Design Article
Improving automotive lighting designs with high-performance LED drivers
By Jeff Gruetter, Linear Technology Corp.
1/3/2013 6:04 PM EST
New HB LED Driver Controller
Linear Technology’s LT3795 LED driver is a boost DC/DC LED driver that can deliver over 100W of LED power. Its 6V to 110V input voltage range makes it ideal for a wide variety of HB LED applications found in automotive, commercial truck and even avionic systems. Its high side current sense design can be configured in boost, buck-boost, SEPIC and buck mode architectures offering a wide range of design flexibility. Additionally, its output voltage can be set from 0V to 110V enabling to drive a wide range of LEDs in a single string, while offering very robust and simple short-circuit protection in a boost design.
A typical 50W headlight application is shown in Figure 3 below. This application uses a boost architecture to deliver up to 87V of LED voltage which can drive as many as 20 LEDs with LED currents in excess of 600mA. Most DRLs use between 8 and 15 LEDs in the 350mA range, whereas low beam headlights will use approximately 50V of 1A LEDs and high beams can use as much as 2A @ 50V. The LT3795 can be used for both of these applications.
The LT3795 incorporates a high-side current sense, enabling its use in boost, buck, buck-boost or SEPIC and flyback topologies. Additionally, it offers both input and output current limiting and monitoring for added reliability and design flexibility. The LT3795 can also deliver efficiencies of over 94% in boost mode, minimizing the need for external heat sinking. A frequency adjust pin enables the user to program the frequency between 100kHz and 1MHz, optimizing efficiency while minimizing external component size and cost. Spread spectrum frequency modulation can be activated for improved electromagnetic compatibility by lowering switching noise by 20db.
The LT3795 has an integrated short-circuit protection feature that stops the regulator from switching when a short circuit is detected and disconnects the LED array from the power path. Furthermore, it reports a short-circuit condition on the SHORTLED pin. After short-circuit protection is enabled, the LT3795 can be programmed to utilize a hiccup mode to see if the short-circuit has been corrected or a latch off mode which requires the EN/ULVO to be toggled to restart the device. This feature offers very robust short-circuit protection for both boost and buck-boost applications, regardless of the output voltage. Additionally, the LT3795 also offers open LED protection and reporting so the LEDs aren’t damaged, if one of the LEDs if one in the string experiences and open-circuit.
The LT3795 delivers LED current accuracy of +3% which ensures constant lighting in an LED string while + 2% output voltage accuracy offers several LED protection features and also enables the converter to operate as a constant voltage source. The LT3795 can utilize PWM dimming to offers dimming ratios as high as 3,000:1 or analog dimming for ratios up to 20:1. Additional features include output disconnect, input and output current limits/ monitors, and integrated fault protection. Its high level on integration and thermally enhanced TSSOP-28 package offer a very compact HB LED driver solution.
Conclusion
The continual acceleration of HB LED applications, especially those found in automotive DRLs and headlamps, is being driven by an insatiable demand for higher performance and cost effectiveness. These demands must be enabled by new HB LED driver ICs. As a result, these LED drivers must provide constant current in order to maintain uniform brightness, regardless of input voltage or LED forward voltage variations, operate with high efficiency, offer very wide dimming ratios and have a variety of protection features such and short-circuit and open-circuit protection to enhance system reliability.
Of course, these LED driver circuits must also offer a very compact, low-profile and thermally efficient solution footprint. Fortunately, Linear Technology is continually redefining its family of LED drivers to meet these challenges with HB LED driver ICs like the LT3795. In addition, we have developed an entire family of high current LED driver ICs aimed specifically at automotive application, ranging from advanced forward lighting headlamps to LCD backlighting.
As automotive lighting systems continue to demand higher performance LED drivers, designers will have innovative IC solutions to satisfy them.
About the author: Jeff Gruetter is Senior Product Marketing Engineer, Power Products
Linear Technology Corp.
Linear Technology’s LT3795 LED driver is a boost DC/DC LED driver that can deliver over 100W of LED power. Its 6V to 110V input voltage range makes it ideal for a wide variety of HB LED applications found in automotive, commercial truck and even avionic systems. Its high side current sense design can be configured in boost, buck-boost, SEPIC and buck mode architectures offering a wide range of design flexibility. Additionally, its output voltage can be set from 0V to 110V enabling to drive a wide range of LEDs in a single string, while offering very robust and simple short-circuit protection in a boost design.
A typical 50W headlight application is shown in Figure 3 below. This application uses a boost architecture to deliver up to 87V of LED voltage which can drive as many as 20 LEDs with LED currents in excess of 600mA. Most DRLs use between 8 and 15 LEDs in the 350mA range, whereas low beam headlights will use approximately 50V of 1A LEDs and high beams can use as much as 2A @ 50V. The LT3795 can be used for both of these applications.
Figure 3. 93% Efficient 50W (87V,
575mA) Buck LED Driver with
Input Current Limit & Spread Spectrum Frequency Modulation
Input Current Limit & Spread Spectrum Frequency Modulation
The LT3795 incorporates a high-side current sense, enabling its use in boost, buck, buck-boost or SEPIC and flyback topologies. Additionally, it offers both input and output current limiting and monitoring for added reliability and design flexibility. The LT3795 can also deliver efficiencies of over 94% in boost mode, minimizing the need for external heat sinking. A frequency adjust pin enables the user to program the frequency between 100kHz and 1MHz, optimizing efficiency while minimizing external component size and cost. Spread spectrum frequency modulation can be activated for improved electromagnetic compatibility by lowering switching noise by 20db.
The LT3795 has an integrated short-circuit protection feature that stops the regulator from switching when a short circuit is detected and disconnects the LED array from the power path. Furthermore, it reports a short-circuit condition on the SHORTLED pin. After short-circuit protection is enabled, the LT3795 can be programmed to utilize a hiccup mode to see if the short-circuit has been corrected or a latch off mode which requires the EN/ULVO to be toggled to restart the device. This feature offers very robust short-circuit protection for both boost and buck-boost applications, regardless of the output voltage. Additionally, the LT3795 also offers open LED protection and reporting so the LEDs aren’t damaged, if one of the LEDs if one in the string experiences and open-circuit.
The LT3795 delivers LED current accuracy of +3% which ensures constant lighting in an LED string while + 2% output voltage accuracy offers several LED protection features and also enables the converter to operate as a constant voltage source. The LT3795 can utilize PWM dimming to offers dimming ratios as high as 3,000:1 or analog dimming for ratios up to 20:1. Additional features include output disconnect, input and output current limits/ monitors, and integrated fault protection. Its high level on integration and thermally enhanced TSSOP-28 package offer a very compact HB LED driver solution.
Figure 4. LED Efficiency of Figure 3
Using the LT3795
Figure 5. Short-Circuit Protection in
Hiccup & Latchoff Mode for Circuit in Figure 3
Conclusion
The continual acceleration of HB LED applications, especially those found in automotive DRLs and headlamps, is being driven by an insatiable demand for higher performance and cost effectiveness. These demands must be enabled by new HB LED driver ICs. As a result, these LED drivers must provide constant current in order to maintain uniform brightness, regardless of input voltage or LED forward voltage variations, operate with high efficiency, offer very wide dimming ratios and have a variety of protection features such and short-circuit and open-circuit protection to enhance system reliability.
Of course, these LED driver circuits must also offer a very compact, low-profile and thermally efficient solution footprint. Fortunately, Linear Technology is continually redefining its family of LED drivers to meet these challenges with HB LED driver ICs like the LT3795. In addition, we have developed an entire family of high current LED driver ICs aimed specifically at automotive application, ranging from advanced forward lighting headlamps to LCD backlighting.
As automotive lighting systems continue to demand higher performance LED drivers, designers will have innovative IC solutions to satisfy them.
About the author: Jeff Gruetter is Senior Product Marketing Engineer, Power Products
Linear Technology Corp.
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anne-francoise.pele
4/12/2013 8:55 AM EDT
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