AN1523 datasheet(10/24 Pages) STMICROELECTRONICS

10 / 24 page

AN1523 APPLICATION NOTE

10/24

Turn-Off

Even at turn off the transition is clean, without any ab-

normal behaviour like overshoots or glitches both on

the output voltages. Checking the full load condition,

a restart attempt is present on the Vcc voltage: it is

due to the circuit Q1, Q2,R9, R10, R11, C6 connect-

ed to the COMP pin. During the switching off phase

the energy in the bulk capacitor is no more refreshed,

then the voltage on it starts to decrease. This pro-

vides for an increasing of the COMP pin voltage due

to the loop intervention which is regulating the output

voltage while the input voltage is decreasing. At a

certain value the COMP voltage is able to switch on

Q1 and then Q2, thus disconnecting the transformer

from Vcc, so that the L6590 stops the operation. Be-

cause the circuit is switched off externally, the bulk

capacitor has still some energy stored and when the

Vcc has dropped below the Vccoff the IC detects that

residual input voltage higher than its Drain start volt-

age (Vdsmin). Hence the L6590 reactivates the inter-

nal current source like in a normal start-up, and the

voltage on the Vcc pin tends to increase again. But

checking the Fig. 17 it is important to note that the

Vcc value is far from the Start Threshold voltage (Vc-

con), then no any perturbation appears on the output,

avoiding any problem..

Figure 16. WAKE UP TIME

Vin = 115 Vrms - 50 Hz @FULL LOAD

CH1:

VPIN1 - DRAIN VOLTAGE

CH2:

VPIN3 -Vcc

CH3:

+5VOUT

Figure 17. TURN-OFF

Figure 18. TURN-OFF

Vin = 115 Vrms - 50 Hz @FULL LOAD

Vin = 115 Vrms - 50 Hz @STAND-BY

CH1:

VPIN1 – DRAIN VOLTAGE

CH1:

VPIN1 – DRAIN VOLTAGE

CH2:

VPIN3 - Vcc

CH2:

VPIN3 -Vcc

CH3:

+5VOUT

CH3:

+5VOUT

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