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AND8349/D
Automotive Applications:
The Use of Discrete
Constant Current
Regulators (CCR) for
CHMSL Lighting

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APPLICATION NOTE
An innovative use of a new Patent Pending Discrete
Technology (Constant Current Regulator – CCR) will beshown 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 orsink. Current regulation can be achieved for Anode-Cathodevoltages 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 arrangementsallow for higher load current applications. (Figure 7)
The CCR reduces the complexity of resistor biased
designs for sensitive loads such as LED strings connected inseries (Figure 4). Simply apply a voltage greater than
Voverhead to achieve an accurate regulated current.

LEDs are being proliferated into many automotive
lighting applications. The Center High Mount Stop Lamp
(CHMSL) is one of several automotive applications forLEDs. 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 havemade 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 approachto maintain consistent luminosity over the wide variation ofbattery 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

IREG CURRENT REGULATION (mA)

80

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.

70
60
50
40
30
20
10
0

−10
−20
−30
−10

0

10
20
30
VAK, ANODE−CATHODE (V)40

50

Figure 1. CCR IV Characteristics© Semiconductor Components Industries, LLC, 2014

January, 2014 − Rev. 3

1

Publication Order Number:AND8349/D

AND8349/D
an 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 typicalbiasing 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 Biascurrent 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.

A CCR is a nearly ideal current source providing constantcurrent regardless of applied voltage above its operating
minimum. In simple terms, a CCR can be considered
a nonlinear voltage controlled resistor.
The Power Dissipation (Pd) in an LED is Pd = I V. Since
the CCR acts as a voltage controlled resistor while theresistor biasing fixes the resistor value, the LED Powerdissipation is shown to be nearly constant over a variable
battery range. (Figure 3)
For automotive CHMSLs, a constant current source forLEDs reduces stress conditions caused by overdriving withcurrent as compared to resistor biasing. A Reverse
protection diode (MBRS140T3 in Figure 4) prevents
a reverse voltage condition which can permanently damage240

40
TA = 25°C
35

Circuit Current withCCR Device
Pd LEDs (mW)

I (mA)

Circuit Current
with 250 W

20

LED Power withCCR Device

200

30
25

TA = 25°C

220

Representative Test Data
for Figure 6 Circuit, Currentof LEDs, FR−4 @ 300 mm2,
1 oz Copper Area

15

180
160
140
LED Powerwith 250 W

120

Representative Test Data
for Figure 6 Circuit, Pd ofLEDs, FR−4 @ 300 mm2,
1 oz Copper Area

100
80

10

60
9

10

11

12

13

14

15

9

16

10

11

12

13

14

Vin (V)Vin (V)

Figure 2. Series Circuit Current

Figure 3. LED PowerMBRS140T3

CCR

CCR

Figure 4.

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2

CCR

CCR

15

CCR

16

AND8349/D
D8

R1

D1
205

MBRS140T3G

HF3−R5570
D5

Anode

Q1

Q2

Qx

CathodeVSUPPLY
13.5Vdc

HF3−R5570
D6

+

Vin

−

HF3−R5570
D7

0

LED

LED

LED

HF3−R5570

HF3−R5570

HF3−R5570

LED

LED

LED

HF3−R5570

HF3−R5570

HF3−R5570

LED

LED

LED

HF3−R5570

HF3−R5570

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 Vfwd 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:
Vsupply –Vsw_bat – Vrpd – (I_led R1) – (3 Vfwd) = 0 V
Vsw_bat = 0 V
Vsupply = 13.5 V Typical
Vrpd = 0.8 V
Vfwd = 2.20 V
I_led = 30 mA
R1 +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 diodeLED’s = (Vin − (QX VF + D1 VF)) / LED VF
Example: Vin = 12 Vdc, QX VF = 3.5 Vdc, D1VF = 0.7 V
LED VF = 2.2 Vdc @ 30 mA(12 Vdc − 4.2 Vdc)/2.2 Vdc = 3 LEDs in series.

For application versatility, if more Current drive isrequired, the CCRs can be connected in parallel to boost theregulated current.

13.5 V * 0.8 V * 3(2.20 V)
30 mA

D1

(eq. 1)+ 203 W or 205 W (Standard 1% Value).

AnodeThis method for setting the current with a specific resistor
is well known. By knowing the LEDs worst case Vfwd, andthe 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 theintensity. 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 todetermine how many series LEDs it can drive.

Q1

Q2

Qx

Cathode
+
−

Vin

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 diodeExample: Vin = 12 Vdc, QX VF = 3.5 Vdc, D1VF = 0.7 V
LED VF = 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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AND8349/D
MBRS140T3G(Reverse Protection Diode)
Vbat (+)

Bias Resistor(250 W)

Jumper(s) selects DUT

NSI45030T1G
(CCR)
VLED
Test Point

HF3-R5570(3 red LEDs)

Jumper(s) to add/remove
LED from circuit

Vbat (-)

NSI45030T1G(CCR)

Figure 8.
CCR Demo Board

have a slight negative trend as the power dissipation
increases. This negative trend reduces the power dissipationin the CCR compared to the increasing power dissipation fora 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 thethermal environment of the application and allow for device
thermals as specified in the device data sheet.

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 ina CCR vs. Power dissipation in a Bias Resistor over battery
voltage variation from 9 V to 16 V. The CCR Power is lessthan 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 fora CCR device. ON Semiconductor’s CCR is designed to400

Ireg, CURRENT REGULATION (mA)

35

350

Pd, (mW)

300
250
200
Pd CCR (mW)

150
100

Pd Resistor (mW)

50
0
9

10

11

12

13

14

15

30

25

20

15

10

16

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Vin (V)

VAK, ANODE−CATHODE (V)Figure 9. CCR Pd vs. Resistor Pd

Figure 10. CCR−IV Characteristics @ 255C

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10

AND8349/D
THERMAL ESTIMATES FOR THE SOD-123 CCR DEVICE

PD, POWER DISSIPATION (mW)

800
700

PD max @ 855C
500 mm2 2 oz
500 mm2 1 oz

600

300 mm2 2 oz
500

300 mm2 1 oz

400
100 mm2 2 oz
300
200
100
−40

500 mm2 2 oz Cu

241 mW

500 mm2 1 oz Cu

228 mW

300 mm2 2 oz Cu

189 mW

300 mm2 1 oz Cu

182 mW

100 mm2 2 oz Cu

117 mW

100 mm2 1 oz Cu

108 mW

100 mm2 1 oz
−20

0

20

40

60

80

TA, AMBIENT TEMPERATURE (°C)

Figure 11. Power Dissipation vs. Ambient
Temperature @ TJ = 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
Vfwd 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 approachto maintain consistent luminosity over the wide variation ofbattery 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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AND8349/D
APPENDIX A
SOD−123 Devices are:SOT−223 Devices are:

NSI45020T1G, Steady State Ireg(SS) = 20 mA ±15%
NSI45025T1G, Steady State Ireg(SS) = 25 mA ±15%
NSI45030T1G, Steady State Ireg(SS) = 30 mA ±15%
NSI45020AT1G, Steady State Ireg(SS) = 20 mA ±10%
NSI45025AT1G, Steady State Ireg(SS) = 25 mA ±10%
NSI45030AT1G, Steady State Ireg(SS) = 30 mA ±10%

NSI45025ZT1G, Steady State Ireg(SS) = 25 mA ±15%
NSI45030ZT1G, Steady State Ireg(SS) = 30 mA ±15%
NSI45025AZT1G, Steady State Ireg(SS) = 25 mA ±10%
NSI45030AZT1G, Steady State Ireg(SS) = 30 mA ±10%

APPENDIX B
Application Note

Title

AND8391/D

Thermal Considerations for the ON Semiconductor Family of Discrete Constant Current
Regulators (CCR) for Drivings LEDs
How To Use Thermal Data Found in Data Sheets
Predicting the Effect of Circuit Boards on Semiconductor Package Thermal Performance
Predicting Thermal Runaway

AND8220/D
AND8222/D
AND8223/D

The products described herein (NSI45030T1G) has patents pending.
ON Semiconductor andare registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks,
copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC
reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any
particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without
limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications
and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC
does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for
surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where
personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and
its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly,
any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufactureof the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.

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