Tunable White LED Drivers

About this Document

This application note provides LED driver design engineers and field application engineers with comprehensive guidance on implementing cost-effective, highly efficient LED drivers using Infineon's BCR601 and BCR602 linear constant current regulators.

The document demonstrates how to replace the traditional buck converter second stage with a linear regulator featuring active headroom control (AHC) without sacrificing efficiency. This approach achieves the highest possible light quality with a very cost-effective solution, particularly beneficial for tunable white and multichannel drivers.

Linear Constant Current Regulators in LED Drivers

As light quality requirements increase, LED driver topologies that provide almost perfect DC current at the output are gaining popularity. The BCR601 and BCR602 linear regulators with active headroom control can achieve efficiencies on par with buck stages while delivering superior light quality.

The AHC system contains two regulation loops:

• The first loop stabilizes the LED current to a value determined by the sense resistor and internal reference
• The second feedback loop reduces the overhead voltage close to the minimum possible by controlling the output voltage of the first stage

Key advantages of this approach include:

• Pure DC LED current eliminates flicker and stroboscopic effects
• Compliance with IEEE 1789-2015 recommendations under all operating conditions
• Analog dimming down to 3% without PWM-like modulation
• Efficiencies above 95% with proper design

LED Current Setting and Dimming

The regulator has a non-dimmed typical reference voltage of 400mV. The nominal LED current can be reduced through the dimming pin (MFIO) either by applying a DC voltage or using a variable resistor.

Efficiency Optimization

Total power loss of the LED current regulator with AHC consists of losses in the sense resistor and the MOSFET. Efficiency can be optimized by:

• Using the highest possible LED voltage for a given output power
• Reducing output voltage ripple of the first stage
• Proper selection of output capacitance
• Optimal setting of headroom voltage

Protection Features

BCR601/602 include comprehensive protection:

• Overvoltage Protection: Built-in OVP stage with configurable threshold
• Overtemperature Protection: Internal sensor reduces LED current at 140°C junction temperature
• Hot-plug Protection: Limits current spikes when connecting LED strings to active drivers
• Short-circuit Protection: Optional external circuit for output short protection

Multichannel LED Driver with BCR601 plus BCR602

The linear regulator approach can be easily extended to multiple output channels by adding BCR602 stages. BCR602 is similar to BCR601 but without AHC, making it ideal for additional channels in a smaller SOT-23-6 package.

In multichannel systems, the optimal strategy is to always use the channel with the lowest headroom voltage for regulation. This ensures all channels receive adequate voltage while maintaining efficiency.

The network for regulating the lowest V_HR consists of one resistor and a small signal diode per channel. This approach ensures proper operation across varying LED currents in tunable white applications.

Key considerations for multichannel designs:

• Dimming behavior is identical across channels
• OVP is handled by BCR601 for the entire application
• Short-circuit protection requires attention to Zener diode power dissipation
• Hot-plug of LEDs is not supported in multichannel applications

Tunable White Reference Design

The reference design demonstrates a complete tunable white LED driver implementation using BCR601 and BCR602. The board can be combined with Infineon's modular board concept (REF-XDPL8219-U40W) or any SSR PFC flyback primary stage.

Board Specifications

Parameter	Symbol	Min.	Typ.	Max.	Unit
Input voltage range	VDD	33		60	V
Targeted LED voltage	VLED	33		53	V
Overvoltage protection	VOVP	54		57.9	V
Regulated headroom	VHR		1.8		V
LED current per channel	ILED	15		470	mA
Efficiency	η			94.5	%

Circuit Description

The reference design schematic includes:

• Current setting resistors for each channel
• Output short-circuit protection circuits
• Headroom voltage setting network
• Decoupling capacitors for noise reduction
• OVP voltage divider
• Stable supply for optocoupler circuit
• 3.3V regulator for controller supply

Electrical Performance

The BCR601-based regulator demonstrates excellent performance characteristics:

LED Current Stability

The reference design maintains stable LED current across varying numbers of connected LEDs, with minimal deviation from the target current.

Regulator Efficiency

Efficiency measurements show performance above 94% with proper design, competing effectively with switching regulator approaches while providing superior light quality.

Light Quality Analysis

The linear regulator approach delivers exceptional light quality with virtually no modulation:

• Modulation index less than 0.1% at maximum LED current
• AC content remains constant with varying light output
• Minimal flicker and stroboscopic effects across the dimming range
• Compliance with stringent light quality standards

Measurements with a spectral light meter show excellent performance:

Parameter	689 mA	515 mA	320 mA	130 mA	56 mA	23 mA
Pst LM	0.0	0.0	0.0	0.042	0.066	0.122
SVM	0.042	0.042	0.042	0.042	0.042	0.042

System Performance

When combined with the REF-XDPL8219-U40W primary side, the complete system demonstrates:

• Full load system efficiency above 87% across input line voltages
• Excellent power factor (>0.99) over wide dimming range
• Low input current THD (<10%)
• Stable thermal performance under full load conditions

These results are impressive for a high-power flyback with linear second stage and compete effectively with more complex topologies.

Bill of Materials

The reference design includes a comprehensive bill of materials with components from reputable manufacturers:

Qty	Designator	Description	Manufacturer	Part Number
1	C1	470 µF/63 V/20%	Panasonic	EEU-FC1J471
1	U1	BCR601	Infineon Technologies	BCR601
1	U21	BCR602	Infineon Technologies	BCR602
2	Q2, Q22	BSP716N	Infineon Technologies	BSP716N H6327
1	G41	FX1117ME V33/PG-SOT-223	Infineon Technologies	IFX1117ME V33