l6599atn STMicroelectronics, l6599atn Datasheet - Page 18

• Manufacturer Part Number: l6599atn
• Description: Improved High-voltage Resonant Controller
• Manufacturer: STMicroelectronics

Application information
6.3
18/31
Soft-start
Generally speaking, the purpose of soft-start is to progressively increase the converter's
power capability when it is started up, so as to avoid excessive inrush current. In resonant
converters the deliverable power depends inversely on frequency, so soft-start is attained by
sweeping the operating frequency from an initial high value until the control loop takes over.
With the L6599AT, the converter's soft startup is easily achieved with the addition of an R-C
series circuit from pin 4 (RF
Initially, capacitor C
parallel to RF
since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far
from the regulated value):
Equation 4
The C
(2 V) and, consequently, the current through R
constants R
close to the regulated value and the feedback loop takes over, so that the optocoupler's
phototransistor determines the operating frequency from that moment onwards.
During this frequency sweep phase the operating frequency decays, following the
exponential charge of C
change gets slower and slower. This counteracts the non-linear frequency dependence of
the tank circuit that makes the converter's power capability change little as frequency is
away from resonance, and change very quickly as the frequency approaches the resonance
frequency (see
Figure 10. Soft-start circuit (left) and power vs. frequency curve in a resonant
As a result, the average input current increases smoothly, without the peaking that occurs
with linear frequency sweep, and the output voltage reaches the regulated value with almost
no overshoot.
Typically, R
SS
capacitor is progressively charged until its voltage reaches the reference voltage
SS
SS
half-bridge (right)
min
and C
·C
Figure 10
SS
and the resulting initial frequency is determined by R
. However, before this time interval completes, the output voltage comes
SS
SS
is totally discharged, so that the series resistor R
are selected based on the following relationships:
SS
, right).
. That is, initially it changes relatively quickly but the rate of
min
Doc ID 15534 Rev 1
) to ground (see
f
start
=
3
⋅
CF
⋅
(
SS
RF
Figure 10
1
goes to zero. This typically takes 5 times
min
//
R
SS
, left).
)
SS
SS
and RF
is effectively
min
L6599AT
only,