Application information
After fixing CF according toTable 6:
L6699
Table 6.
Recommended values for CF as a function of the startup frequency fstart
fstart [kHz]
CF [pF]
fstart [kHz]
CF [pF]
150
160
680
560
470
390
330
270
220
230 - 240
250
180
150
120
100
82
170
260
180
270
190 – 200
210
280
290
68
220
300
56
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
RFmin
fmax
1
RFmi n
=
; RFmax =
3·CF·fmin
-1
fmin
Figure 6.
Oscillator waveforms and their relationship with gate-driving signals
CF
D
T
D
T
t
t
HVG
LVG
HB
t
t
AM11380v1
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).
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).
14/38
Doc ID 022835 Rev 2
STMICROELECTRONICS [ ST ]
