L6599 STMicroelectronics, L6599 Datasheet - Page 21

• Manufacturer Part Number: L6599
• Description: High-voltage resonant controller
• Manufacturer: STMicroelectronics

Specifications of L6599

• Dimensions (l/w/h): 132 mm / 52 mm / 22 mm
• Universal Ac Mains Range: 90 VAC to 264 VAC
• High Output Power: 90 W (4.74 A, 19 VDC)
• Very Low Output Noise And Ripple: less than 300 m VP-P
• Very Low Standby Power: less than 0.5 W at 264 V
• Hiccup Mode: Auto recovery with SCP
• Latch Mode: OVP acts while VOUT > 29 VDC

L6599
7.3
Soft-start
Generally speaking, purpose of soft-start is to progressively increase 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, then soft- start is done by
sweeping the operating frequency from an initial high value until the control loop takes over.
With the L6599 converter's soft start-up is simply realized with the addition of an R-C series
circuit from pin 4 (RF
Initially, the capacitor C
in parallel to RF
since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far
away from the regulated value):
The C
(2V) and, consequently, the current through R
time constants R
regulated value and the feedback loop taken over, so that it will be the optocoupler's
phototransistor to determine the operating frequency from that moment onwards.
During this frequency sweep phase the operating frequency will decay following the
exponential charge of C
change will get slower and slower. This counteracts the non-linear frequency dependence
of the tank circuit that makes converter's power capability change little as frequency is away
from resonance and change very quickly as frequency approaches resonance frequency
(see
Figure 28. Soft-start circuit
Figure
SS
capacitor is progressively charged until its voltage reaches the reference voltage
29).
min
R
SS
Fmin
·C
and the resulting initial frequency is determined by R
min
SS
SS
SS
) to ground (see
but, before that time, the output voltage will have got close to the
is totally discharged, so that the series resistor R
, that is, initially it will change relatively quickly but the rate of
R
C
SS
SS
f
RFmin
start
Css
=
---------------------------------------------------------------
3 CF
⋅
4
1
Figure
⋅
(
L6599
SS
R F
28).
(
1
goes to zero. This conventionally takes 5
min
||
R
SS
)
)
Application information
SS
SS
and RF
| Z ( f ) |
is effectively
min
only,
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