RGB led lighting module

Hello

I’ve started a new project: a RGB led lighting module, that can be controlled from a central communication node.

I’ll be using some SMD rgb leds, in 5050 case, that i  got from e-bay.

The Imax is specified to 40mA, but i’ll be running them at 30mA max, to keep the heat down.

The design features are :

  • 50x50mm per module
  • cheap controller, JTAG interface, enough cpu power for high resolution/ speed PWM
  • the ability to run stand-alone, following a pre-programmed sequence
  • RS485 line drivers
  • wide input voltage range

Led driver

The chosen driver is a TS19371CX6, a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The
device can drive up to 126 LEDs (9S14P) from a 12V supply. The output capacitor can be as small as 4.7µF, saving
space versus alternative solutions. A low 95mV feedback voltage minimizes power loss for better efficiency. Additional
feature include over output voltage limiting when LEDs are disconnected. The TS19371 switches at a fixed frequency
of 1.2MHz, allowing the use of tiny, low profile inductors and capacitors to minimize footprint and cost in space
consideration applications for cellular phone backlighting or other hand held equipment.

TS19371CX6 datasheet

I use a 8S2P configuration, 16 RGB leds in total.
The LED driver is used as described in the data sheet. L1 is a 47uH MURATA LQH88PN470M38L.
The current sense resistor is a 2R7 in a 1210 case, chosen to set an ~30mA current trough the LED’s.
The dimming is a simple PWM one, the controller is directly driving the CTRL pin.

CPU

On this version I’m using a LM3S317 ARM Cortex-M3 micro.
The micro. has a core frequency of 25MHz, 16KB flash, 4KB of RAM, 6 PWM outputs, 6 ADC’s, 1 UART channel and a JTAG debugging interface.
3 of the PWM outputs will be used for the LED drivers.
The UART will be connected to the RS485 line drivers.
The internal temperature sensor will be used to measure the temperature of the entire board.
4 of the ADC channels will measure the input voltage and the voltage of each led string. The measured values will be used for status reports and driver protection.
The 3.3V power supply will be generated by a LP2950CDT LDO.

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