AN2141

LED array reference designs

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5.2

32 LED array reference design

The 32 LED array reference design board is shown in Figure 8 and its block schematic

description corresponds to Section 1: Block diagram. All components are placed on the top

side of the board. This board includes an SMPS DC-DC converter using the L5970D step-

down switching regulator for input voltages from 5 V to 35 V. The order code for this

reference design board is STEVAL-ILL003V2.

Figure 8.

32 LED array reference design board

5.2.1

Application schematic

The detailed application schematic of this design is shown in Figure 9. Two LED drivers

(STP16CP05 and STP16CPS05) with 16 outputs each are controlled by one ST7Lite09

microcontroller. Resistors R1 and R2 (one for each driver) set the constant current for all

outputs. Their value is 1.2 k

Ωand they set the constant sink current to 16 mA for each output

(for more information, please refer to the STP16CP05 and STP16CPS05 datasheets). Six

blinking modes are programmed in this application. The first switches the entire display

ON/OFF. The second is a "harmonic" lighting effect, as the light is moved from the margin to

the center of the display and back. During the third mode the numbers from one to nine are

moved on the display. The fourth and fifth modes are implemented to show differences

between the STP16CP05 and the STP16CPS05 LED drivers (as described below). Since

the constant current LED drivers are used in this design, the brightness for each LED must

be the same. Therefore, during the sixth mode, all LEDs are simply turned ON for LED

brightness checking. All features and functions (communication, brightness regulation,

blinking speed regulation, etc.) are similar to the 80 LED array reference design board, with

exception of the addition of an SMPS DC-DC converter. Its output voltage is 4 V and its

maximum current is 1 A (L5970D device). For more details on this converter design, please

refer to the L5970D datasheet. The clock frequency in this application is 83.6 KHz and the

PWM signal for brightness regulation is 654 Hz.