Application information

L6572

16/22

Doc ID 023882 Rev 1

6

Application information

6.1

Target applications

The L6572 is primarily intended to accurately drive, with a minimum number of external

components, integrated compact fluorescent lamps. Its particular current control method

allows a noticeable extension of lamp lifetime and a higher light quality compared to that

obtainable by self oscillating ballasts. Also, the operation of the application during normal

operation is very similar to the above mentioned ballasts, but with an improved repeatability

of the lamp parameters (see Figure 2).

The same advantages can be exploited by separately controlled fluorescent lamp

applications, especially CFLs up to 26 W.

Using further external components, higher power ratings are also reachable.

Furthermore, dimming functionality, in particular two-step dimming, helps to implement

variable light sources.

6.2

Designing a simple fixed power CFLi application using the

L6572

The first step to follow is to define the clock frequency: it can be calculated in order to fit the

preheating time requirements. The following equations give the relationships between the

typical value of clock period, the preheating time and RCL value.

●

●

●

Given the clock frequency, the value of the resonance frequency of the LC cell must be

selected higher than the half of the clock frequency in all possible conditions. Otherwise the

ignition recognition circuitry may operate incorrectly.

●

The range of frequencies related to nominal operation and to the preheating phase can be

now selected respectively below and above FRes. Considering the operating frequency

ranges and the lamp parameters, the value for resonant components can now be selected:

different calculation approaches can be adopted.The value of the sense resistor has to be

chosen in order to guarantee a sufficient signal level for the ZCD function: at minimum the

peak of the resonant current in every condition must be above VZVS. As the reference

current of programming resistors is matched the integrator reference current, a relation

between the programming resistor value and the obtained voltage*time area (VTA) can be

expressed. These relations can be used to evaluate a first tentative value for the

programming resistors RPH, RNO and RDO. A subsequent iterative verification on the real

project is usually required: in fact, the value of the integral is highly dependent on the real

shape of the sense voltage. Deadtime duration somewhat affects the result of the integration

as well.

RCL

k

Ω

0.10327

T

PH

ms

⋅

≅

TCLμs 0.2364 RCLkΩ

⋅

≅

FCL

KHZ

1000

TCLμs

-----------------

≅

F

Res

1

2

π

LC

⋅

⋅

---------------------------------

≅