Choosing the Right Constant Current Source for Power LEDs
To ensure the longevity of power LEDs and minimize power loss caused by a series resistor used in a constant voltage supply, operating LEDs with a truly consistent current is crucial. This approach significantly extends the lamp’s lifespan. To achieve optimal efficiency, the most effective method is employing a switching regulator. In essence, selecting a switching regulator designed specifically for maintaining a constant current, rather than a constant voltage, is essential for this purpose.
Developing an Efficient LED Pocket Torch with Dimming Functionality
In pursuit of creating a highly efficient LED pocket torch, the author focused on maximizing efficiency. This involved the use of high-capacity rechargeable batteries to extend operating life. Additionally, incorporating a dimming function to reduce LED brightness and operating current when full power wasn’t necessary was deemed essential. The dimming function operated in PWM mode, minimizing power losses. The circuit design employed the LT3518 switching regulator IC, optimized for LED operation in buck mode. Achieving over 90% efficiency depended on the input voltage and a sense resistor with minimal voltage drop. The chosen resistor value (68 mΩ for R2) suited the Cree LED employed, with D1 requiring a diode rated at least 6 W.
Implementing Dimming Functionality
The dimming capability was achieved through IC1, allowing LED dimming via a PWM signal applied to pin 7. The PWM signal, provided by IC2 (LTC6992), controlled the duty cycle based on the voltage applied to the MOD input. Adjusting the internal clock frequency of IC2 using resistor R3 enabled a suitable PWM frequency (around 1,150 Hz). To maintain consistent brightness regardless of battery voltage fluctuations, IC3 generated a stabilized 1.24 V control voltage for potentiometer P1. Series resistor R7 ensured compatibility with the input voltage range of LTC6992. Careful consideration was given to capacitor selection, favoring ceramic types with X5R or X7R dielectric to ensure optimal temperature characteristics and low effective series resistance (ESR).
Voltage and Battery Considerations
The supply voltage was limited to 5.5 V due to IC2’s maximum rated supply voltage. Four NiMH rechargeable cells connected in series were chosen, falling within the specified voltage range. To accommodate this voltage range, an LED operating below 4 V was necessary. Devices with multiple chips connected in series, common in power applications, were not suitable due to the lower supply voltage.
During assembly, attention to soldering was crucial. An exposed thermal pad on the PCB for the LT3518 was required. Ensuring the pad extended beyond the IC outline, the pad and IC rear face were tinned first. The pad was heated with a soldering iron, and after the solder melted, the IC was aligned and soldered into place, followed by the pins. Careful soldering was essential for proper functionality.
Internet Links www.linear.com/product/LT3518  www.linear.com/product/LTC6992  www.ti.com/product/tlv431  www.elektor.com/120201