STLED325 datasheet(11/62 Pages) STMICROELECTRONICS

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STLED325

Functional description

Doc ID 17576 Rev 1

11/62

Functional description

The STLED325 is a common anode LED driver controller which can be used to drive red,

green or blue LEDs as the current is adjustable through the external resistor. In the common

anode configuration, the digit outputs source the current to the anodes while the segment

outputs sink the current from the cathodes. The configurable output current can be used to

drive LEDs with different current ratings (red, green or blue). The brightness can be

controlled through the I2C interface as described later. The outputs can be connected

together in parallel to drive a single LED. In this case, two parallel current sources of equal

value drive a single LED. The external resistor value can be set accordingly to determine the

desired output current.

Soft-start limits the inrush current during power-up. The built-in thermal protection turns off

the display when the temperature exceeds 140°C with a small hysteresis of 15°C. The

display is blanked (LEDs are turned off or in high-Z state) on power-up.

3.1

Low power mode of operation

When not used, the STLED325 goes into low power mode of operation wherein the current

consumption drops to less than 1 mA. During this mode, the data configured is maintained

as long as the supply voltage is still present (the contents of the internal RAM need the

supply voltage to be present). Port configuration and output levels are restored when the

STLED325 is taken out of shutdown. For minimum supply current in shutdown mode, logic

inputs should be at GND or VCC.

3.2

I2C serial interface

The interface is used to write configuration and display data to the STLED325. The serial

interface comprises of a shift register into which SDA is clocked on the rising edge of the

SCL after a valid start of communication. When communication is stopped, transitions on

SCL do not clock in the data. During this time, the data are parallel-loaded into a latch. The

8-bit data is then decoded to determine and execute the command.

For an overflow condition, if more bytes are written, then they are ignored whereas if more

bytes are read, then the extra bytes are stuffed with 1’s.

3.3

Initial power up

On initial power-up, all control registers are reset, the display is blanked and the STLED325

is in the low-power mode. All the outputs are in high-impedance state at initial power-up.

The SDA is pulled high by an external pull-up resistor. The display driver has to be

configured before the display can be used.

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