LED Lighting SMPS Topologies

LED Control

Light Emitting Diodes (LEDs) must be driven with a source of constant current. Most LEDs have a specified current level that will achieve the maximum brightness for that LED without premature failures. An LED could be driven with a linear voltage regulator configured as a constant current source. However, this approach is not practical for higher power LEDs due to power dissipation in the regulator circuit. A switch-mode power supply (SMPS) provides a much more efficient solution to drive the LED. An LED will have a forward voltage drop across its terminals for a given current drive level. The power supply voltage and the LED forward voltage characteristics determine the SMPS topology that is required. Multiple LEDs can be connected in series to increase the forward voltage drop at the chosen drive current level. The SMPS circuit topologies adopted to regulate current in LED lighting applications are the same used to control voltage in a power supply application. Each type of SMPS topology has its advantages and disadvantages as presented in the table below. This design guide presents two types of LED driver solutions. First, an analog driver IC can be used independently or together with a MCU for added intelligence. Second, the LED drive function can be integrated into the MCU application.

Common SMPS Topologies Useful for LED Lighting

Regulator Topology VIN to VOUT Relationship Complexity Component Count Comments
Charge Pump -VOUT < VIN < VOUT Low Medium Limited IOUT range, No Inductors
Buck VIN > VOUT Medium Medium Chopped IIN, High side drive
Boost VIN > VOUT Medium Medium Extra parts needed to isolate output from input
SEPIC VOUT < VIN < VOUT Medium High Smooth IIN , Multiple outputs, Two inductors
Buck-Boost VOUT < VIN < VOUT Medium Medium Single inductor, Up to four switches
Flyback Depending on transformer Medium Medium Transformer can provide electrical isolation, Multiple outputs possible
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