PhotonBoy
Flashlight Enthusiast
EDN article
"LEDs usually take their drive from a constant dc-current source to maintain constant luminescence. Most dc/dc converters, however, deliver a constant voltage by comparing a feedback voltage to an internal reference via an internal error amplifier. The easiest way to turn a simple dc/dc converter into a constant-current source is to use a sense resistor to convert the output current to a voltage and use that voltage as the feedback. The problem is that 500 mA of output current with a 1.2V drop—the typical reference voltage—in the sense resistor incurs relatively high power losses and, thus, a drop in efficiency.
One approach is to use an external op amp to amplify the voltage drop across a low-value resistor to the given reference voltage. This method saves converter efficiency but significantly increases the cost and complexity of a simple converter by using additional components and board space. A better approach is to use the LT1618 constant-current, constant-voltage converter, which combines a traditional voltage-feedback loop and a unique current-feedback loop to operate as a constant-voltage, constant-current dc/dc converter. Figure 1 (PDF) shows the LT1618 driving a 1W, white Lumileds ( http://www.lumileds.com ) LXHL-BW02 Luxeon LED...."
"LEDs usually take their drive from a constant dc-current source to maintain constant luminescence. Most dc/dc converters, however, deliver a constant voltage by comparing a feedback voltage to an internal reference via an internal error amplifier. The easiest way to turn a simple dc/dc converter into a constant-current source is to use a sense resistor to convert the output current to a voltage and use that voltage as the feedback. The problem is that 500 mA of output current with a 1.2V drop—the typical reference voltage—in the sense resistor incurs relatively high power losses and, thus, a drop in efficiency.
One approach is to use an external op amp to amplify the voltage drop across a low-value resistor to the given reference voltage. This method saves converter efficiency but significantly increases the cost and complexity of a simple converter by using additional components and board space. A better approach is to use the LT1618 constant-current, constant-voltage converter, which combines a traditional voltage-feedback loop and a unique current-feedback loop to operate as a constant-voltage, constant-current dc/dc converter. Figure 1 (PDF) shows the LT1618 driving a 1W, white Lumileds ( http://www.lumileds.com ) LXHL-BW02 Luxeon LED...."