Introduction to PLED Open LED Protection Devices

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Introduction to PLED Open LED Protection Devices
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Outline

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

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Series Strings of LEDs

• When configured for lighting applications, LEDs
  are usually arranged in series strings. An LED
  string will typically contain from 5 to 20 LEDs.
  
• LEDs 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.

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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.

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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
  LEDs 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 LEDs requires overvoltage and
  overcurrent protection for the constant current
  driver circuit.

Constant Current Source
0V _
Shorted LED
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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
  LEDs 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

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Outline

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

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Open LED Protection

• Solution PLED devices will turn on and conduct
  current around open LED.
• Result Remaining LEDs 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.

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Budget Open LED Protection

• Protect 2 LEDs with one PLED9 device
• One LED failure will result in two LEDs going
  dark.
• Protection cost cut in half.
• Similar scheme for PLED13 works for 3 LEDs,
  cutting protection cost by 2/3rds

PLED9
Open LED
Constant Current Source
6V _
Active PLED9

• LED Driver Datasheet

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Outline

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

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PWM LED Dimming

• The brightness of LEDs is usually controlled by
  Pulse Width Modulation of the LED string current.

• The switching frequency is typically 60Hz
  1000Hz
• PLED Devices have been tested to 10,000Hz

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Outline

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

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LED Devices May HaveIntegrated Zener Diode
Protection
Q Some LEDs 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.
LED die
Zener diode ESD protection
Bond wire
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Outline

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

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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

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Outline

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

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Package Options
QFN

• All PLED Devices are Bidirectional

SMB
QFN
SMB
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Thank You!
Introduction to PLED Open LED Protection Devices