L6599 datasheet(17/36 Pages) STMICROELECTRONICS

17 / 36 page

L6599

Application information

17/36

After fixing CF in the hundred pF or in the nF (consistently with the maximum source

capability of the RFmin pin and trading this off against the total consumption of the device),

the value of RFmin and RFmax will be 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):

A different selection criterion will be given for RFmax in case burst-mode operation at no-load

will be used (see "Operation at no load or very light load" section).

Figure 22. Oscillator waveforms and their relationship with gate-driving signals

In Figure 22 the timing relationship between the oscillator waveform and the gate-drive

signals, as well as the swinging node 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 start-up, or as the

IC resumes switching during burst-mode operation, the low-side MOSFET will be switched

on first to charge the bootstrap capacitor. As a result, the bootstrap capacitor will always be

charged and ready to supply the high-side floating driver.

min

3CFf

min

⋅⋅

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

max

min

max

min

-----------

–

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

HVG

LVG

L6599 数据表(PDF) 17 Page - STMicroelectronics