Mathematical models have always helped determine the best compensation components for a particular design, however, compensating for the W LED current-regulating boost converter is slightly different than compensating for the same converter that is set to regulate the voltage. Measuring the control loop in a conventional manner is quite inconvenient because the impedance of the feedback (FB) pin is not high and the upper FB resistor is lacking. In Reference 1, Ray Ridley demonstrated a simple small-signal control loop model for boost converters with current mode control.
The following explains how the Ridley model should be modified to work with the WLED current-regulating boost converter. It also shows how to measure the control loop of the boost converter.
Loop component
As shown in Figure 1, any adjustable DC/DC converter can be modified to provide a higher or lower regulated output voltage from the input voltage. In this type of configuration, if ROUT is assumed to be purely a resistive load, then VOUT = IOUT x ROUT. When the DC/DC converter is used to power the LED, it controls the current through the LED by adjusting the lower FB resistor, as shown in Figure 2. The traditional small-signal control loop formula is no longer applicable because the load itself (LED) replaces the upper FB resistor. DC load impedance is
and
The VFWD from the diode data sheet or from the measurement is the forward voltage of the ILED, and n is the number of LEDs in series.
Figure 1: Adjustable DC/DC converter for regulating voltage
Figure 2: Adjustable DC/DC Converter for Adjusting LED Current
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