AN4350 datasheet(5/34 Pages) STMICROELECTRONICS

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AN4350

Circuit description and design guidelines

2.1

Preliminary consideration

The HVLED815PF controller is specifically designed to work as a constant current (CC)

LED driver with a primary side regulation (PSR) and a high power factor (HPF) capability.

As shown on the left side of Figure 2, the HVLED815PF device incorporates two control

loops: the current control loop (constant current - CC) trough the ILED pin regulates the LED

output current (ILED = IOUTCC) and the voltage control loop (constant voltage - CV) trough

the COMP pin regulates the output voltage during the open-LED fault condition.

The voltage control loops can then be used to regulate the output voltage when the output

current is lower than the maximum deliverable current (IOUTCC) resulting in an output

characteristic showed on the right side of Figure 2.

Figure 2. Voltage and current control principle (left side) and output characteristic (right side)

Starting from the standard application schematic (see Figure 1: “Application circuit for high

power factor LED driver - single range input” in the HVLED815PF datasheet for the details)

the constant current LED driver can be easily “converted” into

a constant voltage LED driver (CV) still keeping the high power factor and low total harmonic

distortion (THD).

The high power factor is implemented adding through the RPF resistor a contribution

proportional to the VIN input voltage on the CS pin: as a consequence the input current is

proportional to the input voltage during the line period, implementing the high power factor

correction.

In the CC LED driver the ROS resistor has been mainly used to add on the CS pin a positive

contribution proportional to the average value of the input voltage (VIN) in order to keep

a good line regulation. The second advantage of the contribution trough the ROS resistor is

to keep a good total harmonic distortion (THD) over the line range, because adding a small

offset on the CS pin the input current can go to zero when the input voltage is close to zero.

When implementing a CV LED driver the voltage control loop works to keep the output

voltage constant and independent from the input voltage, so the contribution proportional to

the average value of the input voltage is no more needed.

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