Constant-current, constant-voltage converter drives white LEDs
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 shows the LT1618 driving a 1W, white Lumileds (www.lumileds.com) LXHL-BW02 Luxeon LED.
You need no external op amps for this compact approach. The LXHL-BW02 has a forward voltage of 3.1 to 3.5V for 250 mA of current. Although the maximum dc rating of the LED is 350 mA, you can pulse it up to 500 mA for a camera flash. R4 is set for a 250-mA torch or dimming operation. The IADJ (current-adjustment) pin provides the ability to dim the LED during normal operation by varying the resistor setting or injecting a PWM signal. Access to both the positive and the negative inputs of the special internal constant-current amplifier allows you to place the sense resistor anywhere in the converter's output or input path and provide constant output or input current. Without access to both inputs, you would need a ground-referenced sense resistor, some additional level-shifting transistors, or an op amp. In this case, the floating sense resistor's value is only 100 mΩ; at 500 mA, it consumes an average of 50 mW of power. The sense resistor connects directly to the positive and the negative input pins of the LT1618.
Although the LT1618 conventionally serves as a high-frequency boost converter with the load connected between VOUT and ground, this method of tying the load from VOUT back to VIN allows the IC to drive the LXHL-BW02 from a lithium-ion battery input. Tying the load back to VIN allows the forward voltage of the LED (the load voltage) to be either above or below the input voltage as the battery voltage changes. This topology avoids the need for an additional inductor. This design uses one small, low-cost inductor, matching the all-ceramic capacitors and low-profile IC. Tying the load back to VIN increases the inductor current by summing both the input and the output currents. The internal switch losses double, and the overall efficiency of the approach is approximately 70% over the input-voltage range. Even at this efficiency, it is difficult to match the compactness and low cost of this approach.
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