參數(shù)資料
型號: L6599N
廠商: STMICROELECTRONICS
元件分類: 穩(wěn)壓器
英文描述: 0.8 A SWITCHING CONTROLLER, 500 kHz SWITCHING FREQ-MAX, PDIP16
封裝: LEAD FREE, PLASTIC, DIP-16
文件頁數(shù): 8/36頁
文件大小: 636K
代理商: L6599N
Application information
L6599
16/36
7.1
Oscillator
The oscillator is programmed externally by means of a capacitor (CF), connected from pin 3
(CF) to ground, that will be alternately charged and discharged by the current defined with
the network connected to pin 4 (RFmin). The pin provides an accurate 2 V reference with
about 2 mA source capability and the higher the current sourced by the pin is, the higher the
oscillator frequency will be. The block diagram of Figure 22 shows a simplified internal
circuit that explains the operation.
The network that loads the RFmin pin generally comprises three branches:
1.
A resistor RFmin connected between the pin and ground that determines the minimum
operating frequency;
2.
A resistor RFmax connected between the pin and the collector of the (emitter-grounded)
phototransistor that transfers the feedback signal from the secondary side back to the
primary side; while in operation, the phototransistor will modulate the current through
this branch - hence modulating the oscillator frequency - to perform output voltage
regulation; the value of RFmax determines the maximum frequency the half-bridge will
be operated at when the phototransistor is fully saturated;
3.
An R-C series circuit (CSS + RSS) connected between the pin and ground that enables
to set up a frequency shift at start-up (see Chapter 7.3: Soft-start). Note that the
contribution of this branch is zero during steady-state operation.
Figure 22. Oscillator's internal block diagram
The following approximate relationships hold for the minimum and the maximum oscillator
frequency respectively:
L6599
2 V
+
-
4
RFmin
3
CF
S
Q
R
+
-
1 V
4 V
+
-
CF
IR
2KMIR
KMIR
RFmin
RFmax
RSS
CSS
KMIR
0.9V
3.9V
f
min
1
3CFRF
min
-------------------------------------
=
f
max
1
3CF
RF
min
RF
max
||
()
-----------------------------------------------------------------
=
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