Datasheet
Application information L6699
20/38 Doc ID 022835 Rev 2
the operating frequency is essentially determined by the current sunk by the optocoupler's
phototransistor.
During this frequency sweep phase the operating frequency decays following the
exponential charge of C
SS
: then, it initially changes relatively quickly but the rate of change
gets slower and slower. This counteracts the non-linear frequency dependence of the tank
circuit that makes the converter's input impedance change little as frequency is away from
resonance and change very quickly as frequency approaches resonance frequency (see
Figure 13
).
As a result, the DC input current smoothly increases, without the peaking that occurs with
linear frequency sweep, and the output voltage reaches the regulated value with almost no
overshoot.
Ty p i c a l l y, R
SS
and C
SS
is selected based on the following relationships:
Equation 5
where f
start
is recommended to not be over 300 kHz. Please refer to the timing diagram of
Figure 20
to see some significant signals during the soft-start phase.
Figure 12. Soft-start circuit Figure 13. Input impedance vs.
frequency curve in an LLC
resonant half bridge
!-V
,
#SS
2&M IN
2
33
#
33
2&M IN
!-V
,/7% 22%3/.!. #%
&2%15%. #9
F
\:F\
)NITIAL
FR
EQUENCY
3TEADY STATE
FR
EQUENCY
500%22%3/.!. #%
&2%15%. #9
SS
3
SS
min
start
min
SS
R
10 ·3
=C;
1
f
f
RF
=R
-
-