參數(shù)資料
型號(hào)： L6599TR
廠(chǎng)商： STMICROELECTRONICS
元件分類(lèi)： 穩(wěn)壓器
英文描述： 0.8 A SWITCHING CONTROLLER, 500 kHz SWITCHING FREQ-MAX, PDSO16
封裝： LEAD FREE, SOP-16
文件頁(yè)數(shù)： 14/36頁(yè)
文件大小： 633K
代理商： L6599TR
L6599
Application information
21/36
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 (RFmin) to ground (see Figure 27).
Initially, the capacitor CSS is totally discharged, so that the series resistor RSS is effectively
in parallel to RFmin and the resulting initial frequency is determined by RSS and RFmin only,
since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far
away from the regulated value):
The CSS capacitor is progressively charged until its voltage reaches the reference voltage
(2V) and, consequently, the current through RSS goes to zero. This conventionally takes 5
time constants RSSCSS but, before that time, the output voltage will have got close to the
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 CSS, that is, initially it will change relatively quickly but the rate of
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
Figure 27. Soft-start circuit
f
start
1
3CF
R F
min
R
SS
||
()
------------------------------------------------------------------------
=
L6599
Css
4
1
RFmin
RFmin
RSS
CSS
| Z ( f ) |
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