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NE5560N

Switched-mode power supply control circuit

Philips Electronics 

Philips Semiconductors

Switched-mode power supply control circuit

Product specification

NE/SE5560

TYPICAL PERFORMANCE CHARACTERISTICS (Continued)

Typical Frequency Plot vs RT and

f(kHz)

CT

δ

1000

100

90

80

7600

50

40

30

20

10

9

8

7

6

5

4

3

2

1

R=5kΩ

R=10kΩ

R=20kΩ

R=40kΩ

2

2.5

3

3.5

4

4.5 CnF)

100

90

80

70

60

50

40

30

20

10

V16

1

1.5

2

2.5 VZ

Figure 5. Typical Performance Characteristics

SL00364

THEORY OF OPERATION

The following functions are incorporated:

– A temperature-compensated reference source.

– An error amplifier with Pin 3 as input. The output is connected

to Pin 4 so that the gain is adjustable with external resistors.

– A sawtooth generator with a TTL-compatible synchronization

input (Pins 7, 8, 9).

– A pulse-width modulator with a duty cycle range from 0 to 95%.

The PWM has two additional inputs:

Pin 6 can be used for a precise setting of δMAX

Pin 5 gives a direct access to the modulator, allowing for real

constant-current operation:

– A gate at the output of the PWM provides a simple dynamic

current limit.

– A latch that is set by the flyback of the sawtooth and reset by

the output pulse of the above mentioned gate prohibits double

pulsing.

– Another latch functions as a start-stop circuit; it provides a fast

switch-off and a slow start.

– A current protection circuit that operates via the start-stop

circuit. This is a combined function with the current limit circuit,

therefore Pin 11 has two trip-on levels; the lower one for

cycle-by-cycle current limiting, the upper one for current

protection by means of switch-off and slow-start.

– A TTL-compatible remote on/off input at Pin 10, also operating

via the start-stop circuit.

– An inhibit input at Pin 13. The output pulse can be inhibited

immediately.

– An output gate that is commanded by the latches and the inhibit

circuit.

– An output transistor of which both the collector (Pin 15) and the

emitter (Pin 14) are externally available. This allows for normal

or inverse output pulses.

– A power supply that can be either voltage- or current-driven

(Pins 1 and 12). The internally-generated stabilized output

voltage VZ is connected to Pin 2.

– A special function is the so-called feed-forward at Pin 16. The

amplitude of the sawtooth generator is modulated in such a way

that the duty cycle becomes inversely proportional to the

voltage on this pin: δ ~ 1/V16.

– Loop fault protection circuits assure that the duty cycle is

reduced to zero or a low value for open- or short-circuited

feedback loops.

Stabilized Power Supply (Pins 1, 2, 12)

The power supply of the NE5560 is of the well known series

regulation type and provides a stabilized output voltage of typically

8.5V.

This voltage VZ is also present at Pin 2 and can be used for precise

setting of δMAX and to supply external circuitry. Its max. current

capability is 5mA.

The circuit can be fed directly from a DC voltage source between

10.5V and 18V or can be current-driven via a limiting resistor. In the

latter case, internal pinch-off resistors will limit the maximum supply

voltage: typical 23V for 10mA and max. 30V for 30mA.

The low supply voltage protection is active when V(1-12) is below

10.5V and inhibits the output pulse (no hysteresis).

When the supply voltage surpasses the 10.5V level, the IC starts

delivering output pulses via the slow-start function.

1994 Aug 31

7

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