LED driver technology

Direct Drive (DD)

Direct drive (DD) is the simplest and cheapest design. It is a direct path from the battery to the LED.

Sometimes there is a CPU and transistor to create different modes (25%, 50%, 100% …. etc.).

It is used when we want to make very powerful flashlights.

Advantages:

  • Cheap.
  • There is no flickering, so it is ideal for a video light.

Disadvantages:

  • Current is varying greatly with battery voltage. So the LED is dimming.
  • If there is a current limiter resistor, the efficiency is low (lower than a linear driver).
  • Voltage of the battery must be a bit higher than the LED voltage (generally ~3V) but not too much above. Othervise you can’t use this type of driver.

 

Linear Driver (LD)

This is the same as DD but with a “smart” resistor to limit current.

In this case the resistor value is not constant, it is continously adjusted by a linear regulator to make the current constant.

Most drivers use 7135 linear regulators (0.35A per piece).

Efficiency is varying with the battery voltage. The resistor burns off the extra power!

Eff.=Vled/Vbattery

Fully charged battery  Eff.=3.3V/4.2V=78%

Half charged: Eff.=3.3V/3.5V=94%

Discharged: Eff.=3.3V/3.3V =100%

When the battery voltage drops below Vled, the led starts dimming.

What happens if we use more batteries in series? The efficiency drops rapidly as more power wasted on the driver.

Advantages:

  • Simple and cheap design.
  • Constant current for most of the discharge of the battery.
  • There is no flickering, so it is ideal for a video light.

Disadvantages:

  • Voltage of the battery must be a bit higher than the LED voltage (generally ~3V) but not too much above. Otherwise you can’t use this type of driver.

 

Buck Driver

This is a more complicated design, also called as step down driver. It uses an inductor and capacitor to step down the battery voltage.Compared to a linear driver, the efficiency is more or less constant regardless of the battery voltage, which can be much higher than the LED voltage. The efficiency is between 70% and 90%.

An example would be using 4 li-ion batteries in series (16.8V ffully charged) to drive  3 series connect LEDs with a total Vf of around 13.2V. A buck driver will draw lower current from the input than what it will provide at the output. As the battery voltage drops, the buck driver will draw more current, but never more than the output current.

Advantages:

  • Can be used with battery packs that have a much higher voltage than the LED voltage.
  • Relatively high and constant efficiency.
  • PWM can cause problems for video or photograpy due to the relatively low frequency that is used. This low frequency PWM can ‘beat’ with the camera frame time to cause banding as the LED turns on and off within a single frame.

Disadvantages:

  • More expensive and complicated design
  • Voltage of the battery must be at least a bit higher than the LED voltage.

 

Boost Driver

This is a more complicated design, also called as step up driver.

This is very similar the the Buck Driver, but as its name implies, it will increase the voltage (Buck driver decrease the voltage).

It can also be used to power a multiple LEDs in series.

An example would be using 2 li-ion batteries in series (8.4V fully charged) to drive 4 series connect LEDs with a total Vf of around 13.2V. A boost driver will draw higher current from the input than what it will provide at the output. As the battery voltage drops, the boost driver will draw even more current and this tends to put more strain on a battery pack that is already fairly discharged.

Advantages:

  • Can be used with batteries that have a voltage lower than the LED voltage.

Disadvantages:

  • More expensive and complicated design
  • Voltage of the battery must be lower than the LED voltage.
  • PWM can cause problems for video or photograpy due to the relatively low frequency that is used. This low frequency PWM can ‘beat’ with the camera frame time to cause banding as the LED turns on and off within a single frame.

 

Buck Boost Driver

Combined buck-boost drivers tend to be less efficient that a straight buck or boost driver. Where a typical buck or boost driver can run at 90% efficiency (or higher) a buck-boost would likely be around 80%. There are some efficient buck/boost designs, but they cost more to manufacturer.

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