Constant Current Regulator for Driving LEDs
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To introduce On semicoductor’s Constant Current Regulators and its applications
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Constant Current Regulator for Driving LEDs
- 9. CCR Low Turn On Voltage V / I plot crosses through zero The CCR turns on fast immediatedly the voltage goes positive With 0.5V input the current is already at 5mA - 25% ON
- 10. CCR Surge Test Measure 2V 2V 2V 12 V 50 V 1 ms Pulse
- 13. CCRs from ON Semiconductor Parameters NSI450XXT1G SOD-123 (2-terminal) NSI450XXZT1G SOT-223 (2-Terminal) NSI500XXDZT1G SOT-223 (3-Terminal) NSI500XXDDT1G D-PAK (2-Terminal) Max Anode to Cathode Voltage (V AK ) 45V 45V 50V 50V Voltage Overhead 1.8 V 1.8 V 1.8 V 1.8 V Constant Current Ireg @ Vak = 7.5V 10, 20, 25 & 30 mA 25 & 30 mA 20 - 40 mA ADJ 35 - 70 mA ADJ 60 – 100 mA ADJ 90 – 160 mA ADJ Current Tolerance over Voltage ± 15%, ±10% ± 15%, ±10% ± 15%, ±10% ± 15%, ±10% Ambient Operating Temp Range -55 to 85 o C -55 to 85 o C -55 to 85 o C -55 to 85 o C Max Junction Temperature 150 o C 150 o C 150 o C 150 o C Power Dissipation ( 25 o C; 500mm 2 ) 463 mW 1389 mW 1389 mW 2400 mW Power Dissipation ( 85 o C; 500mm 2 ) 230 mW 750 mW 750 mW 1270 mW ESD Rating: HBM – 1C > 1kV > 1kV > 2kV > 2kV
- 17. Example of CCR Circuits 4 SW1 close – LED Dim Vibrator runs at 50% duty cycle SW2 close – LED Full Vibrator stops Select RC constant for 5 mS Dimming with external BJT Vibrator 2V 2V 2V 2V 2V 2V 2V 2V 2V SW2 SW1
- 18. Direct A/C Line LED circuit with CCR A/C 110 V RMS 3.52V 3.52V 3.52V 3.52V 3.52V 3.52V 30 LEDs 25 mA 100 Ω TP 1 TP 2 Current Loop TP 1 - 156 V P-P TP2 - LEDs 108 V, 52% On Current probe 25 mA
- 19. Direct A/C Line LED circuit with CCR -10% +10% 110 V RMS, TP1 - 156 V P-P TP2 - LEDs 108 V, 52% On Current probe 25 mA 100 V RMS, TP1 - 142 V P-P TP2 - LEDs 108 V, 47% On Current probe 25 mA 120 V RMS, TP1 - 170 V P-P TP2 - LEDs 108 V, 56% On Current probe 25 mA
Notes de l'éditeur
- This is an introduction for On Semi Constant Current Regulator for Driving LEDs.
- Welcome to the training module on Constant Current Regulator for Driving LEDs. This training module will introduce On semicoductor’s Constant Current Regulators and its applications.
- LEDs have been used as indicators for many years. Only recently has the efficiency of LEDs been refined. This allows them to become the major source of lighting in many different markets. LEDs have certain characteristics that need to be considered when using them. LEDs are still a diode. Once the voltage across them is greater than the turn on voltage, they turn on very fast. LEDs have a negative forward voltage to temperature coefficient. Reviewing a simple LED circuit with resistors, the LED warms up and forward voltage across LEDs reduces. The reduction of the voltage across LEDs results an increase of voltage across the resistor. The current through the resistor and LEDs increases resulting in more power being dissipated and LED temperature increasing. That leads to the potential for thermal runaway.
- LEDs have a negative light output to temperature coefficient. The warmer LEDs get, the less light it emits. LEDs also change color based on the temperature and current through them.
- To obtain the longest life and highest reliability it is very desirable to drive LEDs with a constant current and keep the temperature as stable as possible. The main function of an LED driver is to limit the current regardless of input and output conditions across a range of operating conditions. AC-DC power conversion and driver regulation can be merged together into a single driver or separated into two stages.
- LED drive circuits vary from being very simple to being very complicated. The most complex one is the switching regulator. It is the most efficiency but is often the most complex, expensive and has potential for EMI. Linear regulators, constant voltage or constant current, have some features of switching regulators but they are less efficient, do not have EMI issue and cost less than switching regulators. At the bottom end is the current limiting resistors. Obviously they have lowest cost and no EMI issues but they only provide regulation if the input voltage is regulated.
- The constant current regulator or CCR is a small step up from the resistor circuit. It offers a simpler and lower cost solution compared to linear and switching regulators with significant improvement of performance compare to resistor circuits.
- CCRs are self biased transistors and can be 2 or 3 terminal devices. The 2 terminal devices are designed and built to provide a specific current, i.e. 30 mA. The 3 terminal devices are adjustable across a small current range by chaning a bias resistor, i.e. 35 mA – 70 mA. The higher the bias resistor, the lower the current.
- These curve traces were taken from 20mA CCR. The top left trace shows if the input voltage goes to negative, the CCR will break down at about -0.5 V. The circuit may need a reverse protection diode. As the input voltage goes positive, the CCR turns on very fast, often as soon as the voltage crossed the zero intersect point. The bottom right trace shows that at 0.5V input voltage, the CCR is already at 25% of the regulating current.
- The CCR can handle high voltage surges. The scope trace was taken from CCR circuit with three LEDs operating at 12 V. The voltage drop across all three LEDs is 6V ( the magenta scope trace). The 50V, 1ms pulse was injected (the yellow scope trace). The CCR blocked the high voltage surges, and completely absorbed additional power being injected by the 50V pulse. The 6V across the LEDs remained constant.
- Comparing the CCR to a resistor bias circuit, the resistor bias circuit, in yellow, shows the LED turn on as soon as the forward voltage across the LED is achieved, and the current through the LEDs is then proportional to the voltage across the resistor. A low voltage gives a low current and a low light output. A high voltage gives a high current and a high light output. But high voltage on LEDs has potential to go into thermal runaway. The CCR bias circuit, in magenta, shows the LEDs will also turn on as soon as the forward voltage across the LED is reached. The current increases rapidly as the voltage increases and then stabilizes at the control current of 25 mA, and remains constant regardless of the voltage increasing. The CCR enables brighter LEDs at low voltages. The LEDs have constant current over a wide voltage range. The LEDs are also protected as the voltage continues to rise.
- Let’s look at how the CCR changes over temperature. The CCR is a very simple and does not have temperature compensation. It has a negative regulating current to temperature coefficient. The graph on the left is for a 20 mA CCR in a SOD-123 package. At an ambient temperature of +25 º C the current is being regulated at 20 mA. At an ambient temperature of -40 º C the regulating current is approximately 23 mA. At an ambient temperature of +85 º C the regulating current drops to approximately 17 mA. The graph on the right is for a 30 mA device in a SOT-223 package. This graph shows a similar change in regulating current to ambient temperature. As the ambient temperature increases the regulating current decreases. The larger SOT-223 is more thermally efficient compared to the SOD-123 package and thus has improved regulation. The negative temperature coefficient of the CCR protects LEDs as temperature and voltage increases.
- ON Semiconductor is introducing a family of CCRs from 20 mA up to 150 mA in packages as small as a SOD123 up to a D-Pak. All of the packages are thermally robust providing a simple design solution. These devices have a 45 V continuous breakdown voltage with an ESD rating above 8 KV. A typical circuit overhead voltage of 1.8 V. The 20, 25 & 30 mA devices in SOD-123 and SOT-223 are available today.
- We will now review some CCR circuits. Circuit A has a single CCR driving multiple strings of LEDs. This circuit requires the LEDs to be matched exactly in forward voltage because they will not share current evenly. The power dissipated in the CCR will be localized in a single package and will require extra thermal design considerations. If one string of LEDs fail the remaining strings will carry the extra current, that could cause them to be damaged. Circuit B has a CCR for each LED string. This circuit does not require the LEDs to be forward voltage matched. The CCR power is dissipated in 3 devices simplifying the design. If one string of LEDs fails it has no impact on the other strings .
- To achieve higher currents CCRs can be connected in parallel. This circuit shows 18 LEDS in 3 strings of 6 running off 24 V. To drive this circuit with 50 mA, a 20 mA and a 30 mA CCR are connected in parallel. The CCRs will current share without any problems.
- The CCR circuit can be easily dimmed by adding a BJT and providing a PWM (pulse width modulation) signal. The light output will be directly proportional to the PWM signal. The average current is less resulting in the average light output being less. The LEDs are still operating at their optimum current.
- A simple dimming circuit can be configured by adding a second BJT to provide an A Stable Multi-vibrator. Switch 1 closed the multi-vibrator runs and the LEDs are on 50%. Switch 2 closes the multi-vibrator stops and the LEDs are on full.
- This circuit is connected directly to 110 V AC. There is a simple full wave bridge feeding the CCR and LED light string. The high voltage input is offset by the large voltage drop with having 30 LEDs connected in series. The resistor was included in this circuit to facilitate measuring the current through the LEDS. The resistor could be used to help offset the voltage drop across the CCR if there is not sufficient drop across the LEDs. At 110 V RMS input, TP1 has a peak to peak voltage of 156 V. TP2 shows that the peak voltage across the LEDs and resistor is controlled at 108 V. The current probe will measure the current through the LEDs. The following slide will show scope traces for 110 V RMS input, 100 V RMS input or -10%, and 120 V RMS input or +10%.
- At 110 V RMS input, TP1(the yellow trace) has a peak to peak voltage of 156 V. TP2 (blue Trace) shows that the peak voltage across the LEDs and resistor is controlled at 108 V. It also shows that the LEDs are on 52% of the time. The current probe (green trace) measures the current through the LEDs at 25 mA. At 100 V RMS input or -10%, TP1 has a peak to peak voltage of 142 V. TP2 shows that the peak voltage across the LEDs is still controlled at 108 V, the LEDs are on 47% of the time. The current probe continues to measures 25 mA. At 120 V RMS input or +10%, TP1 now has a peak to peak voltage of 170 V. TP2 shows that the voltage across the LEDs is maintained at 108 V. It also shows that the on time is 56%. The current probe again shows 25 mA. A +/- 10% change in RMS voltage input results in an approximate +/- 10% change in light output
- In a CCR circuit, the maximum power dissipated depends on the maximum voltage being applied cross it. It is important to select a thermally robust package and ensure the efficient copper on the PCB to dissipate the heat generated. These graphs show that as the ambient temperature increases , the PCB requires a larger copper pad to dissipate the same amount power. From the graphs, we can see the SOT-223 package is thermally efficient package compared to SOT-123 package.
- In summary, the LED in the CCR circuit had a consistent brightness over a wide voltage range, both low and high, and they were also protected at high voltages. Therefore, the CCR allows the user to achieve the expected long life of the LED array. Since LED brightness is determined by operating current, optimum intensity will be attained by using a CCR approach to maintain consistent luminosity over the wide variation of battery voltage. SOT−223 package devices improve power dissipation.
- Thank you for taking the time to view this presentation on “ Constant Current Regulator for Driving LEDs ” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the On Semiconductor site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility. You may visit Element 14 e-community to post your questions.