STP16CPC26: Analog LED-Dimming via R-EXT

[Kai3]

Hi,

We‘re developing a new LED Matrix Display and we interested to implement this 16-Channel Constant Current LED Driver “STP16CPC26�?.

We need approx. 20x LED-Drivers for one LED-Display.

We want to control / adjust the LED current (brightness) of all LED-drivers with one signal from the microcontroller e.g. an adjustable resistor / an adjustable transistor or a small current source or ?

In previous projects we used a PWM signal at the OE-Input. But now we dont't want to use a PWM signal, because of EMI and to high peaks current pulses.

It’s possible to group the R-EXT input of these 20 LED drivers together?

Thanks and Best regards,

Kai

[Peter BENSCH]

Welcome,@Kai​, to the community!

It is possible to control several R-EXTs together by inserting an adjustable resistor to the GND connection of the respective Rset. The AN3981 shows in section 5.3 how this can be done in principle.

Does it answer your question?

Regards

/Peter

[Kai3]

Hi Peter,

thank you for your quick reply!!

Yes, we have also read this application note.

But unfortunately, it didn't work so well with our old available LED display boards (with STP16CPC26).

It is not possible to dim the brightness linearly and also one LED driver is always darker than the other drivers.

Maybe this application example works only with other LED drivers from ST.

We tried a lot … maybe we’ll forget this HW-concept and we’ve to use a driver with analog gain inside – but these components like “LED1642GW�? are more expensive.

Anyone else have a good idea?

Thanks and Best regards,

Kai

[Peter BENSCH]

It is generally difficult to dim LEDs linearly because the brightness depends very much on the respective LED characteristic curve. This in turn depends on the batch, the binning and the temperature, and the respective tolerance of the constant current also plays a role. This also changes with the age of the LEDs, so that the displays also look different at different ages.

For these and other reasons, PWM leads to considerably predictable brightness settings.

Regards

/Peter

[Kai3]

Hi Peter,

thank you for your reply!

Currently we’ve an LED display with 12x STP16CPC26 driving 187 LEDs – all the same BIN.

In this available display we dim via PMW at OE – and it works fine.

But because of EMI and very high current peaks we’re interested in analog dimming for a new project.

We read the AN3981 and modified one existing display – but it doesen’t work in this way.

For this reason, we asked here ….

Sorry I didn't describe our whole process. 

Thanks and Best regards,

Kai

[Peter BENSCH]

This works with PWM because you always switch to full current and only change the pulse width. However, as soon as you try to adjust the brightness by shifting the operating point on the non-linear LED characteristic curve, the difficulties mentioned begin. If the characteristic curve of the current setting is also non-linear, the problems multiply.

Good luck anyway!

/Peter