Datasheet

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 (RF
min
). 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 RF
min
connected between the pin and ground that determines the minimum
operating frequency;
2. A resistor RF
max
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 RF
max
determines the maximum frequency the half-bridge will
be operated at when the phototransistor is fully saturated;
3. An R-C series circuit (C
SS
+ R
SS
) 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
+
-
4RFmin
3 CF
S
Q
R
+
-
1 V
4 V
+
-
CF
I
R
2·K
M
·I
R
K
M
·I
R
R
Fmin
R
Fmax
R
SS
C
SS
K
M
·I
R
0.9V
3.9V
f
min
1
3CFRF
min
⋅⋅
-------------------------------------=
f
max
1
3CF RF
min
RF
max
||
()⋅⋅
-----------------------------------------------------------------=