Техническая Спецификация для STMicroelectronics 19V - 90W Adapter with PFC for Laptop computers using the L6563H and L6699 EVL6699-90WADP EVL6699-90WADP
Модели
EVL6699-90WADP
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 (
(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
frequency. The block diagram of
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.
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
).
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:
frequency respectively:
Equation 1
!-V
,
2&MIN
3
1
2
#&
)
2
q+
-
q)
2
+
-
q)
2
2
&MIN
2
&MAX
2
33
#
33
+
-
q)
2
(
)
max
min
max
min
min
RF
//
RF
·
CF
·
3
1
=
f
;
RF
·
CF
·
3
1
=
f