TDA8385 Datasheet, PDF (4/21 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
PINNING
SYMBOL
RIO
LED
I_ref
T_on(min)
I_peak
DELAY
V_ss
OVP
V_fb
V_sb
V_diff
I_sim
V_fo
GND
DEM
V_P
PIN
DESCRIPTION
1
regulation indicator output
2
LED driver output
3
current reference setting
4
transistor-on setting input
5
peak current setting input
6
delay setting
7
slow start voltage input
8
over-voltage protection
9
feedback voltage input
10
stand-by voltage input
11
differential amplifier output
12
current simulation input
13
feed forward input
14
ground (0 V)
15
demagnetization input
16
positive supply voltage
handbook, 2 columns
RIO 1
16 VP
LED 2
15 DEM
I _ref 3
14 GND
Ton(min) 4
I _{peak 5}
TDA8385
13 Vfo
12 I _sim
DELAY 6
Vss 7
OVP 8
11 Vdiff
10 Vsb
9 Vfb
MCD402
Fig.3 Pinning diagram.
FUNCTIONAL DESCRIPTION
The TDA8385 can be divided into
10 functional blocks as shown in Fig.1
and Fig.2.
Block for Figs 1 and 2
BLOCK
NO.
DESCRIPTION
I
supply references
II
sawtooth generator
III
control part
IV
pulse width modulator
(PWM)
V
LED control
VI
LED driver
VII
slow-start circuitry
VIII
over-voltage protection
IX
stand-by circuit
X
regulation-indicator
output
These 10 functional blocks of Fig.1
and Fig.2 contain sub-sections
numbered 1 to 28 which are
cross-referenced in the following
description.
Supply references (Block I)
The TDA8385 is intended to be used
on the secondary side of the
self-oscillating power supply. It can be
supplied either by an auxiliary winding
of the transformer or an external
supply e.g. 50 Hz transformer.
Charging of the capacitor C_P (see
Fig.16) takes place during transistor
on-time (T_on; see Fig.17). During
stand-by the IC is supplied by the
stand-by voltage V_sb (pin 10). The
operating voltage range is from 7.5 to
20 V. The supply current, inclusive
drive current for the LED, is less than
20 mA. A bandgap based reference
(2.5 V) generates a stabilized voltage
V_stab of 3.9 V to supply all internal
circuits of the IC except the LED
driver. The LED driver is directly
supplied by V_P. The reference block
generates all the reference voltages
in the circuit. By means of a resistor
connected to pin 3, a reference
current (I_ref) is defined.
This current is reflected several times
and is used to obtain IC-independent
settings e.g. T_on(min) setting, delay
setting, charging and discharging of
slow-start capacitor C_ss on pin 7
(see Fig.16).
The power supply is released by the
opto-coupler IC at an input voltage
level, which is high enough to
guarantee correct operation of the
TDA8385 e.g. V_P = 10 V by sensing
the mains voltage V_I. As soon as the
SOPS switching transistor (T1, see
Fig.16) is conductive the capacitor C_P
is charged. As long as the IC supply
voltage is below 7.5 V the LED driver
is blocked (see latch output;
sub-section 28) in order to guarantee
start-up of SOPS.
During the initialization phase the
quick-discharge-switch
(sub-section 27), set input of
flip-flop (13) and reset input of
flip-flop (23) are also activated.
As soon as the voltage of 7.5 V is
reached the control functions of the IC
are operative. Hysteresis on the
initialization level is 2.3 V.
March 1994
5