Darlington Array IC with Enable Pin

[AzharElectronics]

Hi,
I wanted to Drive LEDs in Two Modes (Day/Night) and these LED has discrete status input hence I wanted to use IC something which has Enable Pin. Most of them use ULN IC but ULN IC does not have Enable Pin.
Day Mode and Night Mode has different current requirements.
ULN2803 Data Sheet Link : https://www.ti.com/lit/ds/symlink/uln2803a.pdf

 

[FvM]

By physical nature, each darlington transistor has one base terminal and no enable pin. You need additional logic to achieve what you want.

 

[KlausST]

Hi,

All depends on voltage and currents.
If you want us to give some recommendations you need to give those values.
Also you should give some circuit informations. LED connection type (common anode?), what device controls the LEDs (microcontroller?)

LED brightness control can be made by:
* controlling DC current
* controlling duty cycle

DC current control with ULNxxxx.
Use two ULN channels for each LED (segment).
* one with a series resistor for low LED current
* one with a series resistor for high LED current
Or use adjustable common anode voltage...


duty cycle control:
* simplest with microcontroller. No additional hardware required.

Klaus

 

[betwixt]

One ULN2803 with dimming control is the obvious solution but if the difference between day and night mode is just the LED currents you can parallel the open collector outputs with additional resistors in series with one of them. It is up to you to make sure only one is actively driven at a time.

Note that the ULN low side drivers are just an array of transistors sharing a common emitter pin, in theory, if you can isolate that pin and ensure nothing is driven 'backwards' by the applied voltage, you can use it as an enable pin. Beware that the pin will carry the combined current of all the output pins.

Brian.

 

[AzharElectronics]

Hi Klaus,
Voltage range I have is from 5V to 28V, but just to keep it simple I have plan to use +12V (I can be flexible with voltage range). LED Connection type is Common Anode.
Current for Day Mode, Red LED = 10.6 ± 0.9 mA (LED Drop Vf = 2V @14mA)
Current for Night Mode, Red LED = 1 ± 0.1 mA (LED Drop Vf = 1.6V @0.1mA)
Current for Day Mode, Green LED = 11.05 ± 0.05 mA (LED Drop Vf = 3.1V @14mA)
Current for Night Mode, Green LED = 0.1 ± 0.01 mA (LED Drop Vf = 2.7V @0.1mA)
Controlling DC Current look simple as my circuit is simple and does not have microcontrollers (I have restriction to use microcontroller in my application).
I have plan to use Two ULN2803 with an N-Channel MOSFET to drive Enable/Disable corresponding to Day/Night Mode. And Yes, Two Series Resistor for each LED will help for Low and high current in turn connected to each ULN ICs.
Di
Can you please recommend some circuit for above requirement also some supporting article for adjustable common anode voltage ?

 

[KlausST]

Hi,

I wonder why your current tolerance is that tight....and why the red ratio is 10.6 / 1 = 10.6 while the green is about ten times bigger: 11.05 / 0.1 = 110.5

Now you are talking about different LEDs with different Vf. This makes it more difficult.

I´m confused. Which solution do you want to go?

Klaus

 

[AzharElectronics]

As Suggested by FvM I am using additional Logic AND gates (CD4081B) for Day and Night Mode Selection.
Also I am using ULN2003AI https://www.ti.com/product/ULN2003AI/technicaldocuments.
what is current that will flow in off State of ULN2003AI ? (As data sheet does not specify off state leakage current directly)
I want to connected cathode of NVG LED to ULN2003AI and LED will Turn On at 500µAmp, I am worried that ULN’s leakage current will turn ON the LED. Also I am driving LED from +6.6 Voltage Regulator https://gaia-converter.com/docs/ds/MGDD08W.pdf.
Hence need solution to get rid of leakage current ? (Any additional analog circuit to added or ULN IC itself has some feature, else any other solution)
Any help or suggestion would be appreciated.

 

[KlausST]

Hi,

what is current that will flow in off State of ULN2003AI ? (As data sheet does not specify off state leakage current directly)

For sure the datsheets gives these currents.

if you are talking about base current: "Off state input current"
if you are talking about collector current: "Collector cutoff current"

Klaus

 

[AzharElectronics]

Hi,
Thank You for the information, got confused with Cutoff word.
Any answer to my second question i:e "solution to get rid of leakage current ? (Any additional analog circuit to be added or ULN IC itself has some feature, else any other solution)

 

[FvM]

I presume you are worrying about hypothetical leakage currents. All specifications are worst case values for maximal Vce of 50V, did you check the actual leakage with 6.6V supply? In the unlikely case that the leakage current is really too high, you need to select a better switch device, or install output pull-up resistors that absorb the leakage.