AN993 datasheet(11/27 Pages) STMICROELECTRONICS

11 / 27 page

AN993

Device block description

Doc ID 5656 Rev 10

11/27

Equation 10

Equation 11

These equations fit well with the measured values, especially in the frequency range of 30 to

100 kHz. Figure 8 shows a comparison between the measured and computed data of fmin

(at Cf = 470 pF).

To summarize:

If Rpre is not connected at pin 2, there is no Δf, so fmax and fmin have the same value.

Furthermore, there is no Imax, that is to say there is no Ifs current. In this case, Cpre is

charged only once by Ipre = 2.3 µA up to 3.5 V (Figure 7 shows the first voltage rise only).

When Cpre is discharged, no Ifs current is present to recharge it. If Rpre is connected after

the preheating and ignition phases, pin 1(Cpre) is at 4.8 to 5 V with a current capability of a

few µA (1/6 IRpre). If Rpre is not connected, Cpre is at GND level in the normal operation

phase.

Figure 7.

Cpre voltage and frequency

shifting

Figure 8.

Operating frequency at Cf =

470 pF

Table 2.

Parameter dependencies

Parameter

Set by choosing...

Tpre

Cpre

Tsh

Cpre

Δf = f

max - fmax

Rpre

fmin

Rign and Cf

AM01315v1

time

freq

V(Cpre)

ignition

preheating

AM01316v1

100

120

ign (k

Ω)

100

120

measured values

computed values

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