Light-emitting diodes (LEDs) have emerged in recent years as viable sources of light and are no longer used solely as status-light indicators for electronic equipment. Advances in technology have provided LEDs that are typically three times more efficient than incandescent bulbs. LEDs are also extremely durable and have lifetimes exceeding tens of thousands of hours.

Power LEDs for lighting applications are designed to be driven with a constant current source. It is common to see standard current drive levels of 350mA and 700mA among different LED manufacturers.

The forward voltage across the LED can, however, vary depending on the type and number of junctions connected in series. Many manufacturers of power LEDs will provide multiple junctions, integrated into a single module.

One simple method that can be used to drive an LED is to install a resistor, in series, to limit the current. A linear voltage regulator or operational-amplifier (op-amp) circuit can also be connected in a constant-current configuration. However, these linear methods will not have enough efficiency at the required power levels.

A Switch-Mode Power Supply (SMPS) provides a much more efficient solution for driving the LED. An SMPS can buck or boost the input voltage to the correct level, to provide the desired LED current. The system input-voltage range and the required LED forward voltage will determine the SMPS topology that is selected.

Figure 1: Buck-boost converter topology

Buck-boost converter
The buck-boost converter topology is used when the supply voltage may be above or below the required output voltage and is especially useful for battery applications. This topology is also known as a fly-back or inverting regulator. A buck-boost converter can be implemented as shown in Figure 1 above.

This implementation has the advantage that a simple, low-side MOSFET driver circuit can be used. The topology shown in Figure 1 will generate a positive voltage, referenced to the input-voltage rail. The downside of this buck- boost implementation is that the load is not referenced to the circuit ground.

A simplified circuit design for an LED driver is shown in Figure 2 below, using a mixed signal, high voltage 8bit microcontroller, such as Microchip Technology's PIC16HV785. The output of the circuit is referenced to the battery voltage, not to ground. The output of the inverter is connected to the LED anode and produces a voltage that is greater than the input voltage.

Figure 2: Simplified LED-driver circuit using a PIC16HV785 microcontroller.

The PIC16HV785 mixed signal microcontroller combines an 8bit microcontroller core with several on-chip analogue peripherals. These include a high speed, two-phase PWM circuit, ideal for current-mode control of switch-mode power supplies, and two on-chip op-amps that can be used to amplify the voltage across the current-sensing resistors.

This allows the use of very small sensing resistors, which reduces circuit losses and increases overall efficiency. The on-chip high voltage shunt regulator eliminates the need for an external 5V regulator when operating from higher supply voltages.

The PIC16HV785 also integrates a digital Capture, Compare and PWM (CCP) module, two analogue comparators, a 10bit A/D converter, an 8MHz internal clock circuit, internal precision voltage reference, and a programmable Brown-Out Reset (BOR) circuit. All of the pins of the op amps and comparators are externally accessible, so that any circuit configuration can be implemented.

The current-sensing op amp is connected as a differential amplifier, to obtain an accurate measurement of the voltage across the current- sense resistor. The current is measured in the return of the power source, to simplify the requirements of the circuit. R1, R2 and C1 form a low-pass filter to reduce any switching noise that may be present. The cut-off frequency of this filter must be chosen above the converter switching frequency, to avoid limiting the control loop response.