Boost LED Performance Via Current Source

For optimum performance, high brightness LEDs should be driven by a current source rather than by a voltage source. This article examines the design of a constant-current LED driver that can be used for driving a string of series-connected LEDs. The number of series-connected LEDs can vary from one to several. To drive the LEDs, the power-stage topology is a modified buck boost converter.
There are many integrated driver solutions in the industry; one possible integrated driver solution to implement the controller subsystem is using the MAX16818 from Maxim Integrated Products. This is an average current mode controller with a transconductance amplifier for the current error amplifier. The current-sense voltage across the current-sense resistor is amplified internally by a factor of 34.5. The transconductance of the current error amplifier is 550μS and the peak-to-peak saw tooth is 2V. In this circuit, the input current is sensed by a resistor, Rs in the return leg.
The value of the current-sense resistor is set by the average current limit required. The maximum voltage across the LEDs is where n is the number of LEDs and is the maximum voltage drop in the LEDs at the full-load current, if the maximum output power is P max = VLED max * If and the efficiency is η. Thus, the maximum input current is:

I in max = P max/ η x V in max

The minimum value of the average current limit threshold is 24mV. The current-sense resistor value can be calculated as:

Rs ≤ 0.024/ I in max * Ω

To prevent oscillations in the PWM comparator section of the controller, the slope of the signal on the negative input of the comparator should be less than the slope of the saw tooth on the positive input. The slope of the saw tooth is given by Vsfs. while the gain of the current error amplifier is given by

Gca = 34.5 * gm * Rc.

In this equation, gm is the transconductance of the CEA The output of this amplifier goes to the negative input of the pulse-width modulation (PWM) comparator. Positive input of the PWM comparator is the saw tooth that has a peak-to-peak amplitude of Vs with a switching frequency, fs. This is the AC gain of the current-error amplifier from the current-sense voltage across Rs to the output of the amplifier at the high frequencies below the pole from the compensation capacitor, Cp. This is the gain of interest at the PWM comparator section.