AN1686 APPLICATION NOTE

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reduced by 18%, which means that the core losses will be reduced by about 38%. The estimated total loss in-

crease is only 0.1 W, hence the hot-spot temperature increase (as compared to the original design) should be

under 3 °C. It is then not necessary to change the wire size.

The specification of the modified transformer is summarized in Table 2.

Table 3. List of modifications to the original design

achieve an actual overcurrent setpoint independent of the input voltage. Their value is directly proportional to

the sense resistor R15 and inversely to the primary inductance. Since both parameters are changed R3 and R4

needs adjustment. The new value for each of them will be 2.2 M

Ω ·(0.319/0.47)·(400/360)=1.6 MΩ.

Table 3 summarizes all of the modifications described in this paragraph.

Circuit for delayed shutdown upon overpower/short circuit

The objective of the circuit of figure 2 is to provide a double programmable delay before shutting down the L5991

following on either an overload (a longer delay) or a short circuit (a shorter delay).

Figure 2. Circuit for delayed shutdown upon overpower/short circuit: electrical schematic

R23 and R24 define a voltage threshold, which the output of the E/A (COMP) will be compared to. If the L5991

is running at normal frequency, that is, the pin ST-BY is internally connected to the 5V reference VREF, Q2 is

ON and the voltage at pin 2 of the LM393 is:

(1)

If the L5991 is running at low frequency (because of a light load) the pin ST-BY is floating and, being connected

to the oscillator through a resistor [2], oscillates between 1 and 3 V, well below the threshold of the zener D6.

Q2 is then OFF and V2 equals VREF, hence the voltage at pin COMP, V(COMP), cannot exceed V2 as long as

Part

Original value

New Value

Note

C1

100 µF

150 µF

R15

0.47

Ω

0.319

Ω = (0.47 Ω // 1 Ω)

Same power rating

T1

See [1]

See table 2

R2, R3

2.2 M

Ω

1.6 M

Ω

Same power rating

4

VREF

D7

1N4148

7

DIS

COMP

8

2

3

1

6

5

IC3A

LM393

IC3B

LM393

Q1

BC327

R23

10 k

Ω

R24

24 k

Ω

R25

4.3 k

Ω

R26

100 k

Ω

R27

22 k

Ω

C18

10 µF

ST-BY

Q2

BC337

D6

4V7

R28

3.3 k

Ω

V2

VREF

R24

R23

R24

+

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⋅

=