Introduction to PLED Open LED Protection Devices

Transcription

1 Introduction to PLED Open LED Protection Devices

2 Outline LED Lighting Series Strings of LED s Voltages in LED Strings LED Failure Modes Open LED Protection PWM Dimming LED s with Integrated Zeners Key Parameters Package Options 2

3 Series Strings of LED s When configured for lighting applications, LED s are usually arranged in series strings. An LED string will typically contain from 5 to 20 LED s. LED s are much easier to control if they are arranged in a series string because: They all share a common current. Better control of LED group brightness. More uniform LED-to-LED brightness. Constant Current LED String Driver 3

4 Voltages in LED Strings The maximum output voltage of the constant current source is called the compliance voltage. Individual LED voltages will range from about 2.5V to 3.5V. LED voltage will vary with power or brightness. LED voltage will vary with temperature. LED voltage will vary with color. The compliance voltage must be higher than the sum of all of the LED voltages. LED Power LED Voltage Max LED Current 1W ~3V 350mA 2W ~3V 700mA 3W ~3V 1000mA Constant Current Source + ~3V _ 4

5 Shorted LED Effects Most ESD or lightning surge events will destroy the semiconductor, causing an LED to short out. A shorted LED will go dark, but the remaining LED s stay illuminated because the current still passes through the shorted LED. The total string voltage is reduced because the shorted LED will have nearly zero volts across it. Preventing shorted LED s requires overvoltage and overcurrent protection for the constant current driver circuit. Constant Current Source Shorted LED + ~0V _ 5

6 Open LED Effects High temperatures or excessive vibration may cause an internal LED wirebond to break a become an open circuit. An open LED will go dark and all of the remaining LED s in the string will also go dark because no current can pass through the open LED. The full compliance voltage of the constant current source will appear across the open LED. A single LED failure causes entire string to fail, resulting in product returns tarnishing of brand warranty expense Wirebond Broken Constant Current Source + Compliance Voltage _ 6

7 Outline LED Lighting Series Strings of LED s Voltages in LED Strings LED Failure Modes Open LED Protection PWM Dimming LED s with Integrated Zeners Key Parameters Package Options 7

8 Open LED Protection Solution: PLED devices will turn on and conduct current around open LED. Result: Remaining LED s stay illuminated. Replacing bad LED will restore original function. In many applications, the failure of a single LED may not be critical or even noticeable. Note that PLED power dissipation is lower than the LED, lowering total system power. Constant Current Source Active PLED6 PLED6 Open LED + ~1.3V _ 8

9 Budget Open LED Protection Protect 2 LED s with one PLED9 device One LED failure will result in two LED s going dark. Protection cost cut in half. Similar scheme for PLED13 works for 3 LED s, cutting protection cost by 2/3rds LED Driver Datasheet Constant Current Source Active PLED9 PLED9 Open LED + ~6V _ 9

10 Outline LED Lighting Series Strings of LED s Voltages in LED Strings LED Failure Modes Open LED Protection PWM Dimming LED s with Integrated Zeners Key Parameters Package Options 10

11 PWM LED Dimming The brightness of LED s is usually controlled by Pulse Width Modulation of the LED string current. 50% 20% 85% ON OFF ON OFF ON OFF The switching frequency is typically 60Hz 1000Hz PLED Devices have been tested to 10,000Hz 11

12 Outline LED Lighting Series Strings of LED s Voltages in LED Strings LED Failure Modes Open LED Protection PWM Dimming LED s with Integrated Zeners Key Parameters Package Options 12

13 LED Devices May Have Integrated Zener Diode Protection Q: Some LED s have a zener diode on board. Do I still need a PLED device? A: Yes. The on-board zener is capable of ESD protection only. It cannot handle the power required by open LED protection. Zener diode ESD protection Bond wire LED die 13

14 Outline LED Lighting Series Strings of LED s Voltages in LED Strings LED Failure Modes Open LED Protection PWM Dimming LED s with Integrated Zeners Key Parameters Package Options 14

15 Key Performance Parameters Minimum Breakdown Voltage Higher than max LED(s) voltage Maximum Breakdown Voltage Lower than LED supply compliance voltage Maximum Continuous Current Higher than LED continuous current 15

16 Outline LED Lighting Series Strings of LED s Voltages in LED Strings LED Failure Modes Open LED Protection PWM Dimming LED s with Integrated Zeners Key Parameters Package Options 16

17 Package Options QFN All PLED Devices are Bidirectional SMB QFN SMB 17

18 Introduction to PLED Open LED Protection Devices Thank You! 18