Datasheet

L6699 Application information
Doc ID 022835 Rev 2 13/38
6.1 Oscillator
The oscillator is programmed externally by means of a capacitor (CF), connected from pin 3
(CF) to ground, that is 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; the higher the current sourced by the pin, the higher the oscillator
frequency. The block diagram of
Figure 5 shows a simplified internal circuit that explains the
operation.
The network that loads the
RF
min
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 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 modulates the current
through this branch - therefore modulating the oscillator frequency - to perform output
voltage regulation; the value of RFmax determines the maximum frequency the half
bridge is 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
the setting up of a frequency shift at startup (see
Section 6.3: Safe-start procedure ).
Note that the contribution of this branch is zero during steady-state operation.
Figure 5. Oscillator's internal block diagram
The following approximate relationships hold for the minimum and the maximum oscillator
frequency respectively:
Equation 1
!-V
,
6


2&MIN
#&
3
1
2


6
6


#&
)
2
q+
-
q)
2
+
-
q)
2
2
&MIN
2
&MAX
2
33
#
33
+
-
q)
2
()
maxmin
max
min
min
RF//RF · CF · 3
1
=f;
RF · CF · 3
1
=f