NE5560 Philipss, NE5560 Datasheet - Page 13

• Manufacturer Part Number: NE5560
• Description: Switched-mode power supply control circuit
• Manufacturer: Philipss

Philips Semiconductors
Remote On/Off Circuit (Pin 10)
In systems where two or more power supplies are used, it is often
necessary to switch these supplies on and off in a sequential way.
Furthermore, there are many applications in which a supply must be
switched by a logical signal. This can be done via the
TTL-compatible remote on/off input on Pin 10. The output pulse is
inhibited for levels below 0.8V. The output of the IC is no longer
blocked when the remote on/off input is left floating or when a
voltage >2V is applied. Start-up occurs via the slow-start circuit.
The Output Stage
The output stage of the NE5560 contains a flip-flop, a push-pull
driven output transistor, and a gate, as indicated in Figure 18. The
flip-flop is set by the flyback of the sawtooth. Resetting occurs by a
signal either from the PWM or the current limit circuit. With this
configuration, it is assured that the output is switched only once per
period, thus prohibiting double pulsing. The collector and emitter of
the output transistor are connected to respectively Pin 15 and Pin
14, allowing for normal or inverted output pulses. An
internally-grounded emitter would cause intolerable voltage spikes
over the bonding wire, especially at high output currents.
This current capability of the output transistor is 40mA peak for V
collector against overvoltages. The max. voltage at the emitter (Pin
14) must not exceed +5V. A gate, activated by one of the set or
reset pulses, or by a command from the start-stop circuit will
immediately switch-off the output transistor by short-circuiting its
base. The external inhibitor (Pin 13) operates also via this base.
Demagnetization Sense
As indicated in Figure 18, the output of this NPN comparator will
block the output pulse, when a voltage above 0.6V is applied to Pin
13. A specific application for this function is to prevent saturation of
forward-converter transformers. This is indicated in Figure 19.
Feed-Forward (Pin 16)
The basic formula for a forward converter is
This means that in order to keep V
cycle must be made inversely proportional to the input voltage. A
pre-regulation (feed-forward) with the function ~1/V
feedback-loop design.
1994 Aug 31
0.4V. An internal clamping diode to the supply voltage protects the
Switched-mode power supply control circuit
V
OUT
dV
n
IN
(n
transformer ratio)
V
OUT
6
V
15
at a constant value, the duty
DISCHARGE
IN
SET
can ease the
Figure 17. Start-Stop Circuit
DEAD
TIME
RESET
CE
CHARGE
13
INCREASES
This loop now only has to regulate for load variations which require
only a low feedback gain in the normal operation area. The
transformer of a forward converter must be designed in such a way
NOTES:
The signal V
a forward converter as shown below.
B
FLYBACK
13
.6V
can be derived from the demagnetizing winding in
FROM START STOP
SET
RESET
V
Z
Figure 18. Output Stage
5.6V
SAWTOOTH
1.1V
V
H
Z
NE/SE5560
P1 P2
Product specification
+
–
“I” “I” “n”
+
–
V
1
0.6V
S1
15
14
13
SL00376
SL00377