TDA8385 Philips Semiconductors, TDA8385 Datasheet - Page 5

• Manufacturer Part Number: TDA8385
• Description: Control circuit for a Self-Oscillating Power Supply SOPS
• Manufacturer: Philips Semiconductors

Philips Semiconductors
Sawtooth generator (Block II)
C
(
The current of the power supply
switching transistor is detected on the
secondary side by an indirect method
of current sensing.
Information of the collector current (I
is obtained by integrating the voltage
of an auxiliary winding of the
transformer during transistor on-time
(T
is charged during T
source I
reflection of the current which flows
into pin 12. This current is obtained by
connecting an external resistor R12 to
the auxiliary transformer winding.
During transistor on-time this current
is related to the input voltage V
During transistor off time (T
capacitor C is discharged by switch
sw1. This switch is active during the
total T
voltage V
sawtooth is a measure for the
collector current of the switching
transistor T1.
For the voltage V
Where: p = reflection factor;
(2)
March 1994
V
I
p
V
SEE FIGS
sim
URRENT SIMULATION
c
c
on
Control circuit for a Self-Oscillating
Power Supply (SOPS)
=
=
=
). An external capacitor C on pin 5
=
I
-------- -
I
sim
off
I
------------------------ -
(1) gives:
--- -
C
12
p
sim
p
sim
time. In this way a sawtooth
c
5
=
C
n
----- -
n
. The current I
is formed across C. This
n
----- -
n
AND
h
p
T
0.2
h
p
on
---------- -
R12
---------- -
R12
16)
V
V
c
I
I
yields:
on
handbook, full pagewidth
by the current
T
on
sim
off
is the
) the
I
.
(1)
(2)
(3)
c
)
initialization
Fig.4 Latch initialization as a function of supply voltage V
latch
V c
V I
Fig.5 Determination of the peak current I
6
T1
T on
n p
I c
L
5.2
n h
7.5
T off
V c
R12
C
I 12
V P (V)
12
5
t
Preliminary specification
MCD404
20
c
.
operation
sw1
TDA8385
I sim
MCD403
P
.