TDA8385 Datasheet, PDF (8/21 Pages) NXP Semiconductors – Control circuit for a Self-Oscillating Power Supply SOPS

Philips Semiconductors
Control circuit for a Self-Oscillating
Power Supply (SOPS)
Preliminary specification
TDA8385
Pulse width modulator (Block IV)
The pulse width modulator compares
the control voltage V_r with the
sawtooth voltage V_sim. If V_sim > V_r
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.
EXAMPLE
If the load decreases, V_O increases
and therefore V_r decreases. This
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_O will decrease.
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
(sub-section 8)
2. Comparator (18)
3. V_P(min) detector
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.
LED driver (Block VI)
The LED driver (pin 2) is blocked if the
supply voltage V_P is in the
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_ss
and therefore the voltage V_mv and the
peak current I_c slowly increase at
start-up.
By means of sub-section 27 the slow
start voltage V_ss is clamped to the
voltage V_fb. If the feedback voltage is
reduced, e.g. as overload, the
slow-start capacitor is discharged to
the level of V_fb. In this way a slow
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_P is below
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_c(max) setting by
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.
OPERATION WHEN THE IC IS
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_ss is
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_O cannot influence the slow-start
discharge procedure.
OPERATION WHEN IC IS
SOPS-SUPPLIED (SEE FIGS 9 AND 10)
When the voltage on pin 8 exceeds
2.5 V the slow-start capacitor is
slowly discharged. During discharge
of C_ss the supply capacitor C_P is also
discharged. Because the capacitors
C_P and C_ss have almost the same
value and the supply current I_P
(≈15 mA) is much larger than the slow
discharge current (≈50 µA), the LED
will be switched off by means of the
V_P(min) detection circuit (5.2 V). At that
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.
March 1994
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