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DKNguyen
DKNguyen
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To add the relevant information from the link posted by Brian O'Regan as full answer:

The document refers to three common/popular circuits for digital LED drives:

  1. Series drive
  2. Shunt
  3. Shunt with over and under-drive

1. Series

schematic

simulate this circuit – Schematic created using CircuitLab

  • Q1 directly switches the LED

Pro: Low average power supply current
Con: Low speed (< 30-50 Mb/s)

2. Shunt

schematic

simulate this circuit

  • Q1 shunts the LED - so a quick discharge == fast turn-off time

Pro: Higher speed (several times faster then 1)
Con: Higher power dissipation (circuit draws more current when LED than when LED is on!)

3. Shunt with Over & Under Drive

schematic

simulate this circuit

extends 2.

  • C1 increasesdecreases the switching times of Q1
  • R3, R4 and C2 provide over-drive at turn-on and under-drive at turn-off
  • typical RC time constant for R3 + C2 == rise/fall time of LED

Pro: higher resulting speeds than 2.
Con: carefully chosen values needed - otherwise destructive

summary:

  • For high-performance LEDs and driver design, optical rise times can be as short as 1.5ns.
  • Most LEDs have slower turn-off times.
  • Here with careful design 2.5ns optical turn-off time can be reached.
  • It is often a good idea to have a small (few percent of peak drive current) pre-bias current to improve dynamic response and so the LED never gets reverse biased.

With all these concepts, operating speeds to about 270 Mb/s can be reached for production ready setups.

All this information is only sourced from the linked document. No self experimentation has been done.

I felt that this was a too big an edit of the original answer; if that's wrong I am happy to move the information over into an edit.

To add the relevant information from the link posted by Brian O'Regan as full answer:

The document refers to three common/popular circuits for digital LED drives:

  1. Series drive
  2. Shunt
  3. Shunt with over and under-drive

1. Series

schematic

simulate this circuit – Schematic created using CircuitLab

  • Q1 directly switches the LED

Pro: Low average power supply current
Con: Low speed (< 30-50 Mb/s)

2. Shunt

schematic

simulate this circuit

  • Q1 shunts the LED - so a quick discharge == fast turn-off time

Pro: Higher speed (several times faster then 1)
Con: Higher power dissipation (circuit draws more current when LED than when LED is on!)

3. Shunt with Over & Under Drive

schematic

simulate this circuit

extends 2.

  • C1 increases the switching times of Q1
  • R3, R4 and C2 provide over-drive at turn-on and under-drive at turn-off
  • typical RC time constant for R3 + C2 == rise/fall time of LED

Pro: higher resulting speeds than 2.
Con: carefully chosen values needed - otherwise destructive

summary:

  • For high-performance LEDs and driver design, optical rise times can be as short as 1.5ns.
  • Most LEDs have slower turn-off times.
  • Here with careful design 2.5ns optical turn-off time can be reached.
  • It is often a good idea to have a small (few percent of peak drive current) pre-bias current to improve dynamic response and so the LED never gets reverse biased.

With all these concepts, operating speeds to about 270 Mb/s can be reached for production ready setups.

All this information is only sourced from the linked document. No self experimentation has been done.

I felt that this was a too big an edit of the original answer; if that's wrong I am happy to move the information over into an edit.

To add the relevant information from the link posted by Brian O'Regan as full answer:

The document refers to three common/popular circuits for digital LED drives:

  1. Series drive
  2. Shunt
  3. Shunt with over and under-drive

1. Series

schematic

simulate this circuit – Schematic created using CircuitLab

  • Q1 directly switches the LED

Pro: Low average power supply current
Con: Low speed (< 30-50 Mb/s)

2. Shunt

schematic

simulate this circuit

  • Q1 shunts the LED - so a quick discharge == fast turn-off time

Pro: Higher speed (several times faster then 1)
Con: Higher power dissipation (circuit draws more current when LED than when LED is on!)

3. Shunt with Over & Under Drive

schematic

simulate this circuit

extends 2.

  • C1 decreases the switching times of Q1
  • R3, R4 and C2 provide over-drive at turn-on and under-drive at turn-off
  • typical RC time constant for R3 + C2 == rise/fall time of LED

Pro: higher resulting speeds than 2.
Con: carefully chosen values needed - otherwise destructive

summary:

  • For high-performance LEDs and driver design, optical rise times can be as short as 1.5ns.
  • Most LEDs have slower turn-off times.
  • Here with careful design 2.5ns optical turn-off time can be reached.
  • It is often a good idea to have a small (few percent of peak drive current) pre-bias current to improve dynamic response and so the LED never gets reverse biased.

With all these concepts, operating speeds to about 270 Mb/s can be reached for production ready setups.

All this information is only sourced from the linked document. No self experimentation has been done.

I felt that this was a too big an edit of the original answer; if that's wrong I am happy to move the information over into an edit.

Grammar, punctuation, using better English phrasiology.
Source Link
edit approved Apr 23, 2018 at 15:37
DiBosco
edit approved Apr 23, 2018 at 15:37
DiBosco
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To add the relevant information from the link posted by brian o'reganBrian O'Regan as full answer:

The document refers to three common/popular used circuits for digital ledLED drives:

  1. Series drive
  2. Shunt
  3. Shunt with over and under-drive

1. Series

schematic

simulate this circuit – Schematic created using CircuitLab

  • Q1 directly switches the LED

proPro: lowLow average power supply current
contraCon: lowLow speed (< 30-50 Mb/s)

2. Shunt

schematic

simulate this circuit

  • Q1 shunts the LED - so a quick discharge == fast turn-off time

proPro: higherHigher speed (several times faster then 1)
contraCon: higherHigher power dissipation (circuit draws more current when led of thenLED than when ledLED is on!)

3. Shunt with Over & Under Drive

schematic

simulate this circuit

extends 2.

  • C1 increases the switching times of Q1
  • R3, R4 and C2 provide over-drive onat turn-on and under-driverdrive at turn-off
  • typical RC time constant for R3 + C2 == rise/fall time of ledLED

proPro: higher resulting speeds than 2.
contraCon: carefully chosen values needed - otherwise destructive

summary:

  • forFor high-performance ledsLEDs and driver design, optical rise times can be as short as 1.5ns.
  • Most LEDs have slower turn-off times.
  • Here with careful design 2.5ns optical turn-off time can be reached.
  • itIt is often a good idea to have a small (few percent of peak drive current) pre-bias current to improve dynamic response and so the ledLED never gets reverse biased.

--> with With all these concepts, operating speeds to about 270 Mb/s can be reached for production ready setups.

all theseAll this information areis only sourced from the linked document. no ownNo self experimentation has occurredbeen done.

iI felt that this was a totoo big an edit onof the original answer -answer; if thatsthat's wrong iamI am happy to move the information over into an edit.

To add the relevant information from the link posted by brian o'regan as full answer:

The document refers to three common/popular used circuits for digital led drives:

  1. Series drive
  2. Shunt
  3. Shunt with over and under-drive

1. Series

schematic

simulate this circuit – Schematic created using CircuitLab

  • Q1 directly switches the LED

pro: low average power supply current
contra: low speed (< 30-50 Mb/s)

2. Shunt

schematic

simulate this circuit

  • Q1 shunts the LED - so a quick discharge == fast turn-off time

pro: higher speed (several times faster then 1)
contra: higher power dissipation (circuit draws more current when led of then when led on!)

3. Shunt with Over & Under Drive

schematic

simulate this circuit

extends 2.

  • C1 increases the switching times of Q1
  • R3, R4 and C2 provide over-drive on turn-on and under-driver at turn-off
  • typical RC time constant for R3 + C2 == rise/fall time of led

pro: higher resulting speeds than 2.
contra: carefully chosen values needed - otherwise destructive

summary:

  • for high-performance leds and driver design optical rise times can be as short as 1.5ns.
  • Most LEDs have slower turn-off times.
  • Here with careful design 2.5ns optical turn-off time can be reached.
  • it is often a good idea to have a small (few percent of peak drive current) pre-bias current to improve dynamic response and so the led never gets reverse biased.

--> with all these concepts operating speeds to about 270 Mb/s can be reached for production ready setups.

all these information are only sourced from the linked document. no own experimentation has occurred.

i felt that this was a to big edit on the original answer - if thats wrong iam happy to move the information over into an edit.

To add the relevant information from the link posted by Brian O'Regan as full answer:

The document refers to three common/popular circuits for digital LED drives:

  1. Series drive
  2. Shunt
  3. Shunt with over and under-drive

1. Series

schematic

simulate this circuit – Schematic created using CircuitLab

  • Q1 directly switches the LED

Pro: Low average power supply current
Con: Low speed (< 30-50 Mb/s)

2. Shunt

schematic

simulate this circuit

  • Q1 shunts the LED - so a quick discharge == fast turn-off time

Pro: Higher speed (several times faster then 1)
Con: Higher power dissipation (circuit draws more current when LED than when LED is on!)

3. Shunt with Over & Under Drive

schematic

simulate this circuit

extends 2.

  • C1 increases the switching times of Q1
  • R3, R4 and C2 provide over-drive at turn-on and under-drive at turn-off
  • typical RC time constant for R3 + C2 == rise/fall time of LED

Pro: higher resulting speeds than 2.
Con: carefully chosen values needed - otherwise destructive

summary:

  • For high-performance LEDs and driver design, optical rise times can be as short as 1.5ns.
  • Most LEDs have slower turn-off times.
  • Here with careful design 2.5ns optical turn-off time can be reached.
  • It is often a good idea to have a small (few percent of peak drive current) pre-bias current to improve dynamic response and so the LED never gets reverse biased.

With all these concepts, operating speeds to about 270 Mb/s can be reached for production ready setups.

All this information is only sourced from the linked document. No self experimentation has been done.

I felt that this was a too big an edit of the original answer; if that's wrong I am happy to move the information over into an edit.

Source Link
Stefan Krüger s-light
Stefan Krüger s-light
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To add the relevant information from the link posted by brian o'regan as full answer:

The document refers to three common/popular used circuits for digital led drives:

  1. Series drive
  2. Shunt
  3. Shunt with over and under-drive

1. Series

schematic

simulate this circuit – Schematic created using CircuitLab

  • Q1 directly switches the LED

pro: low average power supply current
contra: low speed (< 30-50 Mb/s)

2. Shunt

schematic

simulate this circuit

  • Q1 shunts the LED - so a quick discharge == fast turn-off time

pro: higher speed (several times faster then 1)
contra: higher power dissipation (circuit draws more current when led of then when led on!)

3. Shunt with Over & Under Drive

schematic

simulate this circuit

extends 2.

  • C1 increases the switching times of Q1
  • R3, R4 and C2 provide over-drive on turn-on and under-driver at turn-off
  • typical RC time constant for R3 + C2 == rise/fall time of led

pro: higher resulting speeds than 2.
contra: carefully chosen values needed - otherwise destructive

summary:

  • for high-performance leds and driver design optical rise times can be as short as 1.5ns.
  • Most LEDs have slower turn-off times.
  • Here with careful design 2.5ns optical turn-off time can be reached.
  • it is often a good idea to have a small (few percent of peak drive current) pre-bias current to improve dynamic response and so the led never gets reverse biased.

--> with all these concepts operating speeds to about 270 Mb/s can be reached for production ready setups.

all these information are only sourced from the linked document. no own experimentation has occurred.

i felt that this was a to big edit on the original answer - if thats wrong iam happy to move the information over into an edit.