Automotive LED Driver Keeps Backlights Running Under Cold Cranking

Transcription

1 Automotive LED Driver Keeps Backlights Running Under Cold Cranking by L.K. Wong and T.K. Man, National Semiconductor, Hong Kong ISSUE: January 2011 In recent years, LED strings have become a popular method for backlighting LCD panels in automotive applications. In these and other applications, LEDs are chosen over other backlighting options because of their fast response time, high contrast ratio, and low power consumption. In automotive designs, the LED driver controlling the LED strings is powered by the vehicle s battery. To ease the design of the optics and heat sink for the LEDs, the number of LEDs used in each string is normally more than six. So, the voltage required to drive the LEDs will be greater than the battery s nominal voltage of 12 V. One way to drive the LEDs is to use a boost converter in combination with a linear current regulator as shown in Fig Battery V IN Boost converter V OUT LED string 1 LED string 2 LED string n Linear current regulator Fig. 1. An LED driver for automotive LED backlights consists of a boost converter and a linear current regulator. Cold-Cranking Condition Variation of the battery voltage is common in automotive applications. Normally, the output of a 12-V vehicle battery varies from 9 V to 16 V. Therefore, all circuits powered by the battery should be designed to account for the input-voltage variation. Furthermore, if a cold-cranking condition is to be considered, the battery voltage can drop down to 3 V±0.2 V, i.e. 2.8 V in the worst case, for a short duration of around 15 ms. Then, the battery voltage goes up to 6 V, where it remains for a few seconds, before returning to the nominal voltage range within a rise time of a few seconds. Under a cold-cranking condition, the battery voltage profile will be similar for different automotive manufacturers. But with respect to the voltage level and timing, there will be variations among the different manufacturers. One typical example is shown in Fig. 2. This illustrates the case where the cold-cranking condition occurs during the vehicle s load dump and at low temperature. Although the duration of the condition is not long, important automotive equipment like LCD panels and safety electronics are still required to maintain normal operation during this period How2Power. All rights reserved. Page 1 of 6

2 Battery voltage 12V 6V 3V Time 5ms 15ms 50ms 10s 10s Fig. 2. Typical battery voltage profile under a cold-cranking condition. Design Consideration To design LED drivers that continue to operate under the cold-cranking condition, a robust boost converter is required. Referring back to Fig. 1, the variation of the battery voltage affects only the boost converter. If the boost converter can maintain the output voltage under cold cranking, the operation of the linear current regulator will not be affected, and therefore the brightness of the LED strings will remain unchanged. However, since the input voltage is very low under the cold-cranking condition, there are some extra design considerations such as the supply range of the boost controller, input current variation, and thermal management. Operating range of boost controller. Theoretically, the input voltage of a boost converter (not boost controller) can be low provided that the upper limit of the duty ratio is not reached. However, the operating voltage of a boost controller (usually an IC) has a lower limit. Most boost controllers cannot operate at an input voltage lower than 3 V. But under the cold-cranking condition, the battery voltage can be as low as 2.8 V in the worst case as mentioned previously. A conventional method of powering a boost controller at low input voltage (under the IC s normal operation, not start up) uses dual power supply paths. One supply path is simply the battery voltage connected to the input of the boost controller through a diode. The other path is the output voltage of the boost converter connected back to the boost controller input through a diode. These dual power supply paths are shown in Fig. 3. In this case, the boost controller is powered by the battery during power-up; and by the output of the boost converter afterward. Then, the drop of the input voltage no longer affects the operation of the boost controller as long as the boost converter can provide a normal output voltage How2Power. All rights reserved. Page 2 of 6

3 Battery VIN Boost converter GND VOUT Fig. 3. Boost converter with dual power supply paths. Input Current. If the luminance (brightness) of the LED strings is required to remain unchanged, the output power of the boost converter must be maintained. As a result, the input current will increase with the decrease of the input voltage. This implies that under the cold-cranking condition, the peak inductor and MOSFET switch currents will be much larger as compared with nominal operation (when the battery voltage falls in the nominal range). For example, the input current at a battery voltage of 3 V will be about four times the input current at a battery voltage of 12 V. Hence, inductors with larger saturation current and MOSFET switches that can withstand higher turn-on current should be selected. Thermal management. Under the cold-cranking condition, not only will the current be higher, but also the duty ratio of the MOSFET switch will be greater than in nominal operation. This implies a drastic increase in the conduction loss of the MOSFET switch. Although the duration of the cold-cranking condition is short, a significant temperature rise in the MOSFET switch can occur. If the MOSFET switch is integrated in a boost controller IC (which is desirable to minimize component count and solution size), an effective heat dissipation mechanism is critical. Otherwise, poor thermal management may trigger the thermal protection circuit of the integrated circuit and lead to unexpected turn-off of the LEDs. Proposed circuit Fig. 4 shows a popular LED driver circuit for LCD panel backlighting in automotive applications. This circuit, which accounts for the cold-cranking condition, uses the LM3492 from National Semiconductor. The circuit drives two LED strings, each of which consists of six LEDs running at 100 ma. The nominal input voltage of the circuit ranges from 9 V to 16 V, and the nominal switching frequency is 300 khz. L1 33 µh, 3.36A V IN 9V - 16V 2.8V when cold crank CIN1,2 2 X 10 µf 25V CCDHC 0.47 µf DIN RCOMM 52.3 kω C VCC 1 µf CVIN 0.22 µf VIN VCC EN PGND LM3492 GND CDRH129HF-330MC CDHC SW RT VOUT FB IREF ILIM 40V 1A D1 R FB0 R RT 0.1 Ω 392 kω R FB1 C FB 178 kω 10 pf R IREF R FB kω 16.2 kω V OUT 2 X 10 µf, 35V GJ832ER7YA106KA12L CO1,2 6 LEDs per string COMM LGND DIM1/CLK DIM2 IOUT1 IOUT2 Fig. 4. The LM3492 LED driver reference design maintains operation even under cold-cranking conditions How2Power. All rights reserved. Page 3 of 6

4 Basically, the circuit includes a boost converter and a two-channel linear current regulator. In addition, a MOSFET switch is integrated in the LM3492 so that a small solution size can be achieved. The VIN pin of the LM3492 is connected to the input voltage through a diode (D IN ). Meanwhile, within the IC a diode connects the VIN pin to the VOUT pin, which is connected to the output voltage of the boost converter. This second power supply path will power the LM3492 if the battery voltage is lower than the minimum requirement of 4.5 V. For the boost circuit inductor, a component with an inductance of 33 µh with a saturation current of 3.36 A is selected, although a saturation current of 1 A is enough for the normal input voltage range. Measured results Figs. 5 and 6 show waveforms for the LM3492 circuit when the input voltage is 12 V (nominal) and 2.8 V (the worst case under the cold-cranking condition). Table 1 summarizes the measured results when the LM3492 circuit operates under both the nominal and cold-cranking conditions. It can be observed that the LED current remains unchanged even when the input voltage is as low as 2.8 V, meaning that the luminance of the backlight is not affected by cold cranking. Fig. 5. Waveforms of the LM3492 circuit at an input voltage of 12 V. From the waveforms of Fig. 6, it is seen that the duty ratio is larger than 90% at the 2.8-V input voltage. Also, a large input current of 2.61 A is measured, as shown in Table 1. Consequently, the power loss in the internal MOSFET switch is large. A temperature rise of 90 C is measured on the case of the LM3492 under cold cranking versus only a 30 C temperature rise at the nominal input voltage range. With reference to the efficiency in Table 1, the efficiency drops to 56% under the cold-cranking condition, while a high efficiency of around 90% can be obtained at the nominal input-voltage range How2Power. All rights reserved. Page 4 of 6

5 Fig. 6. Waveforms of the LM3492 circuit under the cold-cranking condition. Table 1. Summary of measured results for the LM3492 LED driver reference design. Input voltage Input current LED string voltage LED current Efficiency 2.8V 2.61A 20.7V 0.1A / string 56.7% 6V V 0.1A / string 88.5% 9V 0.51A 20.7V 0.1A / string 90.2% 12V 0.38A 20.7V 0.1A / string 90.7% 16V 0.29A 20.7V 0.1A / string 92.4% Conclusion This article introduces a popular LED driver circuit for LCD panel backlights in automotive applications that require continuous operation of the LEDs even under cold-cranking conditions. Special design considerations including the dual power supply paths, selection of inductor with a higher saturation current, and thermal management have been discussed. Measured waveforms show that the LED current is not affected and the circuit can maintain normal operation even when the battery input voltage drops to 2.8 V. Even while meeting this requirement for low-voltage operation, the boost converter achieves a high efficiency of around 90% while operating in the nominal input voltage range of 9 V to 16 V. About The Author L. K. Wong received his Ph.D. degree at the Hong Kong Polytechnic University in Wong joined National Semiconductor as a senior product application engineer in 2006, working in the area of power electronics including modeling, control, and dc-dc power converter design. Currently, he is focusing on high-efficiency, high-performance LED driver design for general illumination and backlight applications. He is a member of the IEEE How2Power. All rights reserved. Page 5 of 6

6 T. K. Man received the BSc (Hon) degree at Manchester Polytechnic in 1984, MSc degree in engineering at Hong Kong Polytechnic University in 1999 and MSc degree in IC Design Engineering at Hong Kong Science and Technology University in Man has 22 years experience in power supply design, both linear and switching. He joined National Semiconductor in 2004 as a product application manager in Hong Kong. Prior to National, Man worked at Motorola and ON Semiconductor. His research interests include control modeling of SMPS, LED lighting systems and energy harvest technology. He is a senior member of IEEE. For more on designing LED driver circuits, see the How2Power Design Guide, select the Popular Topics category, and then search the LED lighting subcategory How2Power. All rights reserved. Page 6 of 6