage will change with varying current draw. Use a voltage regulator, which maintains a stable voltage. For the small current you need, they come in a TO92 package.

Also, why are you using a high side switch? There are multiple advantages to using a low side switch.

Bob
Since the load and timer are separate circuits (connected in parallel at the output). The voltage divider would always limit the current that is going to the timer and the timer would never draw more current than the timer circuit is designed to. If the parallel load draws more current, then it won't affect what the timer draws. I am also trying to make this as inexpensive as possible.

I am using a high side switch because thats just how my latching circuit is set up (3 BJTs and 1 P-ch mosfet). Once the mosfet turns on, BJTs become activated to pull the Gate to ground until the timer sends a signal to deactivate the latch. I suppose I could try to figure out a way to latch a low side switch too.
 

BobTPH

Joined Jun 5, 2013
4,049
Not true. CMOS circuits use more power when changing states and very little in between. A counter will actually see a different current draw on each count, depending on how many bits change. That said, a voltage divider + a capacitor could work in this case, but it is generally not a good practice, and it is actually more components than a three terminal regulator.

Bob
 

Thread Starter

tvist21

Joined Jun 11, 2021
7
Not true. CMOS circuits use more power when changing states and very little in between. A counter will actually see a different current draw on each count, depending on how many bits change. That said, a voltage divider + a capacitor could work in this case, but it is generally not a good practice, and it is actually more components than a three terminal regulator.

Bob
An issue with using a regulator on this project is that I want this timer to be able to work on a 12V system and a 24V system. I could configure the linear regulator to work just under 12V but there would be a significant amount of wasted power when the same timer is used on the 24V system. I could try to find a switching regulator that would work for this range, however, this will add a significant amount to the cost compared to just using a voltage divider. Designing my own switching regulator to save cost would required FCC testing if I sold this in a product and I don't have time for that. I think setting up a voltage divider to provide ~10 to 20mA (+ decoupling capacitors) is my best bet.
 

BobTPH

Joined Jun 5, 2013
4,049
A voltage divider wastes more power than a linear regulator!

Edited too add:

And a voltage divider will not provide the same output voltage with two different input voltages, whereas the regulator will. So both if your arguments actually favor the regulator.

Bob
 

Thread Starter

tvist21

Joined Jun 11, 2021
7
A voltage divider wastes more power than a linear regulator!

Edited too add:

And a voltage divider will not provide the same output voltage with two different input voltages, whereas the regulator will. So both if your arguments actually favor the regulator.

Bob
The timer accepts a wide range of input voltages so that should be fine. The resistors will be limiting the current so much that very little power will be wasted.
 

Deleted member 115935

Joined Dec 31, 1969
0
I am using a CD4060 as a timer and I need to know how much current it requires to properly size my voltage divider resistors. The datasheet does not specify a current requirement so I'm unsure. Does anyone know a general rule of thumb when it comes to powering different kinds of ICs? CMOS?What about TTL?
can you show us a circuit, even a hand drawn sketch please

sounds like you have a resistor divider on the power supply, which is disastrous,
 
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