L6699D datasheet(14/38 Pages) STMICROELECTRONICS

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Application information

L6699

14/38

Doc ID 022835 Rev 2

After fixing CF according to

Table 6:

Value of RFmin and RFmax is selected so that the oscillator frequency is able to cover the

entire range needed for regulation, from the minimum value fmin (at minimum input voltage

and maximum load) to the maximum value fmax (at maximum input voltage and minimum

load):

Equation 2

Figure 6.

Oscillator waveforms and their relationship with gate-driving signals

A different selection criterion is given for RFmax if burst-mode operation is to be used (see

Section 7: Operation at no load or very light load).

Figure 6 the timing relationship between the oscillator waveform and the gate-drive

signal, as well as the midpoint of the half bridge leg (HB) is shown. Note that the low-side

gate-drive is turned on while the oscillator's triangle is ramping up and the high-side gate-

drive is turned on while the triangle is ramping down. In this way, at startup, or as the IC

resumes switching during burst-mode operation, the low-side MOSFET is switched on first

to charge the bootstrap capacitor. As a result, the bootstrap capacitor is always charged and

ready to supply the high-side floating driver (see

Section 12: Bootstrap section).

Table 6.

Recommended values for CF as a function of the startup frequency fstart

fstart [kHz]

CF [pF]

fstart [kHz]

CF [pF]

150

680

230 - 240

180

160

560

250

150

170

470

260

120

180

390

270

100

190 – 200

330

280

210

270

290

220

300

;

min

max

min

max

min

mi n

AM11380v1

HVG

LVG

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