Application information

L6572

18/22

Doc ID 023882 Rev 1

Equation 5

The second relation is also valid for dimmed operations.

A small capacitor placed in parallel with Rph (around 10 pF), helps to improve the

preheating phase.

Deadtime can also be fixed during this step.

A useful relationship to find the right value of Rdt, expressed in k

Ω, given the value of the

selected deadtime, expressed in µs is:

Equation 6

Being a non dimmable application, the RDO pin and TSD pin have to be connected to

ground. To be noted that CFLi applications up to 25 W are equipped by MOSFETs having

small gate charges: therefore no gate resistance is required and the gate pin of the

MOSFETs can be directly connected to driver output.

6.3

Two-step dimmable CFLi application

Once a fixed power application is designed, the dimmability feature can be added by

activating TSD pin and RDO.

The first pin is intended to sense the interval between AC turn-off and on, usually using only

Ra. As soon as the AC line is turned off, resonant current decreases and consequently the

half-bridge stops switching due to Tdis function. Once this occurs the supply voltage drops

below turn-off threshold.

The present voltage across the half-bridge is decreasing, but it is greater than zero keeping

the TSD structure well supplied thanks to Ra. The smaller the Ra, the longer the time the

TSD voltage remains above the dimming enable threshold and the slower the required turn-

off turn-on action to enable dimming.

If, during the turn-off interval TSD drops below the threshold, the internal memory cell is

reset, otherwise it remains in its previous state.

At subsequent turn-on, after preheating, this state is checked to determine which power

level is required.

The dimmed area is selected by Rdo: its value is expected to be lower or equal to Rno.

Frequently, the starting voltage across the half-bridge has a wide voltage ripple, and its

nominal value is variable as well. For these reasons, a precise design guidance of the

selection of Ra cannot be provided. The quickest way to proceed is represented by direct

measurements on the real application. Selection of Rdo can be done using the same

approach used to determine Rno.

R

ph k

Ω)

(

A

ph

μVs

×

()

20

⋅

≅

R

no k

Ω)

(

A

no

μVs

×

()

20

⋅

≅

R

dt

10.8

T

dt

⋅

=