Many applications for LED illumination require wide dimming ratios. This can be accomplished simply via an adjustable current source as show below. The current source can be varied by a number of different means, and a large LED current range can be achieved. The primary problem with this technique is that the power dissipation can be quite high. The voltage source V1 has to be chosen to be high enough to accommodate the largest LED voltage drop and the headroom required by the current source. LED manufacturers typically specify a maximum voltage that is higher than the average value, forcing the designer to use a higher than necessary input voltage. Applications that use multiple LEDs in series multiply the voltage tolerance issue.
Another technique of extending the dimming ratio is to employ a pulse width modulation (PWM) signal to the LED to pulse it on and off. If this pulsing can be done at a high enough rate, typically a few hundred Hz, the pulsing is not visible to the human eye. With this technique the dimming ratio can be extended to 3000:1.
While the pulsing is not visible to the human eye, it can be an issue for other applications. For instance, the scan rate of a digital video recorder can interact with the LED flashing to produce undesirable artifacts.
Hybrid Solution Using LT8614 Silent Switcher and LT3083 Linear Regulator
One potential technique allows a wide dimming ratio to be controlled by a strictly analog means. It uses a hybrid approach where a switching regulator holds the voltage across an adjustable current source constant, and puts the LED voltage inside a feedback loop so that variations in the LED voltage have no effect on the LED current. The voltage across the current source does not have to support the LED voltage drop and its variations, and thus can be optimized for the best choice of power dissipation vs. dimming ratio. The following example demonstrates the concept using the LT8614 as the voltage supply and the LT3083 as the current source.
The LT8614 is a step-down regulator that features a Silent Switcher architecture designed to minimize EMI/EMC emissions while delivering high efficiency at frequencies up to 3MHz. Assembled in a 3mm × 4mm QFN, themonolithic construction with integrated power switches and inclusion of all necessary circuitry yields a solution with a minimal PCB footprint. The LT3083 is a 3A low dropout linear regulator that can be paralleled to increase output current or spread heat on surface mounted boards. Architected as a precision current source and voltage follower, this new regulator finds use in many applications requiring high current, adjustability to zero, and no heat sink.
In this case an input voltage of 12V is used, and the LED current is varied over a 0A to 3A range via a 0V to 300mV signal applied to the SET terminal of the LT3083. The LT3083 OUT terminal follows the SET terminal, thus a constant current flows in R5 via the following IR5=VSET/R5.
The current in the IN terminal of the LT3083 follows the current in the OUT terminal very precisely until a minimum load value of about 500µA is reached.
The LT3083 needs 510mV worst case dropout from IN to OUT, as well as 300mV for R5 at maximum current. The LT8614 holds the voltage at the IN terminal of the LT3083 at exactly 0.97V which keeps the power dissipation of the LT3083 at tolerable levels, peaking at about 2.4W at 3A output current.
The LED cathode is held at 0.97V by the LT3083, but the anode voltage goes to whatever voltage is required due to the feedback action of the LT8614. Additional LEDs can be put in series with D1 and the current will not change until the dropout voltage of the LT8614 is reached.
Hybrid Solution Using LT8614 Silent Switcher and LT6015 Op Amp
The LT3083 is a convenient part to realize the current source function, but as detailed above it does have a minimum load requirement. In addition there is a slight offset between the SET and OUT terminals, which is typically a few hundred microvolts but can be as high as ±6mV over temperature. This offset places a lower limit on the dimming ratio. If a greater dimming ratio is required a discrete current sink using a low offset voltage op amp can be used. One such realization uses the LT6015 precision op amp as the control element.
The LT6015 is a rail-to-rail input operational amplifier with input offset voltage trimmed to less than 50μV. This amplifier operates on single and split supplies with a total voltage of 3V to 50V and draw only 315μA per amplifier. The Over-The-Top® input stage of the LT6015 is designed to provide added protection in tough environments.
The LT6015 has a maximum voltage offset of ±250µV over temperature, and the current source only needs about 100mV dropout. This allows the full scale voltage to be raised to 600mV, and permits dimming ratios in the range of 1000:1.