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AN1729 APPLICATION NOTE

Enhanced Burst-Mode Operation

The start-up circuit plays an important role and it is important to look at its schematic redrawn in fig. 4.

The circuit is basically a current generator including a high-voltage MOS (Q5) that can be disabled through the

switch Q6 driven by the output of the error amplifier of the L6565 (COMP). Note that the inverting input of the

error amplifier (INV) is connected to ground, while the non-inverting one is internally connected to a voltage ref-

the controller is off, COMP is floating.

turns off Q5. Thus, the current flowing in the start up network during normal operation is only that through the

two resistors RH1 and RH2 (4.7M

Ω each), therefore a really low power is dissipated (10 mW @ 220 Vac).

As previously said, the start-up circuit is arranged as a constant current generator, with the advantage of having

a start-up time of the converter independent of the mains voltage. The analytic expression of the current deliv-

ered by the generator is:

(Eq.1)

duction threshold gate-source voltage of the MOS Q5 (STQ1NC60).

Figure 4. Start up circuit

In the circuit shown in the schematic of figure 3, the L6565 internal E/A is not used in the feedback loop. During

loop is closed summing the feedback signal to the current sense signal through a resistor (R9 in the schematic)

between the current sense resistor and the current sense pin of the L6565. This arrangement allows realizing

the enhanced burst-mode operation during standby through the circuit whose schematic is redrawn in figure 5.

If a large current is forced to flow in the resistance R9, the voltage on the current sense pin will exceed the sec-

ond overcurrent protection threshold and this will disable the gate driver. To re-enable the driver, first the IC

I

st up

–

V

z

V

Drop

V

th

–

+

RH3

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

=

Mains

Bulk

Capacitor

RH1

RH2

RH3

C4

L6565

Pin1 Inv

Q5

STQ1NC60

Q6

BC547

DZ1

D10

1N4148

R24 100K

R24 22K