AN3111

Circuit design

Doc ID 16815 Rev 2

7/23

The proposed circuit runs at constant peak current. The current sensing voltage is set for 1

V, which is the clamping voltage of the current sensing comparator of the L6562A. The

voltage of the LED string is sensed on the INV pin of the L6562A through a coupled winding.

The turn ratio of the coupled inductor is designed so that the feedback voltage is lower than

2.5 V in normal operating conditions, so the error amplifier is saturated at maximum level

which sets the max. current sensing voltage to 1 V. Therefore, the peak current of the

inductor is fixed at a level determined by the sensing resistor value. As the peak current of

the inductor is constant, the input power is constant. The LED voltage is considered

constant, and the current into the LED can also be considered constant. If the load (the

LEDs) is open, the reflected output voltage on the INV pin is above a certain level, and the

controller shuts down to provide open load protection.

If the inductor current is controlled at constant level all the time, then the power factor is very

poor. The multiplier of the L6562A is used to reshape the current amplitude near the line

voltage zero crossing to improve the power factor. The rectified sine waveform is sampled at

the MULT pin. The output of the multiplier, which is the current setting, is lower than setting

the level near the line voltage zero crossing. In this way, the power factor of the converter is

improved significantly.

Design procedure:

●

Input voltage:

●

Output voltage: 18 LEDs in series, Vout=54 V

●

Output current: Iout=350 mA

The design variable is the peak current of the inductor (Ipk), and the inductance (L).

When Q1 is turned on, the inductor is charged to Ipk. The ON time is:

Equation 1

The OFF time is:

Equation 2

The period of the switching cycle is:

Equation 3

The duty cycle (D) is:

Equation 4

Vin

θ

()

2Vin

θ

()

sin

⋅

⋅

=

Vin

120V

=

,

)

t

(

Vin

Ipk

*

L

)

(

Ton

=

θ

Vout

Ipk

*

L

)

(

Toff

=

θ

Vout

Ipk

*

L

)

(

Vin

Ipk

*

L

)

(

Toff

)

(

Ton

)

(

T

+

θ

=

θ

+

θ

=

θ

)

(

Vin

Vout

Vout

)

(

T

)

(

Ton

)

(

D

θ

+

=

θ

θ

=

θ