TDA8385 Philips Semiconductors, TDA8385 Datasheet - Page 8

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

Philips Semiconductors
Pulse width modulator (Block IV)
The pulse width modulator compares
the control voltage V
sawtooth voltage V
output sub-section 8 is HIGH the LED
is switched on and then the switching
transistor is switched off. In this way
the output voltage is controlled.
E
If the load decreases, V
and therefore V
causes the LED to start conducting
prematurely, which implies that the
switching transistor is turned off
sooner. The consequence is that the
collector peak current decreases and
hence less energy is stored in the
transformer and V
LED control (Block V)
If either output of sub-section 8 or
output of sub-section 16 are HIGH the
LED is conductive. In order to
improve the start-up behaviour of the
power supply, the demagnetization
signal of sub-section 12 will only
activate the LED driver if flip-flop (13)
has previously been set. The set
signal is generated in the following
three ways.
1. Pulse width modulator
2. Comparator (18)
3. V
Set signal (2.) and (3.) are added as
extra security to guarantee a
demagnetization pulse in the event of
the switching transistor not having
enough base current. In that situation
e.g. at start-up, no comparator signal,
set signal (3.) is generated by
sub-section 8.
March 1994
XAMPLE
Control circuit for a Self-Oscillating
Power Supply (SOPS)
(sub-section 8)
P(min)
detector
r
decreases. This
O
sim
will decrease.
r
. If V
with the
O
increases
sim
V
r
LED driver (Block VI)
The LED driver (pin 2) is blocked if the
supply voltage V
initialization phase (see Fig.4). The
output stage is a push-pull stage,
which can sink 5 mA and source
10 mA.
Slow-start circuit (Block VII)
The slow-start circuit is active at
start-up, over voltage protection or
after an overload (short-circuited),
and stand-by mode. The voltage V
and therefore the voltage V
peak current I
start-up.
By means of sub-section 27 the slow
start voltage V
voltage V
reduced, e.g. as overload, the
slow-start capacitor is discharged to
the level of V
start-up is also guaranteed after an
overload, short-circuit situation or
after a stand-by mode. The circuit of
sub-section 27 is not active during an
over voltage protection.
When the supply voltage V
the reset-level of 5.2 V
(sub-section 28) the slow-start
capacitor is quickly discharged.
The slow-start input (pin 7) can also
be used for I
connecting a resistor to this pin.
Over voltage protection
(Block VIII)
The operation of the over voltage
protection circuit is, in the event of the
IC being SOPS-supplied, quite
different from when the IC is
externally supplied.
fb
. If the feedback voltage is
c(max)
fb
c
ss
. In this way a slow
slowly increase at
P
is clamped to the
9
is in the
setting by
mv
P
is below
and the
ss
O
EXTERNALLY SUPPLIED
When the voltage on pin 8 exceeds
2.5 V the slow-start capacitor is
slowly discharged. During discharge
the LED is permanently conducting.
Discharge is stopped when V
below 115 mV. Flip-flop (23) will then
be reset and the circuit is ready again
for a new slow-start procedure.
During an over voltage sub-section 27
is not active so that the output voltage
V
discharge procedure.
O
SOPS-
When the voltage on pin 8 exceeds
2.5 V the slow-start capacitor is
slowly discharged. During discharge
of C
discharged. Because the capacitors
C
value and the supply current I
( 15 mA) is much larger than the slow
discharge current ( 50 A), the LED
will be switched off by means of the
V
moment the switching transistor will
be switched on again until the 7.5 V
level is reached. During this
hysteresis interval the slow-charge
capacitor is quickly discharged. At the
7.5 V level the LED will be switched
on again because flip-flop (23) output
is still HIGH.
The same procedure will be repeated
several times until the slow-start
capacitor reaches the 115 mV reset
level. At that moment the slow-start
procedure is started again.
If there is still an over voltage the
procedure will be repeated.
Figure 10 is a detailed exposure of
Fig.11.
O
P
P(min)
PERATION WHEN THE
PERATION WHEN
and C
cannot influence the slow-start
ss
the supply capacitor C
detection circuit (5.2 V). At that
SUPPLIED
ss
have almost the same
Preliminary specification
(
SEE FIGS
IC
TDA8385
IS
IC
IS
9
AND
P
ss
P
is also
is
10)