AND8349-D Datasheet, PDF (1/6 Pages) ON Semiconductor – Automotive Applications The Use of Discrete

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
70
60
50
40
30
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10
0
−10
−20
−30
−10
0
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50
V_AK, ANODE−CATHODE (V)
Figure 1. CCR IV Characteristics
© Semiconductor Components Industries, LLC, 2009
1
May, 2009 − Rev. 1
Publication Order Number:
AND8349/D