AND8349/D
Automotive Applications
The Use of Discrete
Constant Current
Regulators (CCR) For
CHMSL Lighting
Prepared by: Brian Blackburn (FAE),
Mike Sweador (AE)
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
LEDs are being proliferated into many automotive
lighting applications. The Center High Mount Stop Lamp
(CHMSL) is one of several automotive applications for
LEDs. Interior lighting is another area where LEDs are very
well suited due to their small size and high efficiency. Recent
advancements in higher efficiency LEDs at lower costs have
made these light sources the technology of choice for
automotive lighting.
Since LED brightness is determined by operating current,
optimum intensity may require a constant current approach
to maintain consistent luminosity over the wide variation of
battery voltage possible in automotive electrical systems.
Battery voltage typically is 13.5 V; however, it can range
from as low or less than 9 V in a faulty charging system to
24 V for several minutes in a double battery jump scenario.
The list of potential automotive lighting LED applications
includes:
•
CHMSL Arrays
•
Instrument Cluster Backlighting
•
Switch Cluster Backlighting and Tell−Tales (Icon
Lighting)
•
Dome Lighting
•
Mirror Lights
•
Fog Lights
•
Convenience Lighting
•
RGB Ambient Lighting
•
Emergency Flashlight
Each application requires specific attention to light output
and optical design, LED circuit topology, driver current
requirements, and thermal management. It is the intent of
this article to concentrate on CHMSL LED circuit
requirements, and to discuss thermal management as it
applies to the driver circuitry.
An innovative use of a new Patent Pending Discrete
Technology (Constant Current Regulator – CCR) will be
shown to have several distinct advantages for controlling the
LED operating current compared to the common method of
selecting a bias resistor to adjust the LED operating current.
Figure 1 shows a typical I-V curve for the NSI45030T1G
device. The CCR is a current regulator that offers
outstanding regulation for LEDs and other applications
requiring a low cost, stable current source. Unlike costly
switching regulators, a CCR is relatively EMI free, does not
require startup circuitry, and operates as a current source or
sink. Current regulation can be achieved for
Anode−Cathode voltages ranging from as little as 1.8 V. No
external components are required to regulate the typical
value of 30 mA. Since these are CCR sources, parallel
arrangements allow for higher load current applications.
(Figure 7)
The CCR reduces the complexity of resistor biased
designs for sensitive loads such as LED strings connected in
series (Figure 4). Simply apply a voltage greater than
V
overhead
to achieve an accurate regulated current.
80
I
REG
CURRENT REGULATION (mA)
70
60
50
40
30
20
10
0
−10
−20
−30
−10
0
10
20
30
V
AK
, ANODE−CATHODE (V)
40
50
Figure 1. CCR IV Characteristics
©
Semiconductor Components Industries, LLC, 2009
May, 2009
−
Rev. 1
1
Publication Order Number:
AND8349/D
AND8349/D
A CCR is a nearly ideal current source providing constant
current regardless of applied voltage above its operating
minimum. In simple terms, a CCR can be considered a
nonlinear voltage controlled
resistor.
The Power Dissipation (P
d
) in an LED is P
d
= I V. Since
the CCR acts as a voltage controlled resistor while the
resistor biasing fixes the resistor value, the LED Power
dissipation is shown to be nearly constant over a variable
battery range. (Figure 3)
For automotive CHMSLs, a constant current source for
LEDs reduces stress conditions caused by overdriving with
current as compared to resistor biasing. A Reverse
protection diode (MBRS140T3 in Figure 4) prevents a
reverse voltage condition which can permanently damage an
40
35
30
I (mA)
25
20
15
10
LED and protects the CCR from conducting in the reverse
bias mode (Figure 1).
A basic CHMSL configuration with 3 Red LEDs in series
is shown in Figure 4. A CCR provides a uniform intensity
over full line voltage swings and greatly reduces LED power
dissipation as compared to common resistor biasing. At 9 V
battery input, a CCR provides a higher current than a typical
biasing resistor value would provide (Figure 3). At 16 V a
stable, constant current is supplied by the CCR.
Figure 2 shows a comparison of CCR vs. Resistor Bias
current over battery voltage variation from 9 V to 16 V. The
LED current, and therefore intensity, is constant with the
CCR device compared to the resistor bias.
T
A
= 25°C
Circuit Current with
CCR Device
Pd LEDs (mW)
240
220
200
180
160
140
120
100
80
16
60
9
T
A
= 25°C
LED Power with
CCR Device
Circuit Current
with 250
W
Representative Test Data
for Figure 6 Circuit, Current
of LEDs, FR−4 @ 300 mm
2
,
1 oz Copper Area
9
10
11
12
13
14
15
LED Power
with 250
W
Representative Test Data
for Figure 6 Circuit, Pd of
LEDs, FR−4 @ 300 mm
2
,
1 oz Copper Area
11
12
13
14
15
16
10
V
in
(V)
V
in
(V)
Figure 2. Series Circuit Current
Figure 3. LED Power
MBRS140T3
CCR
CCR
CCR
CCR
CCR
Figure 4.
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2
AND8349/D
D8
R1
205
D1
HF3−R5570
D5
MBRS140T3G
Anode
Cathode
Q1
Q2
Qx
V
SUPPLY
13.5Vdc
HF3−R5570
D6
+
−
LED
LED
HF3−R5570
LED
HF3−R5570
V
in
HF3−R5570
HF3−R5570
D7
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
0
LED
HF3−R5570
Figure 5.
Figure 5 shows a typical resistive bias for a single
CHMSL LED string. The resistor value is calculated to take
into account the V
fwd
across the series connected LED
string. If a specific supply voltage, such as 13.5 V, is used,
a specific resistor can be chosen to supply a 30 mA drive
current.
Example for a 3 Red LED String:
V
supply
–V
sw_bat
– V
rpd
– (I_
led
R
1
) – (3 V
fwd
) = 0 V
V
sw_bat
= 0 V
V
supply
= 13.5 V Typical
V
rpd
= 0.8 V
V
fwd
= 2.20 V
I_
led
= 30 mA
R
1
+
13.5 V
*
0.8 V
*
3(2.20 V)
30 mA
(eq. 1)
Figure 6. Typical Application Circuit
(30 mA each LED String)
Number of LED’s that can be connected is determined by:
D1 is a reverse battery protection diode
LED’s = (V
in
−
(Q
X
V
F
+ D1 V
F
)) / LED V
F
Example: V
in
= 12 Vdc, Q
X
V
F
= 3.5 Vdc, D1VF = 0.7 V
LED V
F
= 2.2 Vdc @ 30 mA
(12 Vdc
−
4.2 Vdc)/2.2 Vdc = 3 LEDs in series.
For application versatility, if more Current drive is
required, the CCRs can be connected in parallel to boost the
regulated current.
D1
Anode
Q1
Q2
Qx
+
203
W
or205
W
(Standard 1% Value).
This method for setting the current with a specific resistor
is well known. By knowing the LEDs worst case V
fwd
, and
the light intensity required, a specific range of resistor values
can be chosen. However, as the supply voltage varies from
9 V to 16 V, the current changes in the LED which affects the
intensity. With the same 205
W
resistor and 9 V supply,
rearranging the equation and solving for I_
led
yields 7.8 mA.
Assuming all of the parameters remain constant and the
supply voltage is elevated to 16 V, an I_
led
value of 42 mA
is calculated. Again, the intensity of the LED is affected.
A CCR from ON Semiconductor would keep the current
and intensity constant over this supply voltage range
(Figure 2).
Here is how you can use ON Semiconductor’s CCR to
determine how many series LEDs it can drive.
Cathode
+
−
V
in
LED
HF3−R5570
LED
HF3−R5570
LED
HF3−R5570
Figure 7. Typical Application Circuit
(90 mA each LED String)
Number of LED’s that can be connected is determined by:
D1 is a reverse battery protection diode
Example: V
in
= 12 Vdc, Q
X
V
F
= 3.5 Vdc, D1VF = 0.7 V
LED V
F
= 2.6 Vdc @ 90 mA
(12 Vdc
−
(3.5 + 0.7 Vdc))/2.6 Vdc = 3 LEDs in series.
Number of Drivers = LED current/30 mA
90 mA/30 mA = 3 Drivers (Q1, Q2, Q3)
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3
AND8349/D
MBRS140T3G
(Reverse Protection Diode)
Vbat (+)
Jumper(s) selects DUT
Bias Resistor
(250
W)
NSI45030T1G
(CCR)
V
LED
Test Point
Vbat (-)
Jumper(s) to add/remove
LED from circuit
NSI45030T1G
(CCR)
HF3-R5570
(3 red LEDs)
Figure 8.
CCR Demo Board
This demo board (Figure 8) is the circuit shown in
Figure 6. It is used to generate several curves and can be used
to validate the CCR operation.
Figure 9 shows a comparison of power dissipation in a
CCR vs. Power dissipation in a Bias Resistor over battery
voltage variation from 9 V to 16 V. The CCR Power is less
than a Bias Resistor at higher operating voltages. At higher
Battery voltage, a higher wattage Power resistor would be
required increasing the circuit cost.
Figure 10 shows a typical Current / Voltage curve for a
CCR device. ON Semiconductor’s CCR is designed to have
400
350
300
P
d
, (mW)
250
200
150
100
50
0
9
10
11
12
13
14
15
16
P
d
CCR (mW)
P
d
Resistor (mW)
a slight negative trend as the power dissipation increases.
This negative trend reduces the power dissipation in the
CCR compared to the increasing power dissipation for a bias
resistor (Figure 9) and helps to prevent thermal runaway.
Since reduction in current is small, the change in LED
intensity is minimal.
Figure 11 shows thermal estimates for the NSI45030T1G
device with various heatsink footprints. If the heatsink area
is increased, the ambient operating temperature may be
increased. It is up to the circuit designer to understand the
thermal environment of the application and allow for device
thermals as specified in the device data sheet.
35
I
reg
, CURRENT REGULATION (mA)
30
25
20
15
10
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10
V
in
(V)
V
AK
, ANODE−CATHODE (V)
Figure 9. CCR P
d
vs. Resistor Pd
Figure 10. CCR−IV Characteristics @ 25
5
C
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4
AND8349/D
THERMAL ESTIMATES FOR THE SOD-123 CCR DEVICE
800
P
D
, POWER DISSIPATION (mW)
700
600
500
400
300
200
100
−40
100
mm
2
2 oz
500 mm
2
2 oz
500 mm
2
1 oz
300 mm
2
2 oz
300 mm
2
1 oz
P
D
max @ 855C
500 mm
2
2 oz Cu
500 mm
2
1 oz Cu
300 mm
2
2 oz Cu
300 mm
2
1 oz Cu
100 mm
2
2 oz Cu
100 mm
2
1 oz Cu
241 mW
228 mW
189 mW
182 mW
117 mW
108 mW
100 mm
2
1 oz
−20
0
20
40
60
80
T
A
, AMBIENT TEMPERATURE (°C)
Figure 11. Power Dissipation vs. Ambient
Temperature @ T
J
= 1505C for Variable Copper
Heat Spreader
Summary:
Simple, Economical and Robust (SER), the solid state
CCR will allow the user to achieve the expected long life of
their LED array.
CCRs will improve the efficiency and extend the life of
CHMSL LEDs. They will minimize design time and speed
up time to market.
Eliminating the large range of resistor values that must be
uniquely chosen to compensate for the LED’s variation in its
V
fwd
is the best benefit to CCR LED biasing.
Since LED brightness is determined by operating current,
optimum intensity will be attained by using a CCR approach
to maintain consistent luminosity over the wide variation of
battery voltage in automotive electrical systems.
SOT−223 package devices are also available which
improve power dissipation. See application note
AND8391/D for a through thermal discussion for both the
SOD−123 and SOT−223 packages.
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