Technical application guide - The LEDset (Gen2) interface (05/14)

www.osram.com/ledset
05/2014
Technical application guide
The LEDset (Gen2) interface
Light is OSRAM
The LEDset (Gen2) interface | Contents
Contents
1 Introduction
03
1.1 Features and benefi ts
03
1.2 Differences between LEDset Gen1 and Gen2:
What’s new in the LEDset Gen2?
03
3 LEDset applications
11
3.1 Current setting by external resistor
11
3.2 Thermal derating/overtemperature protection
12
3.2.1 Application solution 1
12
2 LEDset specifi cations
04
3.2.2 Application solution 2
13
2.1 General overview
04
3.2.3 Application solution 3
15
2.2 LEDset characteristics
04
2.2.1 General description
04
2.2.2 LEDset implementation in OSRAM’s SSL system
05
2.3 Technical details
05
2.3.1 How to select the proper Rset value to get the desired Iout
06
2.3.2 Connection of multiple LED modules
07
2.3.3 Thermal protection for LED modules
07
2.3.4 Terminals
08
2.3.5 Output current accuracy and ground path resistance
08
2.3.6 Insulation
09
2.3.7 Cable length
09
2.3.8 Marking
09
2.3.9 Incorrect wiring
09
Please note:
All information in this guide has been prepared with great
care. OSRAM, however, does not accept liability for possible errors, changes and/or omissions. Please check
www.osram.com/ledset or contact your sales partner for
an updated copy of this guide.
2
The LEDset (Gen2) interface | Introduction
1 Introduction
LED technology is changing the world of general lighting.
In luminaire design, however, the various benefits of LEDs,
e.g. their high level of flexibility in operating luminaires, can
only be achieved with perfectly matched power supplies.
This is further complicated by the rapid improvement of the
efficacy and current capability of LED technologies, which
ask for even greater adaptability of the corresponding
power supplies.
OPTOTRONIC ® power supplies with LEDset interfaces can
meet this demand for greater adaptability by supporting a
wide power and current range and by their future-proof design, which makes them ready for coming LED generations.
Purpose of this application guide:
The purpose of this application guide is to provide basic
technical information on the LEDset Gen2 interface,
focusing on application solutions that illustrate the specific
functions of this new interface and show how these can
be used. The application solutions demonstrate that the
LEDset Gen2 interface opens up many opportunities for
customizing your LED-based luminaire: The simplicity and
flexibility of LEDset gives you the freedom to develop new
luminaire system features.
1.2 Differences between LEDset Gen1 and Gen2:
What’s new in the LEDset Gen2?
LEDset Gen2 is the enhanced interface between
OPTOTRONIC ® LED power supplies and LED modules
(such as OSRAM PrevaLED®). It can be identified by the
power supply product name, including the letters “LT2” at
its end (while LEDset Gen1 ends with “LT” only).
LEDset behavior has been changed in order to obtain the
following advantages:
— To add the parallel modules operation, especially for
linear and area SSL systems, while optimizing the operating range with spot and downlight systems
— To simplify assembly (only one additional wire instead
of three)
The table below shows the improvements of the LEDset
Gen2 compared to the previous version:
Table 1: What’s changed in LEDset Gen2?
LEDset Gen1
LEDset Gen2
Current setting
method
Rset resistor
Rset resistor with
new coding
Current coding
Relative
Absolute (within the range
(in % of the
of 0.1 A to 5 A)
maximum output
current of the
power supply)
Typical number of
LED modules in
the system
1
From 1 up to many
(series and parallel
combinations)
1.1 Features and benefi ts
LEDset helps you to meet important market requirements:
—
—
—
—
Future-proof solutions in terms of lumen output
Long-life operation
Luminaire customization
Energy and cost saving
In combination with OSRAM LED power supplies, the LEDset Gen2 interface offers full flexibility and a future-proof
system with the following features and benefits:
Number of wires
for LEDset
3
1
Multi-vendor
Yes (being adopted
by other vendors)
— Simplified wiring for easy setting of the LED driver
current, according to system and load configuration
— Versatile connectivity of several LED modules,
either in parallel or in series (or a mix of both)
— Thermal protection for LED modules
No
(provided by
OSRAM only)
Note:
There is no cross-compatibility and interchangeability
between the first and second generation of LEDset.
Figure 1: LEDset Gen2 application features
LED power supply
For simplicity reasons, the “LEDset” notation will be used
throughout the entire document instead of “LEDset Gen2”.
LEDset implicitly refers to the latest LEDset version.
LED modules
LEDset Gen2
3
The LEDset (Gen2) interface | LEDset specifi cations
2 LEDset specifications
2.1 General overview
The relationship Iout vs. Rset is defined by the following
formula:
LEDset is a low-cost analog interface based on a threewire connection between the LED driver and one or more
LED modules. Only one additional wire – besides the two
LED current supply wires (LED+, LED-) – is used for transferring information from the LED module/s to the LED
power supply.
This interface is designed to allow communication between
the LED module and the LED power supply, performing
LED current setting and thermal protection functionality.
(1)
Iout[A] =
5V
x 1000
Rset[Ω]
Figure 3: LEDset interface wiring
LED module
LED+
l out
The interface supports the following functionalities:
— Absolute output current setting of the constant-current
LED driver (LED module self-recognition)
— Handling of parallel/serial LED module connection
— Thermal protection of the LED module
LEDset
5V
Iset
R set 1
Typical applications of this interface are single or parallel or
serial LED module connections, offering an increasing
choice of modular capabilities and low-cost thermal protections circuits. In case of multiple module connection, all
connected modules must be identical (with the same current
set) and with matched forward voltages.
LED
power
supply
LED-
Figure 2: LEDset interface wiring (block diagram)
Power supply
LED Module
The basic working principle of the LEDset interface is to
measure the current Iset which flows from a LEDset port to
one or more Rset setting resistors which are located on the
LED module(s).
LED+
LEDset
Current setting
(Rset connection)
LED power supplies with LEDset interface are able to
measure Iset and to set the LED power supply output current ILED depending on the measured value of Iset according
to the equation:
Thermal protection
(2)
LED-
2.2 LEDset characteristics
2.2.1 General description
The LEDset interface operates on the basic principle of
Ohm’s law. By selecting the ohmic value of a simple resistor Rset, it’s possible to adjust the output current Iout of the
LED driver as desired.
Iout[A] = Iset[A] x 1000
The Rset resistor can be mounted onto the LED module and
connected to the LED driver by means of a dedicated LEDset wire. Alternatively, it can be used as a discrete part and
plugged into the push-in connector of the LED driver.
Note:
The LEDset interface is not meant to be used as a control
interface (for instance 1...10 V). If this is not observed, both
performance and safety requirements of the installation
may be affected.
4
The LEDset (Gen2) interface | LEDset specifi cations
2.2.2 LEDset implementation in OSRAM’s SSL system
OSRAM offers a complete portfolio of LED modules
(e.g. PrevaLED®) and OPTOTRONIC® LED power supplies
interfaced by LEDset, suitable for both indoor and outdoor
application.
In the above condition, the maximum nominal LED power
supply rated current Iout_max is set by the minimum Rset value
(Rset_min = 5 V/Imax x 1000) and the minimum nominal LED
power supply rated current Iout_min is set by the maximum Rset
value (Rset_max = 5 V/Imin x 1000).
Figure 4: OSRAM SSL components with LEDset
OPTOTRONIC ®
LED power supplies
The output current Iout, selected via the Rset resistor and
within the valid LEDset range, must match the driving
current of the LED components in the module and the
nominal current range of the utilized LED power supply.
LED modules
The interface behavior is compliant with the following table:
Indoor
Table 2: Interface behaviors
Outdoor
R set selection
I out
R set < R set_min (A)
Iout behavior defi ned by product specifi cation.
R set_min < Rset < R set_max (B) Iout[A] =
R set > R set_max (C)
5 V x 1000
R set[Ω]
Iout behavior defi ned by product specifi cation.
For further details and deviations from this basic information, please refer to the datasheet and instruction sheet of
the respective power supply.
2.3 Technical details
With the LEDset interface, the output current can be set
to “absolute” by selecting the ohmic value of a simple
Rset resistor. The interface is intended to cover an output
current range from 0.1 A to 5 A, according to the LEDset
equation (see figure 5, below), the correspondent valid Rset
resistor range is therefore between 50 kΩ and 1 kΩ.
Figure 6: l out vs. Rset
lout vs. Rset (B)
Iout [mA]
5000
4500
4000
3500
Figure 5: LEDset characteristics
3000
(A)
(B)
(C)
2500
lout vs. Rset
Iout [mA]
5000
2000
1500
4500
1000
4000
500
3500
3000
1000
2500
Rset min
Rset max
100000
Rset [Ω]
2000
1500
1000
500
1000
10000
100000
Rset [Ω]
5
The LEDset (Gen2) interface | LEDset specifi cations
2.3.1 How to select the proper Rset value to get
the desired Iout
LEDset allows a stepless selection of the output current
through the simple selection of the proper Rset value and
the connection of a potentiometer or a fixed standard resistor to the LEDset line.
The table below shows the output current values in the entire valid LEDset range if the standard resistor series E24 is
used.
Table 3: Output current values using standard E24 resistor values
R set
E24 [Ω]
Output
current
[mA]
R set
E24 [Ω]
Output
current
[mA]
R set
E24 [Ω]
Output
current
[mA]
R set
E24 [Ω]
51 000
100
22 000
227
9 100
549
3 900
1 282
1 600
3 125
47 000
106
20 000
250
8 200
610
3 600
1 389
1 500
3 333
43 000
116
18 000
278
7 500
667
3 300
1 515
1 300
3 846
39 000
128
16 000
313
6 800
735
3 000
1 667
1 200
4 167
36 000
138
15 000
333
6 200
806
2 700
1 852
1 100
4 545
33 000
151
13 000
385
5 600
893
2 400
2 083
1 000
5 000
30 000
166
12 000
417
5 100
980
2 200
2 273
27 000
185
11 000
455
4 700
1 064
2 000
2 500
24 000
208
10 000
500
4 300
1 163
1 800
2 778
Output
current
[mA]
R set
E24 [Ω]
Output
current
[mA]
Better current accuracy can be reached using two setting
resistors (Rset1 and Rset2) connected in parallel. For the typical LED current values, the table below shows the current
selection through the parallel connection of two E24 series
resistors with the related output current error.
Table 4: Output current values using two parallel setting resistors (Rset1 and Rset2), standard E24 resistor values
I out
[mA]
R set1
E24 [Ω]
R set2
E24 [Ω]
R set
total [Ω] = R set1 // R set2
Output
current
error [%]
100
100 000
100 000
50 000
0
150
43 000
150 000
33 420
-0.252
200
30 000
150 000
25 000
0.004
350
15 000
300 000
14 268
0
500
10 000
–
10 000
0
700
8 200
56 000
7 153
-0.136
1 050
9 100
10 000
4 764
-0.052
1 400
3 900
43 000
3 576
-0.119
1 750
3 000
62 000
2 861
-0.134
2 100
2 700
20 000
2 379
0.088
6
The LEDset (Gen2) interface | LEDset specifi cations
2.3.2 Connection of multiple LED modules
This simple working principle of the LEDset communication
allows the connection of multiple modules to the same interface line. The current delivered by the LED power supply
will, in this case, be set by the equivalent resistance applied
to the LEDset line.
Parallel connection of LED modules
If more than one LED module of the same type is connected
in parallel (see figure 7, below) to one LED power supply,
then the current delivered by the driver will be the sum of
currents required by each module.
(3)
Iout =
( R5 V
+
set1
5V
5V
+...+
Rset2
Rsetn
)
2.3.3 Thermal protection for LED modules
With its simple and flexible properties, LEDset also allows
users to manage the overtemperature protection, simply by
adding an overtemperature protection circuit to the LED
module (see figure 9). The thermal protection unit decreases the setting current in case of an overtemperature
event and thus limits or holds back the LED driver output
current. Several LED modules from OSRAM include this
protection.
Figure 9: LED module with thermal
protection circuit
x 1000
LED module
LED+
Figure 7: Typical setup for multiple-module parallel
configuration
LED module n
LED module 2
Thermal
protection
unit
LED module 1
LED+
LEDset
LED
power
supply
LED
power
supply
R set
R set 1
R set 2
Iset
R set n
5V
Iset
LEDset
LED-
Serial connection of LED modules
The LEDset interface also supports serially connected
LED modules (see figure 8, below). In this configuration,
only one module is connected to the LEDset line of the LED
power supply, and thus only the current-setting resistor and
thermal protection information of that particular module are
detected. Only the LED module which is connected to LEDmay be connected to the LEDset port. For missing wiring,
refer to miswiring section.
(4)
Iout[A] =
5V
x 1000
Rset1
Figure 8: Typical setup for multiple-module serial
configuration
LED module n
LED module 2
LED-
The LEDset interface allows users to strategically define
their module temperature management, thus providing the
possibility to implement their own specific solution with reliable accuracy. In order to ensure the compatibility between
the LED driver and the LED module, user solutions must be
compliant with the absolute maximum ratings shown in the
table below.
Table 5: Absolute maximum ratings for the LED module
LED module
LED module 1
Min.
Max.
voltage voltage
LED+
LEDset
R set 1
Rset 2
Rset n
I set
LED-
5V
LED
power
supply
Maximum output voltage that
V LEDset
design value the LED module can generate
during overheating/thermal derating
conditions through the thermal
protection unit
Minimum input voltage that the
LED module shall withstand during
normal operation conditions
6V
11 V
-
7
The LEDset (Gen2) interface | LEDset specifi cations
2.3.4 Terminals
Figure 12: Example of use with LEDset – (optional) with
isolated resistor suitable also for automatic insertion
Two output ports (LED+ and LED-) are used for the connection of the LED string/s. LEDset is a one-wire interface
and uses the LED- line as the reference ground. The interface is intended for the control of a single-channel, constant-current LED driver with a single or multiple LED string
load. The recommended connector colors and order are
shown in figure 10 and figure 13.
LED+
LED+ wire
LED-
LED- wire
LEDset
Figure 13: LED module terminal configuration and
color code; view from above
LED+ wire
LED+
LED- wire
LED-
LEDset wire
LEDset
LED module terminals
output terminals
LED power supply
Figure 10: LED power supply output terminal
configuration and color code; view from above
LEDset wire
LED(optional)
— LED+ is the LED power supply terminal for the positive
power supply wire connection (color: red).
— LED- is the LED power supply terminal for the negative
power supply wire connection as well as the ground reference for the interface logic (signal ground, color:
black).
— LEDset is the LED power supply terminal for the control
wire connection (color: white).
— LED- (optional) is the LED power supply auxiliary terminal
equipotential with the LED- terminal (color: black). This
connector terminal can be used with a stand-alone resistor or when a second ground reference is adopted to
increase the system accuracy.
LED(optional)
Note:
The LED power supply and LED module terminals order as
well as the color code are suggested but not mandatory.
2.3.5 Output current accuracy and ground path
resistance
The accuracy of the LEDset system is affected by the
voltage drop on the ground return path:
Figure 14: Ground path resistances
Power supply
Figure 11: Example of use with LEDset – (optional)
with standard stand-alone resistor
LED module
LED+
Iout
LEDset
Iset
LED-
Connector
resistance
–
LED+
LED
Cable resistance
Connector
resistance
Ground path voltage drop
+
R set
Iout
The total ground path resistance Rgpr (connectors plus
cable resistances) reduces the effective voltage across the
Rset resistor and consequently the Iset current. This parameter reduces the output current previously selected by
Rset, introducing a current offset error. The real output current can be re-calculated using the following formula:
(5)
Iout_real =
(R
set
5V
+ (1000 x Rgpr)
)
x 1000
8
The LEDset (Gen2) interface | LEDset specifi cations
In order to preserve the LEDset interface accuracy, the
cable and connectors have to be properly selected so that
they maintain the ground path voltage drop below 40 mV at
the maximum current allowed by the power supply (about
50 mΩ with 700 mA of output current).
Note:
When the second ground reference is adopted, using the
optional LED- terminal for the Rset connection, the accuracy
is not affected by the LED return current and the real output current can be calculated using the following formula:
(6)
Iout_real =
(R
set
5V
+ Rgpr
)
x 1000
2.3.6 Insulation
The interface line terminals of the LED power supply have
the same grade of insulation to the mains supply voltage
as the output circuits. The LEDset interface has no specific
protection against electrostatic discharge (ESD), except
where noted otherwise in the product specifications. Therefore, it is recommended that any circuit (e.g. accessible
potentiometer) connected to the LEDset interface port has
a proper insulation against touchable parts.
2.3.7 Cable length
The LEDset wire can be as long as the output supply wires
to the LED modules. Further limitations to cable length
generally derive from EMI emission or immunity issues or
directly from product specification details. For detailed
information, please refer to the datasheet or instruction
sheet of the respective LED power supply.
2.3.8 Marking
LED power supplies and LED modules equipped with LEDset and compliant to LEDset specifications will be marked
with the following logo:
2.3.9 Incorrect wiring
Missing LEDset control wire
LEDset is an interface meant for current setting and
thermal management of an LED module. If the LEDset line
is not connected to the setting and derating circuit of the
LED module, the thermal protection of the module and its
correct current setting will not work. This fault condition
could result in an undetected overheating of the module.
In order to protect the LED module in this condition, the
absence of the control signal is recognized and the driver
behavior then follows the description specified by the
product specification.
LEDset short circuit
In case of short circuit (< 900 Ω) of LEDset (LEDset connected to LED-), the interface recognizes the fault condition
and sets the LED output current as specified by the
product specifications.
Miswiring of LED+, LED-, LEDset
The interface is protected against incorrect wiring connections of the three poles LED+, LED- and LEDset at powerup. Compliance with this requirement is mandatory for LED
power supplies and recommended but not mandatory for
LED modules.
Incorrect wiring (not native) covered by
LEDset interface:
In case of multi-LED module connection, if one or more
LED+ module wires are disconnected from the power supply
but all Rset resistors remain connected to the PSU, the remaining connected modules receive from a higher LED current from the power supply. In this case, the LED modules
can overheat if they are not equipped with thermal protection circuits.
The LEDset logo
Figure 15: Critical condition not covered by LEDset interface
LEDset
LED module 1
LEDset
LED module 2
LEDset
LED module 3
Rset without
thermal
protection
circuit
R set without
thermal
protection
circuit
Rset without
thermal
protection
circuit
LED+
LED power supply
LEDset
With LEDset interface
LED-
9
The LEDset (Gen2) interface | LEDset applications
3 LEDset applications
3.1 Current setting by external resistor
If the application requires a specific fixed output current,
the easiest way to set the output current is to apply a
resistor between the LEDset and LED- terminals.
The LEDset interface is able to generate a constant voltage
output (Vset = 5 V) and thus allows the use of “passive” circuits (i.e. resistors) to achieve the setting current (Iset).
Figure 16: Current setting by external resistor placed on the ECG terminal block (left) and by external resistor
placed on the LED module (right)
LED power supply
LED power supply
LED module
LED module
LED+
LED+
LEDset
Rset
Rset
LEDset
LED(optional)
LED-
LED-
The resistor can be placed either on the terminal block of
the ECG or on the LED module (see figure 16). If the first
solution is adopted (resistor directly plugged into the LED
power supply), the additional LED- terminal has to be used
for the resistor placement.
The LED current can be easily set, in absolute way and in
the correct LEDset range (Rset_min < Rset < Rset_max) , choosing
the correct resistor value through the following formula:
(7)
ILED =
5V
x 1000
Rset
Note:
For resistor values out of the Rset_min to Rset_max range,
please consult the product datasheet, the instruction
sheet or the additional application guide.
10
The LEDset (Gen2) interface | LEDset applications
3.2 Thermal derating/overtemperature protection
Figure 18: PTC resistance versus PTC temperature
3.2.1 Application solution 1
The easiest and cheaper way to implement the thermal
derating on the LED module is to connect the series of
the PTC thermistor and the Rset resistor to the LEDset
terminal. Figure 17 (below) shows this simple circuit.
105
RPTC vs. TPTC
RPTC
Ω
104
5
Figure 17: Thermal derating circuit through a
PTC thermistor
103
5
LED power supply
LED module
102
LED+
-20
0
20
40
60
80
100
120
140
LEDset
PTC
Rset
TPTC
The typical LED current value at ambient temperature can
be calculated using the typical PTC resistance value
RR = 470 Ω as follows:
LED-
(9)
The LED current can be calculated using the following
formula:
(8)
ILED =
5V
x 1000
Rset + RPTC
As the PTC resistance value RPTC rises sharply with increasing temperature after its reference temperature has been
exceeded, the LED current drops when the LED module
temperature exceeds the selected temperature threshold
Tth.
Example 1:
The circuit in figure 17 has been simulated using the following
components:
— PTC EPCOS B59421A0095A062
(SMD_0402, RR = 470 Ω, tsense = 95 °C)
— Rset = 6800 Ω
ILED typ =
5V
x 1000 = 687 mA
6800 + 470 Ω
Using the PTC characterization from the EPCOS datasheet
(see figure 18, above), the LED current can be calculated as
a function of the PTC temperature. The diagram below
(figure 19) shows the LED current versus the PTC temperature,
the real LED current is a curve within the minimum and
maximum of the calculated LED current.
Figure 19: LED current versus PTC temperature
RPTC
Min. LED current
Max. LED current
800
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
0
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120
PTC temperature [°C]
11
The LEDset (Gen2) interface | LEDset applications
The diagram below (figure 20) shows the deviation of the LED
current with respect to the typical nominal value of 687 mA.
If two identical PTCs are used in parallel connection as
shown in figure 21, the LED current can be calculated
using the following formula:
Figure 20: LED current deviation versus PTC
temperature
(10)
LED current deviation [%]
Min. LED current deviation
Max. LED current deviation
10
ILED =
5V
x 1000
R
Rset + PTC
2
Example 2:
The circuit in figure 21 has been simulated using the following
components (the same components as in example 1):
0
-10
— PTC EPCOS B59421A0095A062
(SMD_0402, RR = 470 Ω, tsense = 95 °C)
— Rset = 6800 Ω
-20
-30
-40
-50
The typical LED current value at ambient temperature can
be calculated using the typical PTC resistance value
RR = 470 Ω as follows:
-60
-70
-80
(11)
-90
ILED typ [A] =
5V
6800 +
-100
-30 -20 -10 0
470
2
x 1000 = 710 mA
10 20 30 40 50 60 70 80 90 100 110 120
PTC temperature [°C]
3.2.2 Application solution 2
The accuracy of the thermal derating solution presented in
paragraph 3.2.1 can be increased by simply using two
PTCs in parallel (see figure 21, below). In this case, in fact,
the resistance tolerance due to the PTC components is reduced as the PTCs are connected in parallel, and is halved
if two identical PTCs are used.
Figure 21: Thermal derating circuit through two PTC
thermistors
Power supply
LED module
LED+
PTC
PTC
Rset
LEDset
LED-
12
The LEDset (Gen2) interface | LEDset applications
Using the PTC characterization from the EPCOS datasheet
(see figure 18 on page 11), the LED current can be calculated as a function of the PTC temperature. The diagram
below (figure 22) shows the LED current versus the PTC
temperature, the real LED current is a curve within the
minimum and maximum of the calculated LED current.
The diagram below (figure 23) shows the deviation of the
LED current with respect to the typical nominal value of
710 mA.
Figure 22: LED current versus PTC temperature
Figure 23: LED current deviation versus
PTC temperature
LED current [mA]
Min. LED current
Max. LED current
LED current deviation [%]
800
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
0
Min. LED current deviation
Max. LED current deviation
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120
PTC temperature [°C]
-100
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120
PTC temperature [°C]
13
The LEDset (Gen2) interface | LEDset applications
3.2.3 Application solution 3
The standard LED module temperature control and setting
circuit is shown in figure 24. It consists of the Rset resistor
(in this example two resistors are used in order to obtain
a better accuracy of the current set point) and the thermal
derating circuit. This example refers to a 50-V (±8 %) LED
module with a maximum operating temperature of 76 °C.
Figure 24: Example of a circuit for temperature control – LED module side (VLED = 50 V ±8 %)
LED+
D6
R1
27 k
R4
Vref
3k9
D2
R3
LED module
D1
D3
11 V
C2
3k9
Optional
Optional
(or short)
NTC
47 k
D4
LED power supply
R5
4k7
Q1
Vb
BC846BW
C1
D5
220 p
R2
Rtg
10k5
Ve
Vset
C3
Rset
Thermal derating circuit
LEDset
Optional
(or open)
47
Rset1
Optional
LED-
The resistors R1 and R4 have to be selected in order to
guarantee enough bias current to the thermal derating circuit (see figure 24). In order to decrease power dissipation,
R1 and R4 may be connected to a lower voltage source (for
instance an intermediate LED tap of the total LED string),
provided that the minimum tap voltage is higher than the
D1 Zener diode voltage (cf. the following notes). C2 and C3
are optional capacitors for ESD/immunity filtering, to be
tuned in the final application.
14
The LEDset (Gen2) interface | LEDset applications
Rset 1 is an optional resistor available for fine tuning of the
LED current. R5 is also an optional resistor used for reducing the power dissipation of the R4 resistor during thermal
derating (in the example of figure 24 on page 14, the R5
resistor is necessary in order to use a 1206 R4 case size).
R 3 calculation
The R3 value must be selected lower than the RNTC_th value
calculated at the Tth temperature in order to preserve the system temperature sensitivity. This resistor is used to decrease
the slope of the thermal derating as indicated in figure 26.
R 2 calculation
Figure 25: Current derating simulation – Iout [mA] vs.
LED module temperature [°C]*
(15)
R2 =
Vset + V be_BC846W_th
x (RNTC_th + R3)
Vref_th – Vset – V be_BC846BW_th
where Vset = 5 V
LED current vs. LED temperature
LED current [mA]
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
0
Figure 26: Current derating simulation – Iout [mA] vs.
LED module temperature [°C] – with different R3 and
R2 value combinations*
R3 = 1 kΩ R2 = 7.15 kΩ
ΔT = 4 °C
T1 = 74.5 °C
T2 = 78.5 °C
72.5 73 73.5 74 74.5 75 75.5 76 76.5 77 77.5 78 78.5 79 79.5
LED module temperature [°C]
* Based on the circuit shown in fi gure 14 on page 8
Design step 1 – temperature derating threshold (T th)
setting:
After having selected the temperature threshold Tth, we can
calculate all the system parameters influenced by the temperature:
LED current [mA]
R3 = 4.7 kΩ R2 = 10.5 kΩ
R3 = 6.8 kΩ R2 = 12.5 kΩ
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
0
72.5 73 73.5 74 74.5 75 75.5 76 76.5 77 77.5 78 78.5 79 79.5
LED module temperature [°C]
* Based on the circuit shown in fi gure 26 – fi xed temperature threshold point – R NTC = 6844 Ω at 75 °C
RNTC value at T th:
(12)
RNTC_th = 47 k x e
Bx
( 273.151 + T
–
th
1
298.15
)
V be_BC846BW value at T th:
(13)
V be_BC846BW_th = 0.55 – 2.3 mV/°C x (Tth – 25 °C) 1)
Vref value (using BZX384-B11) at T th:
(14)
Vref_th = 11 + 7.4 mV/°C x (Tth – 25 °C) 2)
where Tth = threshold temperature derating, B = NTC B
parameter (B = 4000 for NTC EPCOS B57423V2473H062).
At the temperature threshold, the R tg current is very low,
and we can approximate its voltage drop to zero.
1) -2.3 mV/°C is the typical temperature coeffi cient of the BC846BW
base-emitter junction – 0.55 V determined using Ebers-Moll equation calculated at working point Tamb = 25 °C at I c = 30 μA.
2) 7.4 mV/°C is the typical temperature coeffi cient of the BZX384-B11.
15
The LEDset (Gen2) interface | LEDset applications
R tg calculation
In order to select the R tg resistor value, it is necessary to
calculate the total equivalent series resistance R tot:
Figure 27: Equivalent circuit analysis – based on circuit of figure 24
R3
LED module
Vref
LED module
Req
LEDset
+
–
Veq
Q1
BC846B
Rtg
Rset
Veq and Req are the Thevenin equivalent circuit parameters
viewed from the base of Q1 and calculated as:
(17) Veq =
(R3 + RNTC_th) x R2
R3 + RNTC_th + R2
Vref x R2
R3 + RNTC_th + R2
R total can be calculated as follows:
(18) R total = R tg +
Req
1 + hFE_min
LEDset
+
–
Veq
Rtotal
Rset
LEDset
Rset
Figure 28 (below) shows the thermal derating behaviour
with different R tg values; the slope of the derating curve is
lower at higher values of R tg.
A disadvantage of using high Rtg values is the influence of
this parameter on the final shutdown temperature T2. If high
values of Rtg are used, the slope of the thermal derating
curve increases when the Rset value is reduced. For this
reason, it is recommended to select the lowest R tg value
that guarantees the thermal derating stability (to be verified
in the corresponding application). A rule of thumb for Rtg
selection is shown below:
(19)
where hFE_min is the minimum static current gain of the
transistor Q1.
In order to preserve the thermal derating behavior in case
of multi-module parallel connections or low Rset value, the
R total value must be selected below 100 Ω.
Q1
BC846B
Vset
Q1
BC846B
Rtg
Vset
R2
(16) Req =
LED module
RNTC
Vset
+
–
R tg >
Req
(1 + hFE_min)
Figure 28: Current derating simulation – Iout [mA] vs.
LED module temperature [°C] at different R tg values*
LED current [mA]
Rtg = 10 Ω
Rtg = 47 Ω
Rtg = 68 Ω
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
0
72.5 73 73.5 74 74.5 75 75.5 76 76.5 77 77.5 78 78.5 79 79.5
LED module temperature [°C]
* Based on the circuit shown in fi gure 14 on page 8 (with fi xed R 2 and R 3)
16
The LEDset (Gen2) interface | LEDset applications
In order to facilitate R2 and R3 selection, a simplified table is
shown below:
Table 6: R 2 , R 3 and R tg_max values according to T th
temperature threshold*
T th [°C]
R NTC_th [Ω]
R 3 [Ω]
R 2 [Ω]
R tg_max [Ω]
45
20 222
13 975
33 349
16
50
16 647
11 505
27 163
31
55
13 786
9 527
22 257
43
60
11 481
7 973
18 342
53
65
9 613
6 644
15 199
61
70
8 091
5 592
12 660
67
75
6 844
4 730
10 599
72
77
80
5 817
4 020
8 916
85
4 966
3 432
7 535
80
90
4 258
2 943
6 395
83
95
3 666
2 534
5 452
86
100
3 170
2 191
4 666
88
105
2 751
1 901
4 009
89
110
2 396
1 656
3 458
91
115
2 095
1 448
2 993
92
120
1 837
1 270
2 600
93
125
1 617
1 117
2 266
94
* Table based on the circuit shown in fi gure 14 on page 8
Design step 2 – R1 selection:
R1 has to be selected in order to ensure the minimum bias
current of the thermal derating circuit and in order to limit
its power dissipation:
Other fi xed data
Minimum Zener diode bias current (Ibias_z):
Maximum interface current (Iinterface_max):
Q1 - BC846BW min static
current gain (hFE_min = Ic/ib):
0.5 mA
5 mA at Iout = 5 A
200 at Ic = 2 mA
R1 calculation
Using the equations (20) and (21), it is possible to find the
minimum and the maximum Zener diode voltage
(Vref_max, Vref_min):
(20) Vref_max = V Zener_nom + V Zener_nom x tol % + Ktemp x (Tmax – 25 °C)
(21) Vref_min = V Zener_nom – V Zener_nom x tol % + Ktemp x (Tmax – 25 °C)
Maximum Zener diode voltage (BZX384-B11):
Vref_max = 11 V + 11 V*2 % + 9 mV/K * (80 °C-25 °C) = 11.715 V
Minimum Zener diode voltage (BZX384-B11):
Vref_min = 11 V - 11 V*2 % + 9 mV/K * (80 °C-25 °C) = 11.275 V
Now it is necessary to calculate the IR1 current (24) as a
sum of three currents: BC846BW base current (22), maximum NTC current (23) and the minimum Zener diode bias
current Ibias_z.
Iinterface_max
hFE_min
(22)
Ib_max_BC846BW =
(23)
INTC_max =
(24)
IR1_min = Ib_max_BC846BW + INTC_max + Ibias_z
Vref_max
R2 + R3
Design example:
Input data
Maximum module temperature (Tmax):
Maximum LED module voltage (VLED+_max ) 3):
Minimum LED module voltage (VLED+_min):
Minimum LEDset voltage (Vset_min):
Maximum LEDset voltage (Vset_max):
Zener diode nominal voltage (V Zener_nom): 11 V
Zener diode voltage tolerance (tol%):
2%
Zener diode temperature
9 mV/K
coefficient max (Ktemp):
Zener diode maximum
300 mW
power dissipation (PD1_Zener_max):
80 °C
54 V 4)
46 V 5)
4.75 V
5.25 V
(BZX384-B11)
(BZX384-B11)
Maximum BC846BW base current (Ib_max_BC846BW ):
Maximum NTC current (INTC_max):
IR1min ≈
25 μA
746 μA
1.3 mA
(BZX384-B11)
(BZX384-B11)
3) V LED+ is the voltage between the LED+ and LED- poles.
4) In case of intermediate LED voltage source connection,
use its maximum value.
5) In case of intermediate LED voltage source connection,
use its minimum value.
17
The LEDset (Gen2) interface | LEDset applications
Under the condition of VLED+_min > Vref_max:
(25)
Req has to be selected fulfilling the two conditions (31) and
(32): In order to save power dissipation, Req has to be
selected as close as possible to the Req_max value:
VLED+_min – Vref_max
IR
R1_max =
1_min
R1 has to be selected fulfilling the two conditions (26) and
(27); in order to save power dissipation, R1 has to be selected as close as possible to the R1_max value:
(26)
(27)
R1 < R1_max
P R1 =
(VLED+_max – Vref_min)
R1
2
< PR1_max
where PR is the maximum permitted power dissipation
by the selected R1 resistor according to its package size
(cf. resistor datasheet) and PR1 is its calculated maximum
power.
1_max
If these conditions cannot be fulfilled, please select a
bigger R1 case in order to increase the PR value.
1_max
After the R1 selection, it is necessary to verify the Zener
diode power dissipation constraint (28):
(28) PD1_Zener =
(VLED+_max – Vref_min)
x Vref ≤ PD1_Zener_max
R1
where PD1_Zener_max is the maximum power dissipation
allowable by D1.
The user has to verify the maximum power dissipation by
D1 using equation (28), according to its maximum allowed
power dissipation (cf. Zener diode datasheet). If this condition isn’t met, please select a bigger D1 package size in
order to increase the PD1_Zener_max value.
(31)
Req < Req_max
(32)
PReq =
(VLED+_max – VCEsat_BC846BW – Vref_min)2
≤ PReq_max
Req
PReq is the total calculated power dissipation of R4 + R5 and
PReq_max is the maximum power dissipation allowed by the
two resistors. In order to save a number of components, it
is possible to consider R5 = 0 Ω and so PReq_max = PR4_max. The
R4 package has to be selected in order to fulfill condition
(32). If this condition is not fulfilled, please select a bigger
R4 case in order to increase the PR4_max value or use the
series R5 resistor in order to share the power dissipation
(selecting R5 and R4 according to equation (29) as shown
in the following example).
Example:
Thermal protection design example
Tth = 75 °C
[email protected] °C = 6844 Ω
V [email protected] °C = 0.435 V
[email protected] °C = 11.37 V
VCEsat_BC846BW = 90 mV
R1 = 27 kΩ – SMD case: 0805 – Pmax = 125 mW 6)
R2 = 10.5 kΩ – SMD case: 0805 – Pmax = 125 mW
R3 = 4.7 kΩ – SMD case: 0805 – Pmax = 125 mW
R4 = 3.9 kΩ – SMD case: 1206 – Pmax = 250 mW
R5 = 3.9 kΩ – SMD case: 1206 – Pmax = 250 mW
Design step 3 – R4 and R 5 selection:
Considering the following equation:
(29) Req = R4 + R5
Req has to be selected in order to assure the maximum interface current (Iinterface_max = 5 mA) with VLED+_min, the
BC846BW collector-emitter saturation voltage VCEsat_BC846BW
and the R tg voltage drop. The maximum Req resistor value
has to be calculated as indicated in equation (30):
(30) Req_max =
VLED+_min – Vset_max – VCEsat_BC846BW – R tg x Iinterface_max
Iinterface_max
6) Pmax is the maximum power dissipation allowable by the resistor,
considering its power thermal derating.
18
05/14 OSRAM CRM MK AB Subject to change without notice. Errors and omissions excepted.
www.osram.com/ledset
For more product-specific information, please go to:
— OPTOTRONIC ®: www.osram.com/optotronic
— Light management systems: www.osram.com/lms
— PrevaLED®: www.osram.com/prevaled
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80807 Munich, Germany
Phone +49 89 6213-0
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