AN4460 datasheet(21/54 Pages) STMICROELECTRONICS

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AN4460

Firmware implementation

The following section offers a view of the firmware implementation. For more details about

the firmware, refer to the source code. The compiled portion of the firmware core is

3.5 Kbyte and the code size is ~12 Kbyte. Most of the code size is allocated to the

debugging machine and general user interface (see Section 7.3 on page 28).

The PWM algorithm and the OCP controls are entirely implemented in the SMED block,

leaving the CPU free for control tasks such as: slow varying algorithmic compensations, the

command line, and interrupt management.

5.1

Dimming algorithm

The dimming algorithm controls the brightness of the LED string by modulating the amount

of time, the LED is switched ON during a ~5 ms period. This period defines the dimming

cycle, which operates with a ~200 Hz frequency. The longer the LED is ON during a single

cycle, the brighter the LED results to human eyes. To turn ON a LED, the algorithm simply

activates the SMED associated by setting the SMED's SWx event to active state.

Once the desired on-time period is passed, the LED is turned OFF and the algorithm waits

for the beginning of the next dimming cycle to turn ON the LED again.

To optimize power, the dimming cycle in channel x is 90 degrees shifted respect to channel

x-1. LED chains can be configured with different on-time length; however the dimming start

period is fixed by design. In this way, the LEDs are always turned ON at different times,

reducing current peaks on the power supply.

The LED on-time is instead a multiple of 20 µs and is based on the STMR timer, which itself

has 1 µs granularity.

The software stores the time-on length in the onoff[8][2]structure, where the first byte

indicates the channel and the PWM action (turn ON or OFF the LED) and the second byte

specifies the time (up to 256 time units = 256 x 20 µs = 5120 µs) before the next event.

The next example shows the activity of LED channels with different dimming on-times:

DIMM1 is active at 87,5%, DIMM2 is active at 25%, DIMM3 is active at 75% and DIMM4 is

active at 37.5%. When the dimming is ON the SMED output PWM is active and the LED

light is ON.

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