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
T
–
P
dmax
● P
● T
● T
● R
- maximum power dissipation [W]
dmax
- ambient temperature [°C]
a
- maximum junction temperature [°C]
jmax
- thermal resistance junction to ambient [°C/W]
thja
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 (V
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:
jmaxTa
-------------------------- -=
R
thja
) minus the
c
Equation 2
#outputs
P
totIcUc
● P
● I
● U
● I - constant LED current set by external resistor [A]
● #outputs - number of outputs
● V
● n
● V
- power dissipation of chip [W]
tot
- supply current for driver [A]
c
- supply voltage for driver [V]
c
- LED supply voltage [V]
c
- number of serial connected LED for each output
i
- LED forward voltage [V]
F
IVcniVF–()
+⋅=
∑
i1=
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