Datasheet

Application information L6699
14/38 Doc ID 022835 Rev 2
After fixing CF according to Tabl e 6 :
Value of RF
min
and RF
max
is selected so that the oscillator frequency is able to cover the
entire range needed for regulation, from the minimum value f
min
(at minimum input voltage
and maximum load) to the maximum value f
max
(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 RF
max
if burst-mode operation is to be used (see
Section 7: Operation at no load or very light load ).
In
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 f
start
f
start
[kHz] CF [pF] f
start
[kHz] CF [pF]
150 680 230 - 240 180
160 560 250 150
170 470 260 120
180 390 270 100
190 – 200 330 280 82
210 270 290 68
220 220 300 56
1
f
f
RF
=RF;
f · CF · 3
1
=RF
-
min
max
min
max
min
mi n
AM11380v1
CF
HVG
t
t
LVG
t
HB
t
T
D
T
D