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IC Drivers for the LED screens

An LED driver is a microchip supplying constant current to LEDs. IC drivers are based on integrated command circuits. In the simplest form it may be an “on-off” command. In more complex “intellectual” drivers commands include the possibility to control LED screen brightness via PWM.

With simple drivers, PWM function has to be handed over to controllers that will send a command to drivers about switching LEDs “on” and “off”. Advantages are evident: simple drivers are cheap. Disadvantages are many: controllers become more complex, it becomes difficult to generate the necessary number of brightness levels and refresh frequency. To achieve high image quality controllers must operate in high frequency mode and transfer data to drivers. But these frequencies have technological limitations.

“Intellectual” drivers take over some of the above functions and simplify operation of controllers. But the LED screen cost goes up. To justify this cost increase, the control system must be sufficiently sophisticated to support all new functions of IC drivers and thus relay high quality image on screen.

To sum up, IC drivers have the following essential features:

  • number of channels;
  • output current (range, stability);
  • clock frequency;
  • PWM free-running capability;
  • control current output for each PWM channel;
  • internal oscillator;
  • number of channels (interface width);
  • additional feedback capability (overheating, LED open/short);
  • digital dot-correction;
  • digital brightness correction;
  • IC driver package: SOP, SSOP, QFN.

LED drivers are developing following several trends:

  1. Upgrading of “simple” drivers. Additional functions are introduced: e.g. diagnostics and brightness control or increased frequency of data transfer.
  2. Upgrading of “intellectual” drivers. New functions appear and quantitative parameters improve (frequency, number of PWM levels). Advanced LED drivers support 16-bit PWM at frequencies nearing 20 МHz and wide range of service and feedback functions.
  3. Numerous specialized drivers with certain fixed functions appear: for instance, DM163 with 24 channels, 8-bit PWM and 6-bit brightness correction. This also concerns drivers with in-built clock support, e.g. MBI5050.

It is a tradition already to make drivers with 8 or 16 channels. However this is not always suitable for LED screens because modern pixel usually consists of 3 LEDs which complicates the topology of the PCB. Technologically, it would be more convenient to place one driver in the center of a 4-pixel cluster.

Unfortunately, most companies involved in design and manufacture of IC drivers tend to make “all-in-one” products. Sophisticated modern drivers, they have the widest possible range of functions. Customers rarely use all these functions but have to pay for them. For example, an excellent DM634 boasts 16-bit PWM and 7-bit general brightness correction. The latter function is superfluous at best, because with 16-bit PWM and modern LED screen control system it is possible to ensure general brightness correction via other methods.

The problem of external interface stands separately. Different drivers enable data transfer and PWM control in different ways – not always convenient from the point of view of the control system. With no unified approach to design of drivers and control system, the problem gradually deepens: each type of drivers requires different interface block. Thus, practically, each LED screen has a unique control system.

Customers buying LED screens know little if anything about LED drivers. They learn about the acceptable screen parameters in terms of refresh and brightness levels – but these parameters are frequently misleading. Driver consumes a lot of energy and heats a lot. With poor module design, IC drivers will overheat and lead to LED and module failures.

For a developer of LED screen control system to choose a driver is a difficult choice between necessary screen parameters and price.

 
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