Two overriding issues should be considered when driving high-brightness (HB) LEDs. The first is to ensure that a fairly constant current is available with minimal variation, since a fluctuation in current can cause an apparent change in brightness. The LED having a nonlinear response to current can exacerbate this. The second issue is to control the temperature of the LED. Temperature affects the brightness and longevity of an LED, so it is important that lighting systems enable heat to be effectively dissipated to control LED temperature. In addition, it may be necessary to monitor temperature and reduce the current through the LED. This has the effect of lowering its temperature and optimizing light output, efficiency, and longevity.
There are broadly three different kinds of LED control: resistor-based, linear control, and switching control. All three may be represented by Figure 1.
LEDs are typically arranged in a string of up to 12. Each LED will have a forward bias voltage VF of typically 3–4 volts, and with a string of 12 this would require up to 48 volts. LEDs have ratings starting at about 350mA and go up to 3A plus. Resistor-based control is inexpensive and very simple to implement, but the downside is that current can vary when the VF changes. A string of up to 12 LEDs could have a significant variation in forward bias voltage, which in turn varies the voltage drop across the resistor. Additionally, placing a resistance in series with the LEDs is inefficient and will generate heat. Linear control is simple to implement but is also relatively less efficient, and will still cause heat to be dissipated. The optimal solution for HBLEDs uses switching control, which is highly efficient but relatively more expensive and normally requires some form of CPU. An example of this methodology can be found in Cypress Semiconductor Corp.’s PowerPSoC family of controllers.
With switching control drive, a constant current regulator is used to control the current through the LED (see Figure 2). At time t=0, the FET switch is turned on and current starts to flow through the LED string. The inductor limits the rate of increase in current. The RSENSE resistor is very small and detects the current passing through the string. When the differential voltage drop across RSENSE is large enough, the current sense amplifier detects that the threshold is reached and the FET switch is turned off. Energy stored in the inductor still allows current to flow through a diode in the return path. The current through the string will diminish as energy stored in the inductance is dissipated. When a lower voltage threshold is detected across RSENSE, the FET is switched on and current ramps up again. The size of the ripple in the current is determined by the hysteresis in the switching characteristics of the comparator. In PowerPSoC this feature is programmable and is set by digital-to-analog converters (DACs). Current ripple is generally not detectable by the human eye as long as the ripple is <20%.
LED shortcomings need to be taken into account when applying this methodology. LEDs yield different values of brightness, power, and chromaticity in manufacturing and are put into different buckets (or bins). LEDs with a tighter specification are more expensive; LEDs with a looser spec are cheaper but more difficult to control their color and brightness.
This makes it difficult to achieve repeatable color points. Applying the same current to two sets of red/green/blue LEDs from varying chromaticity bins will produce visibly different hues of the same color. Similarly, applying the same current to LEDs from two flux bins will produce two levels of brightness. Therefore, engineers require flexibility and programmability within their switching regulators to account for these variations.
Additionally, LED output degrades over temperature and different color LED outputs degrade at different rates. This makes it difficult to maintain the same color over time, especially taking into account the heating of the fixture due to LED power dissipation. Thermal feedback capability in a switching regulator ensures that color variation is kept to a minimum. PowerPSoC implements this feature, allowing engineers to de crease the DAC values on the upper and lower threshold levels for RSENSE current to reduce power dissipation.
Dimming of LEDs also poses a problem. The color temperature of LEDs varies over current, and luminescence is a non-linear response to current, making precision analog dimming difficult. To do this effectively requires knowledge of the transfer function of the LED to map a linear ramp to the non-linear response. For multicolored LEDs this is complex. Most controllers use digital dimming by a PWM to get around this. The PWM mark space ratio and frequency is used to control the brightness of the LED by turning off the FET switch, causing the LED current to go to zero. The LEDs would then remain off until the PWM output changes. Then the FET control is passed back to the comparator function, avoiding any problems associated with non-linearities. The switching frequency must also be above 150Hz to avoid flicker. Too high a frequency of PWM will not allow the current ramp to go to zero, and switching losses in the FET might become apparent over time.
One alternative method of digital dimming is Cypress’ Precise Illumination Signal Modulation (PrISM) used in the PowerPSoC controller. A normal PWM utilizes a counter and a programmable combinatorial decode to drive its output. PrISM utilizes a pseudo-random source based on a shift register and 2-input EXOR with feedback into the shift register. This enables de signers to take the values of the counter over the PWM period ‘T’ and mixes them in an apparently random fashion. The value present on the shift register then feeds into programmable combinatorial logic, which creates a pseudo- random series of pulses. Over the period ‘T’ this modulation guarantees the overall high and low times are maintained. PrISM is a form of pulse density modulation, which effectively reduces peak EMI emissions, especially when switching several strings of LEDs.
LED control is far from straightforward, and most engineers will come across some of the issues raised here. Embedded intelligence is required to control these LEDs. PowerPSoC represents an attempt to address designers’ issues with a programmable controller that can handle the many variations in LED performance over time, over changes in temperature, and between bins. An easy-to-use tool with lots of pre-programmed data about LEDs eliminates much of the complexity involved in managing LED performance.