AN1729 APPLICATION NOTE

6/10

When the micro controller of the TV gives the standby command, the switch SW1 is open and the 14V output

turns on the small signal bipolar transistor Q3. As a result, a big current (as compared to the current that flows

during normal operation) is forced to flow in the LED of the opto-coupler and thereby in both the phototransistor

and R9. As explained before, this big current will disable the gate driver of the L6565.

When the gate driver is disabled the quiescent current is unchanged and, since no energy is coming from the

the start-up generator will be enabled and a new start-up cycle will begin. The resulting behaviour will be a low

frequency intermittent operation that we will refer to as "enhanced burst-mode operation". With such operation

the regulation loop used in normal operation of the converter is skipped; another kind of regulation takes place.

is the voltage drop of the diode D8 (1N4148) (we can neglect the voltage drop on resistor R21). At this point the

gate driver of the controller is disabled and switching is stopped. The length of this idle period is fixed by the

discharge time of the capacitor C4 as above explained. During this idle period the output voltages will go down

and the minimum values that they reach depends on their individual load.

In order to obtain some design equations we can divide the burst-mode period in three time intervals:

– Phase 1. From the moment when the L6565 controller has been just turned off and the start-up circuit

– Phase 2. From when the L6565 is turned on until the transistor Q3 is turned on and the L6565 stops.

threshold and the L6565 is turned off.

Our purpose is to obtain simple equations, so some approximations will be introduced. An optimisation can be

done, looking at the measurement results, in order to have as good performance as possible.

– Phase 1.

current Mos Q5 is on and the capacitor C4 is charged through it and the resistor RH3. The starting value of the

voltage on C4 is the UVLO off threshold. The voltage value on C4 when the L6565 is turned on is the turn-on

(Eq. 2)

– Phase 2.

and the gate driver of the L6565 disabled. The duration of this switching phase is typically much shorter than

the others and will be neglected.

– Phase 3.

current of the L6565; the time for discharging the C4 capacitor is:

T

ch

C 4

–

V

th

on

–

V

th

o ff

–

–

() C4

⋅

I

st

u p

–

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

Vcc

Hys

RH3 C 4

⋅⋅

V

Z

V

Dro p

V

th

–

+

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==