AN993 datasheet(10/27 Pages) STMICROELECTRONICS

10 / 27 page

Device block description

AN993

10/27

Doc ID 5656 Rev 10

Where:

Equation 4

Rign and Rpre are the resistors connected to pin 4 and pin 2.

At the end of the preheating time (t = Tpre), the L6574 oscillates at fmax, set by:

Equation 5

This means that the preheating frequency depends on both Rpre and Rign.

At the end of the frequency shifting (t = Tpre + Tsh), the second term of Equation 3

decreases to zero and the switching frequency is set only by Imin (that is, Rign).

Equation 6

Since the second term of Equation 3 is equal to zero, we have:

Equation 7

Note that there is no fixed voltage threshold across Cpre in which the ignition phase finishes

(that is, the end of the frequency shifting): Tsh depends on Cpre, Imax, gm, and Ifs

(Equation 7). This fact is also verified in Figure 7. By making Tsh independent of Imax, the Ifs

current has been designed to be a fraction of Imax, therefore:

Equation 8

In this way, the frequency shifting time depends only on the capacitor Cpre. The typical value

of the kfs constant (frequency shift timing constant) is 0.15 s/µF, that is:

Equation 9

So when choosing Cpre, both Tpre and Tsh are set.

The frequencies fmin and fmax depend on the resistors Rpre and Rign, but also on the

capacitor Cf (oscillator frequency setting -> capacitor at pin 3). fmin is set by choosing Cf and

Rign, then with Rpre the Δf = fmax - fmin is set.

Simplified equations can be used.

AN993 Datasheet(PDF) 10 Page - STMicroelectronics