STP08CP05TTR: Clarification on Using External Current Control for LED Dimming

[Anjaneyulu]

Hi Sir,

We have connected the REXT resistor with a value of 241 Ohms, which sets the current to 80 mA. I would like to confirm whether it is possible to use an external current control circuit to dim the LED.

For example, in the case of STP08CP05TTR, with VDD = 3.3V REXT = 241 Ohms, can I control the LED current from 20 mA to 80 mA using an external current control circuit to adjust the brightness?

In this setup, I plan to apply a constant 5V as the LED supply voltage.

Additionally, I would like to clarify the following:

1. Is this device a constant current sinking driver?
2. Is the current set by REXT the maximum allowable current or the constant current used for sinking?
3. If I apply 5V with 20 mA using an external current control circuit, what will happen? Will there be any issues such as voltage drops or other effects apart from dimming?

Kindly confirm if this approach is suitable.

[Peter BENSCH]

Welcome @Anjaneyulu, to the community!

It is generally NOT a good idea to adjust the brightness of LEDs via the current. The brightness not only depends non-linearly on the current, but also on parameters such as temperature and the manufacturing technology of the LEDs. In the case of "white" LEDs with a yellow-looking layer in front of the LED chip, there is also a colour location shift of the white point.

For these reasons, the variable brightness of LEDs is not controlled by current, but by PWM.

Hope that answers your questions?

Regards
/Peter

[Anjaneyulu]

I am using an IR LED with a 940nm wavelength and need to control its brightness. Can I use external current control with the STP08CP05TTR IC for this purpose?

[Peter BENSCH]

You are welcome to try controlling the intensity of the IR LEDs using a variable REXT. Since you will probably want to control several LEDs, you should note that the characteristic current-brightness curves may differ, which means that the individual LEDs may have different brightness levels.

To answer your original questions:

1. Yes, the LED outputs are constant current drivers.
2. REXT determines this constant current.
3. Please explain where you connect 5V, where 20mA flows and what kind of external control unit you have in mind.

However, I would also like to point out that the output channels of the STP08CP05T regulate the LED current linearly, i.e. they have to dissipate the difference between the voltage at the LED anode and the forward voltage of the LED (usually 1.5-2V for IR LEDs), which means that the set LED currents results in a correspondingly high power loss in the chip that has to be dissipated.

Regards
/Peter