UM0422
User manual
Constant current controller for high brightness LEDs
Introduction
The recent improvements in high power light-emitting diode (LED) technology with 100
lumens per LED chip and efficiency higher than that of incandescent lamps bring solid state
lighting close to reality. Light-emitting diodes (LEDs) have become very popular in the area
of general lighting as a replacement technology for low voltage lighting. Customers are
recognizing the advantages of using LED lighting which include long operating life, no fragile
glass, and low voltage DC operated.
LEDs are now available in higher and higher wattages per package (1 W, 3 W and 5 W) with
currents up to 1.5 A. At these current levels, the traditional means of limiting current with a
resistor are not sufficiently accurate nor efficient. It is well known that the brightness of LEDs
is proportional to the forward current. The best way to supply LEDs is to control the forward
current in order to get a good match of the output light. LED manufacturers specify the
characteristics (such as lumens, beam pattern, color) of their devices at a specified forward
current (IF) but they do not specify these characteristics at a specific forward voltage (VF). In
order to better control the LED current under several application conditions,
STMicroelectronics has developed the STCS1, a device which not only guarantees a
constant LED current but also provides useful features that make the device suitable for
different applications such as advertising signs, automotive interior and exterior lighting, and
emergency vehicle warning lights. This document describes how to use the STCS1
evaluation board. It includes a schematic diagram, bill of materials (BOM), and test data.
Table 1.
Device summary
Description
STCS1A evaluation board
STCS1ADEMO1
Order code
Figure 1.
STCS1A evaluation board
February 2008
Rev 2
1/14
www.st.com
Contents
UM0422
Contents
1
2
3
4
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Detailed description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Pin functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Description of the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1
4.2
Input/output connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
Schematic and bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1
5.2
Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Bill of materials (BOM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6
7
Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1
Thermal information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2/14
UM0422
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
STCS1A evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Constant voltage control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
STCS1A application diagram for 0.5 A LED current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Schematic - power section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Schematic - PWM oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Assembly layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Maximum power dissipation vs T
A
for MLP 3x3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Maximum power dissipation vs T
A
for PowerSO-8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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Background
UM0422
1
Background
In designing a power supply for white high brightness LEDs, the main requirements are
efficiency, size, and cost of the complete solution.
LEDs are current-driven devices whose brightness is proportional to their forward current.
Forward current can be controlled in two ways: voltage mode and current mode. The first
method uses the LED V-I curve to determine what voltage has to be applied to the LED in
order to generate the desired forward current. This is typically accomplished by applying a
voltage source and using a ballast resistor as shown in
Figure 2.
The output voltage
required for the circuit in
Figure 1
is given in
Equation 1.
This method does not provide
adequate current control or the system efficiency necessary for high current LEDs.
Figure 2.
Constant voltage control
Vin
Vin
Vout
R1
VFB
Voltage Regulator
R2
Rled
Equation 1
R
1
V
OUT
=
V
FB
⎛
1
+ ------
⎞
=
n
•
V
FMAX
+
I
LED
•
R
LED
-
⎝
-
R
2
⎠
Controlling current accurately is a challenge when each LED has a large manufacturing
tolerance in its forward voltage. The best way to drive the LEDs is to control the forward
current in order to eliminate the brightness variations due to forward voltage spread of the
LEDs.
Figure 3
shows the STCS1 application diagram. In this case the controller regulates
the voltage across a current-sense resistor rather than regulating the output voltage.
4/14
UM0422
Figure 3.
STCS1A application diagram for 0.5 A LED current
Detailed description
V
IN
4.5V to 40V
BAT46ZFILM
R
IN
100Ω
C
BYP
0.1µF
VCC
PWM
EN
DISC
DRAIN
ON
OFF
ON
OFF
STCS1A
SLOPE GND FB
C
SLOPE
10nF
R
FB
0.2O
C
DRAIN
0.47µF
Load disconnection
(Open Drain output)
2
Detailed description
The STCS1 is designed to supply high power LEDs under the severe conditions of
automotive and industrial applications resulting in constant brightness and extended LED
lifetime.
The STCS1 is a BiCMOS constant current controller designed to provide a precise constant
current starting from a variable input voltage source. STCS1 is well suited for applications
requiring high voltage input and it is able to withstand automotive load-dump events up to
40 V. An external shunt resistor in the ground path of the connected LEDs is used to sense
the LED current. A regulation loop holds the voltage drop at the shunt resistor on a constant
level of 100 mV ( typ.). The potential difference across the sense resistor is compared to a
fixed reference voltage and the error is both amplified and used to drive the internal power
series pass device. Selecting the sense resistance permits adjusting the output current level
up to 1.5 A with a precision of
±10%.
The integrated PWM input of the STCS1 allows LED
brightness regulation by pulse width modulation. The luminosity spread arising from the
LED production process can be compensated via software by an appropriate duty cycle
applied to the PWM pin. The STCS1A version has a specific pin which allows the user to set
of the slope of the edges of the LED current during the PWM dimming mode.
The device assembled on the demonstration board is the STCS1A, so the user will be able
to test the this feature. It is possible to disable this feature by removing the C
SLOPE
capacitors. An open drain pin output provides information on load disconnection.
A more detailed description of STCS1 device can be found in the STCS1 datasheet.
5/14
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