ST AN2141 Application note

AN2141

Application note

LED array reference design board

Introduction

New high brightness LED (HB LED) applications such as displays, information panels, signs, traffic signals, automotive lighting and advertising are becoming more and more popular. For these applications new HB LED drivers with constant current outputs have been designed. The constant LED current guarantees the same brightness for all LEDs and provides a cost advantage in lighting system solutions, as there is no need for external resistors for each individual LED. This document describes a simple design solution to drive an array of high brightness LEDs, using the STP16CP05 and the STP16CPS05 LED drivers.

Two reference design (one with 80 LEDs and another with 32 LEDs) are proposed to provide a solution for driving high brightness LED arrays. The first has an array of 80 blue HB LEDs arranged in a 5 x 16 matrix. This matrix of LEDs is driven by five 16-channel STP16CPS05 drivers, with which it is possible to implement short, moving text. Also, all of the LEDs in the display can be turned on simultaneously using a switch, in order to demonstrate the uniformity in the brightness of the LEDs, which is achieved by applying the same sink current to all channels. The second reference design is a smaller 4 x 8 LED matrix driven by the STP16CP05 and the STP16CPS05, and includes a DC-DC converter for supply voltages varying from 5 V up to 35 V. For this new 32 LED array reference design, an evaluation board using OSRAM LEDs is available through order code STEVALILL003V2.

The new STP16CP05, STP08CP05 and STP16CPS05 LED drivers are monolithic, low voltage 8-bit or 16-bit shift registers designed for driving LED and LED panel displays. Thanks to these drivers the output LED current is constant and can be very precisely set using just one external resistor to control the light intensity of the LEDs. The STPxxCx05 guarantees up to 20 V output driving capability, allowing designers to connect more LEDs in series. The high 30 MHz clock frequency also satisfies the system requirement of high volume data transmission. Both designs are controlled by the ST7Lite09 microcontroller, which provides full text motion control, brightness regulation through PWM and control of text speed. A 3.3 V supply voltage for the microcontroller and the drivers is provided by a linear voltage regulator (the LE33 or L78L33).

The STP16CPS05 LED driver employs auto power saving to reduce power consumption.

January 2008

Rev 3

1/18

www.st.com

Contents	AN2141

Contents

1	Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
2	Timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Reducing power dissipation on the chip . . . . . . . . . . . . . . . . . . . . . . . .	6
4	Common drain outputs configuration . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
5	LED array reference designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	5.1 80 LED array reference design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10

5.1.1 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.1.2 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.2 32 LED array reference design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2.1 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2.2 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6	The STP16CP05 vs. the STP16CPS05 . . . . . . . . . . . . . . . . . . . . . . . . . .	16
7	References and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
8	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17

2/18

AN2141	List of figures

List of figures

Figure 1. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 2. Timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. General configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 4. Configuration with resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. Common drain outputs configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 6. 80 LED array reference design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 7. Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. 32 LED array reference design board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 9. Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3/18

Block diagram	AN2141

1 Block diagram

The block diagram of the LED array reference design board is shown in Figure 1. The diagram shows a universal STMicroelectronics system solution for driving an LED array. The number of LEDs can be increased by adding additional drivers in cascade. The input voltage is connected to the anodes of all of the HB LEDs. The input voltage value is very important due to the power dissipation in the drivers. A detailed explanation is described in Section 3: Reducing power dissipation on the chip. The LED cathodes are connected to ground through constant current sinks. The value of the constant current is set by only one external resistor. The DC-DC converter is represented by a simple linear controller (LE33 or L78L33) and provides 3.3 V supply voltage to the microcontroller and the LED drivers. The DC-DC SMPS (switch mode power supply), using the L5970D step-down converter, is designed to increase the efficiency, performance and supply voltage range (5 V up to 35 V). A detailed description of the SMPS design can be found in the L5970D device datasheet.

The control unit in this application is a microcontroller which sends data through the serial peripheral interface (SPI) to the shift registers. The data are shifted bit by bit to the next driver with the falling edge of the clock frequency (the maximum communication frequency for this driver is 30 MHz).

The control panel consists of the switch and two potentiometers. The switch changes the modes and the potentiometers allow the changing of the brightness and the text speed (blinking speed).

Figure 1. Block diagram

4/18

AN2141	Timing diagram

2 Timing diagram

The typical timing diagram is shown in Figure 2. The DATA, CLOCK and LATCH waveforms are shown. The data are changed with the falling edge of the clock frequency. For example, in Figure 2 one byte (01101001) can be seen. When all data are written to the drivers through the SPI, the microcontroller sets the latch input terminal (LE) pin to “log 1" and rewrites the data to the storage registers. In the next step the LE pin is grounded and thus the following data can be transmitted to the shift registers without changes in the output stage. The data in the storage registers are converted to the output constant current stages by the output enable (EO) pin. Thanks to the output enable pin, the brightness can be regulated through the PWM signal. Both LED array reference designs have adjustable delay time via potentiometer after implementation of the “latch signal”. Thanks to this feature the blinking speed can be regulated (time between sending data packets is changed).

Figure 2. Timing diagram

5/18

Reducing power dissipation on the chip	AN2141

3 Reducing power dissipation on the chip

One of the most important considerations in this application is the calculation of the maximum power dissipation on the driver chip. The maximum power consumption can be calculated with the ambient temperature and the thermal resistance of the chip. The thermal resistance depends on the type of package and can be found together with the maximum allowed junction temperature in the datasheet for the device. The maximum allowable power consumption of this device is calculated as follows:

Equation 1

P Tjmax – Ta

dmax = --------------------------

Rthja

●Pdmax - maximum power dissipation [W]

●Ta - ambient temperature [°C]

●Tjmax - maximum junction temperature [°C]

●Rthja - thermal resistance junction to ambient [°C/W]

The maximum forward current for each type of LED is provided in the datasheet and must not be exceeded. Each output channel of the driver operates as a linear current sink. As the sink current for each output of LED driver is set as constant by an external resistor, the power dissipation of the chip depends on the value of LED supply voltage (Vc) minus the forward voltage drop of the LEDs. To optimize the power dissipation of the chip, it is recommended to use the lowest possible supply voltage for the LEDs. An example of how to calculate the power dissipation of the chip is shown in Figure 3. The equation for this basic connection is:

Equation 2

#outputs

Ptot = Ic Uc + I ∑(Vc – niVF)

i = 1

●Ptot - power dissipation of chip [W]

●Ic - supply current for driver [A]

●Uc - supply voltage for driver [V]

●I - constant LED current set by external resistor [A]

●#outputs - number of outputs

●Vc - LED supply voltage [V]

●ni - number of serial connected LED for each output

●VF - LED forward voltage [V]

6/18