Understanding protection modes for LED drivers

Driving LEDs, especially high-brightness, higher-power ones, is simple in concept – all you need is a dc/dc converter that can supply the necessary power, at the required current level, typically in the 1-A range. But driving LEDs means delivering significant power, so there are practical issues that must be addressed. These include dealing with LED open circuit, short circuit, and temperature rise. LED overheating is a particular problem as excessive temperatures will, at the very least, greatly shorten an LED’s life and in some cases can result in catastrophic, premature failure of these expensive devices. This article discusses requirements for protecting LED drivers, paying special attention to how LED overheating can be avoided by using a thermal regulator circuit with current foldback.

Background on LED operating characteristics
High-brightness LEDs, especially those intended for lighting applications, typically deliver optimum performance with a forward voltage of around 3 V. However, as can be seen in Fig. 1 , the relationship of voltage to current is nonlinear, so while the LED will start to turn on at a lower voltage, it will rapidly draw a much higher current as the voltage increases above its nominal rating. For this reason, the conventional solution for driving LEDs is to control the current through the device.

Fig. 1: Typical LED drive characteristic.

The relationship between the relative luminous flux of an LED (a measure of its light output) and the forward current is also nonlinear, so doubling the current does not result in double the light output. What’s more, the relative chromaticity, that is, color tone, of an LED is also affected by variations in current, which is another reason for sticking to the specified nominal current rating for an LED. An overdriven “white” LED will tend toward blue and underdriven toward yellow. This is why PWM dimming of LEDs is preferred to simple analog dimming.

Even this isn’t the whole story, since LED temperature is also a factor and a 30% drop in the relative luminous flux as the LED’s junction temperature increases quite typically from 25° to 150°C. Chromaticity also varies with temperature. The challenge then is keeping the LED junction temperature in check, which is why LED manufacturers provide guidance on thermal design and include graphs in their datasheets specifying the maximum forward current versus temperature possible for various values of junction to ambient thermal resistance.

But, I hear you say, “Aren’t LEDs super efficient, surely they don’t produce much heat.” Unfortunately, this isn’t true, and while LED light bulbs are certainly more efficient than incandescent bulbs, producing the same light output for around one fifth of the input power, an LED bulb still only converts around 15% of the energy it consumes to light; the rest is lost as heat. [Note: The reason white-light LEDs are often thought to be cool to operate is because they don’t produce heat as infrared (IR) radiation, which accounts for 83% of the energy used by an incandescent lamp along with 12% lost as heat, leaving just 5% output as visible light.]

Clearly, managing temperature is important, not only to extract the best performance from an LED but also to avoid elevated junction temperatures, which significantly reduces an LED’s life expectancy. Initial projections from the US-based Lighting Research Center suggest that the life of an LED at 85ºC might be just 20,000 hours compared to 80,000 hours at 45°C (where “life” is defined as a 50% reduction in relative light output). And if appropriate thermal design precautions can’t completely avoid excessive temperatures, then the provision of some form of overheating protection may be necessary.

LEDs rarely fail catastrophically, even as a consequence of high-temperature operation, but can experience the normal early- or end-of-life failures that statistically characterize any semiconductor device, resulting in either an open or short circuit. Equally, or indeed perhaps more likely, other system failures may result in an LED driver seeing an open- or short-circuit condition.

In considering requirements for an LED driver, we need to think about protection modes for open and short circuits, and overheating conditions. Adding protection to existing designs is possible but the integration of such features in newer LED driver ICs makes life much easier. However, to better understand these solutions we will look at the XRP7613 LED driver from Exar.

LED driver protection modes
The XRP7613 from Exar is a 1.2-A 36-V step-down, high-brightness, LED driver using a “floating buck” topology. Besides having an input voltage range meeting the needs of the relatively new EMerge Alliance, it includes several features to ensure safe operation under abnormal operating conditions. To appreciate how the  XRP7613 does this, it is useful to refer to its application circuit (Fig. 2 ) and internal block diagram (Fig. 3 ).

Fig. 2: XRP7613 application.

The normal LED current is set using the external sense resistor RSET, which also biases the LED string through the inductor to ground via the internal FET. In the event of an LED open circuit, RSET will pull the voltage sensed on pin ISEN up to VIN, which will cause the device to shut down.

Under an LED short-circuit condition, the device will maintain the LED current as determined by resistor RSET. 
 

Fig. 3: XRP7613 internal block diagram.

Overheating protection is provided using a current foldback technique that applies automatic linear dimming of the LED current as the temperature increases. It is achieved by monitoring the voltage (VTH) created by the potential divider circuit shown in Fig. 2, where RTH is a negative temperature coefficient (NTC) thermistor and RT is a fixed resistor connected to a reference voltage (VREF). The values of RT and the characteristic of RTH are chosen such that as the LED temperature reaches 70°C the voltage at the TH pin reduces to 0.4 V and then reduces further to 0.28 V as the temperature increases to 90°C. This allows the nominal LED drive current to be maintained up to 70°C but progressively reduced up to 90°C after which the LED is turned off, allowing it to cool, until VTH rises to 0.3 V when the LED is turned on again. Clearly, as implied by Fig. 2, the thermistor needs to be in close thermal contact with the LEDs.

 Author biography  

Jon Cronk joined Exar in 2006 as Director of Technical Marketing and Application Engineering. Jon’s experience prior to that spans nearly three decades working in the fields of power supply design and magnetics. He has held various design, applications and marketing roles for leading companies including National Semiconductor and Bell Laboratories. Jon has an MSEE from the Stevens Institute of Technology and a BSEE from Virginia Polytechnic Institute and State University.