
March 1994
8
Philips Semiconductors
Preliminary specification
Control circuit for a Self-Oscillating
Power Supply (SOPS)
TDA8385
Fig.7
Current foldback characteristic; stabilized output voltage (V
O
)
as function of load current (I
O
).
(5)
VO
IO
(4)
(3)
(2)
(1)
MCD406
(1), (2), (3) = V
TCG
.
(4), (5) = V
diff
.
The voltage V
Ton(min)
determines the
minimum on-time of the switching
transistor. This voltage can be
determined externally with a resistor
on pin 4. With this resistor the current
foldback characteristic can be
influenced (see dotted line in Figs 6
and 7).
The minimum on-time is of
importance for the following.
Stand-by operation
Starting-up of power supply
Overload and short-circuit
conditions.
The output of the differential amplifier
(V
diff
), the output of the TCG (V
TCG
)
and the voltage V
ss
+ V
Ton(min)
are
compared in a minimum voltage
clamping circuit (see Fig.1
sub-section 6). The output voltage is
equal to the lowest input voltage.
Some relevant characteristics of the
control part are depicted in Fig.8.
Fig.8 Characteristics of the control part.
The voltage V
determines the collector peak current I
c
of transistor T1. The right-hand curve is passed through at start-up. When the feedback voltage
slowly increases from zero, the peak current starts at I
and rises along the straight line until I
c(max)
is reached. At a slightly higher feedback voltage
the regulation slope is reached, which is approximately V
ref
.
The plateau of the top between the points x and y has to be kept as small as possible.
The voltage V
diff
decreases with the decreasing load. For good no-load operation the peak current has to be made zero with V
diff
.
Due to the characteristic of the TCG open- and short-circuit feedback loop will result in low peak current.
An additional signal on pin 13 can be supplied which is subtracted from the signal V
mv
. This input can be used for feed forward information.
If no feed forward information is used, pin 13 should be connected to ground.
handbook, full pagewidth
MCD407
Vss+
fb
V
Ic (max)
Ic (min)
Ic
Vmv
x
y
Vref
VTCG
V
external peak-current
setting (pin 7)
diff
V
mv
VTon(min)