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
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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
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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=
6/18
AN2141 Reducing power dissipation on the chip
Figure 3. General configuration
STP16CPS05
When only one LED is connected to each output (the most common configuration used in
LED array reference boards) the equation can be simplified as follows:
Equation 3
P
totIc
#outputs
U⋅c–=
–()⋅⋅
IV
cVF
As previously stated, only the LED supply voltage V
an LED supply voltage V
about 0.5 V higher than VF or n*VF for n LEDs connected in
c
is variable. Thus the correct choice is
c
series.
If a PWM signal controls the brightness of the LEDs, the total power dissipation on the chip
is calculated as:
Equation 4
P
totIc
#outputs
U⋅c–=
IVcVF–()D⋅⋅ ⋅
where D is the duty cycle.
If there is no possibility to obtain the proper value of the LED supply voltage in an
application, an external resistor R in series with each LED must be connected to limit the
power dissipation on the chip as shown in Figure 4. The power dissipation on the chip is
consequently decreased as follows:
Equation 5
P
totIc
#outputs
U⋅c–=
IVcVFIR⋅–()–()⋅⋅
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Reducing power dissipation on the chip AN2141
Figure 4. Configuration with resistors
8/18
AN2141 Common drain outputs configuration
4 Common drain outputs configuration
As mentioned above, the STPxxCx05 is focused on driving high brightness LEDs. The
maximum forward current of HB LEDs can vary from 20 mA to several hundred mA.
A typical current value for super HB LEDs is 350 mA. Thanks to the common drain
configuration the outputs can be connected together as shown in Figure 5. This increases
the performance and current capability of the driver. This configuration enables the driving of
several types of HB LEDs even with the limited current-per-output.
Figure 5. Common drain outputs configuration
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LED array reference designs AN2141
5 LED array reference designs
Two LED array reference designs have been developed: one with 80 LEDs using the
STP16CPS05 LED driver, and another with 32 LEDs using the STP16CP05 and
STP16CPS05 LED drivers. In the following sections these two designs and their differences
are briefly described.
5.1 80 LED array reference design
An example of the 80 LED array reference design proposal (no evaluation board is currently
available for this design) is shown in Figure 6. Its block schematic description is provided in
Section 1: Block diagram. The only simplification in this reference design is that it does not
include an SMPS DC-DC converter. Therefore the supply voltage should be in the range of
4 V to 5 V.
Figure 6. 80 LED array reference design
5.1.1 Application schematic
A detailed application schematic of this design is shown in Figure 7. Five STP16CPS05 LED
drivers with 16 outputs are controlled by the ST7Lite09 microcontroller. The microcontroller
provides the clock frequency (SCK pin), the serial data (MOSI pin), the PWM signal (PWM
pin) and the latch signal (PA0 pin). The clock frequency in this application is 633 KHz and
the PWM signal for brightness regulation is 619 Hz. Resistors R1 to R5 (one for each driver)
set the constant current for all outputs. Their value is 1.3 kΩ and they set the constant sink
current to approximately 10 mA (for more information, please refer to the STP16CP05 and
STP16CPS05 datasheets). The analog voltage values from the trimmers R6 and R8 are
measured and converted by the microcontroller’s ADC to the digital reference signal. The
microcontroller sets the PWM signal for brightness and the data time delay for text speed
control accordingly.
10/18
AN2141 LED array reference designs
5.1.2 Bill of material
Table 1. Bill of material
Item Quantity Reference Part
1 8 C1, C2, C3 C4 C5 C7, C8, C9 100 nF
2 1 C6 10 µF
3 1 C10 2.2 µF
480 D1 - D80 LED
5 5 J1, J2, J3, J4, J5 STP16CPS05
6 1 J6 ICP
7 1 J7 Connector
8 5 R1, R2, R3, R4, R5 1k3
9 3 R6, R7, R8 10 k
10 1 SW1 SW push button
11 1 U1 ST7Lite09
12 1 U2 LE33/SO
11/18
LED array reference designs AN2141
Figure 7. Application schematic
STP16CPS05 STP16CPS05
STP16CPS05
STP16CPS05
STP16CPS05
12/18
AN2141 LED array reference designs
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 stepdown 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 kΩ and 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.
13/18
LED array reference designs AN2141
5.2.2 Bill of material
Table 2. Bill of material
Item Qty Reference Part Description Order code
1 1 CONNECTOR1 ICC Programming connector
2 5 C1,C2,C3,C5,C6 100 nF SMD cap. 0805
3 1 C4 10 nF SMD cap. 0805
4 1 C7 220 pF SMD cap. 0805
5 1 C8 22 nF SMD cap. 0805
6 1 C9 10 µF/35 V SMD tantal. cap.
7 1 C10 100 µF/16 V SMD tantal. cap.
8 1 C11 220 µF/16 V Electrolytic cap.
932 D1 - D32
10 1 D33 SMD green LED Green SMD LED
11 1 D34 STPS340U Schottky diode STPS340U
OSRAM SMD blue
LED
LB T68C-P2S1-35 OSRAM - Q65110A1978
12 1 IO1 ST7LITE09 Microcontroller ST7FLite09Y0M6
13 1 I02 STP16CP05 LED driver STP16CP05MTR
14 1 IO3 STP16CPS05 LED driver STP16CPS05MTR
15 1 IO4 78L33 Voltage regulator L78L33ACD
16 1 IO5 L5970D DC-DC converter L5970D
17 4 J1,J2,J3,J4 CON1 Not connected
18 1 J5 CON3 Input connector
19 1 L1 33 µH Inductor Coilcraft - DO3316P-333
20 2 P1,P2 10 k Potentiometer
21 2 R1,R2 1K2 SMD 1206 resistor
22 1 R3 10 k SMD 1206 resistor
23 1 R4 560 Ω SMD 1206 resistor
24 1 R5 6k8 SMD 1206 resistor
25 1 R6 3 k SMD 1206 resistor
26 1 R7 4k7 SMD 1206 resistor
27 1 S1 Switch Button
14/18
AN2141 LED array reference designs
Figure 9. Application schematic
15/18
The STP16CP05 vs. the STP16CPS05 AN2141
6 The STP16CP05 vs. the STP16CPS05
The STP16CPS05 LED driver includes an additional feature called “auto power saving”,
which minimizes the quiescent current if no active data is detected on the latches, and
automatically powers-up the device at the first active data latched. The fourth and fifth
blinking modes are designed on the 32 LED reference design board STEVAL-ILL003V2 to
measure the supply current for the STP16CP05 and STP16CPS05, when no active data is
detected and when active data is latched.
Ta bl e 3 shows that the STP16CPS05 has much lower current (0.067 mA) than the
STP16CP05 (3.11 mA) if no active data is detected. Therefore for large panels it is possible
to save a significant amount of energy using the STP16CPS05 LED drivers with the auto
power saving feature.
Table 3. Current consumption for the STP16CP05 and STP16CPS05 measured on
the STEVAL-ILL003V2
Data status STP16CP05 STP16CPS05
Active data is detected (all outputs are turned ON) 3.03 mA 3.07 mA
No active data is detected (all outputs are turned OFF) 3.11 mA 0.067 mA
16/18
AN2141 References and related materials
7 References and related materials
1. STP16CP05 and STP16CPS05 datasheets
2. ST7Lite0 datasheet
3. L5970D datasheet
8 Revision history
Table 4. Document revision history
Date Revision Changes
28-Apr-2005 1 Initial release.
17-Nov-2005 2
22-Jan-2008 3
Added information about new LED driver STP16CP596. Modified
Ta bl e 2 and Figure 9.
Document reformatted. Replaced references to part numbers
STP16CP596, STP16CL596 and STP08CL596 throughout the
document with the new, improved devices STP16CP05,
STP16CPS05 and STP08CP05, respectively. Added Section 6: The
STP16CP05 vs. the STP16CPS05. Added the order code for the 32
LED reference design board STEVAL-ILL003V2. Clarified that no
board is commercially available for the 80 LED reference design in
Section 5: LED array reference designs
17/18
AN2141
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