5/7

AN1686 APPLICATION NOTE

light load and the L5991 running at the lower frequency, V(COMP) might be greater then the value of V2 defined

by (1), thus triggering the delayed shutdown function erroneously.

Assuming Q2 is ON, when V(COMP) > V2 the output of IC3A (which is open-collector) goes high and C18 is

charged through R26. The time needed for C18 to be charged at V2 is:

When this voltage is reached, the output of IC3B goes high and the voltage at pin DIS of the L5991 is pulled

above 2.5V, thus causing a latched shutdown of the PWM controller and of the converter too. The converter will

stay idle as long as it is supplied by the input power source. To be able to restart the system, the power source

must be removed. The diode D7 is used because the pin DIS is already biased (by a resistor divider to set up

OVP protection) so as to decouple the two circuits.

output of IC3A goes back low thus discharging C18 and stopping this slow shutdown countdown.

In case of short circuit COMP will saturate high and tend to go over 6 V. In that case not only the output of IC3B

will go high but also Q1 will be turned on (V(COMP) will be clamped at about 5.6V), thus C18 will be charged

through R25 and R26. This circuit is analogous to that shown in [3]. The delay will be approximately:

,

charge of C18 through R25 will be stopped; the slow charge through R26 can be stopped too or still go on, de-

One more comment: COMP is saturated high and Q1 turned on (initiating the fast shutdown countdown) every

time the converter is started up, during the ramp-up of the output voltage from zero to the regulated value. Ob-

With the part values shown in fig. 2, the results are the following:

Refer to the appendix of [4] for formulae useful for relating the voltage level on pin COMP to the power level in

a flyback converter, to be able to design the circuit under different conditions.

Figure 3 shows how the circuit of figure 2 is inserted into the modified 45W demo board (the changes are high-

lighted by the shaded regions) and the oscilloscope pictures of figure 4 illustrate the experimental results in case

of both overload and short circuit.

If converter's latched shutdown is not desired, but the user wants the system to automatically recover normal

operation when the overload/short is removed, yet maintaining a low power throughput during the fault, it is very

easy to modify the circuit of figure 2 to achieve this: R27 will be reduced at 2.2 k

Ω and the diode D7 connected

to the ISEN pin of the L5991, so as to invoke its "Hiccup" OCP [2]. The resulting schematic is illustrated in figure

5, its insertion into the 45W AC-DC adapter is obvious.

Td

1

R26 C18

VR EF

VR EF

V2

–

-------------------------------





ln

⋅⋅

=

Td

2

R25 C18

VREF

VREF

V2

–

-------------------------------





ln

⋅

⋅

≈

V2

5

24

10

24

+

-------------------

⋅

3.53 V

==

Td

1

100 10

3

10 10

6

–

5

53.53

–

---------------------





ln

⋅⋅

⋅

⋅

13.4

()

ln

⋅

1.22s

==

=

Td

2

4.3 10

3

10 10

6

–

5

53.53

–

---------------------





ln

⋅⋅

⋅

⋅

4.3 10

2

–

3.4

()

ln

⋅⋅

52m s

==

=