23/28

L6611

threshold separately by properly choosing the resistors of the external divider. The following relationships can

which, solved for R1 and R2, yields:

Both the ACsns threshold and the hysteresis current can be trimmed (see "On board trimming and mode oper-

ating" section).

Figure 21. ACsns circuit and timing diagram

6

APPLICATION EXAMPLE

In applications like desktop PC's, server or web server, the system usually consists of two converters (Main and

Auxiliary) that can be supplied directly from either the AC Mains or a PFC stage. The control and supervision at

the secondary side is usually entrusted to a housekeeping circuit.

The Auxiliary section supplies a stand-by voltage (5V typ.) through a flyback converter. The Main section, in

forward configuration, presents 4 standard outputs (3.3V, +5V, ±12V).

At the secondary side, the housekeeping circuitry governed by the L6611 checks the outputs and sends control

signals to the primary side through three optocouplers. It also generates power good information to the system

while managing all timings during power-up and power-down sequences. In fig. 22 a detailed circuit for the sec-

ondary side is presented; it is possible to note the very low number of external components required.

Simply connecting the power supply outputs to the L6611 relevant pins ensures the protection against over/un-

dervoltage in the Main section.

A crowbar on the auxiliary output is switched on through DFAULT in case of overvoltage.

The L6611 is supplied by the Auxiliary output; the signals sent to the primary side are:

– a "digital" ON/OFF signal through an optocoupler that drives the relevant pin of primary Main controller

to switch the Main converter ON and OFF;

– two analog signals that provide voltage feedback for both the Auxiliary and the Main section, driving the

primary controller pins responsible for the duty cycle modulation.

VB

ON

()

2.5

–

R

1

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

2.5

R

2

--------

50

µA

+

=

VB

OFF

()

R

2

R

1

R

2

+

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

⋅

2.5

=

R

1

VB

ON

()

–

50

µA

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

=

R

2

R

1

2.5

VB

OFF

–

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

⋅

=

+2.5V

GND

I

HYS=50

µA

_

+

AC_GOOD

R

1

R

2

C

1

R

F

B

L6611

ACsns

VB

AC_GOOD

V

ACsns

VB(on)

VB(off)

∆=50µA*R

1

∆

∆

PW_OK

ON

+2.5V

GND

I

HYS=50

µA

_

+

AC_GOOD

R

1

R

2

C

1

R

F

B

L6611

ACsns

VB

AC_GOOD

V

ACsns

VB(on)

VB(off)

∆=50µA*R

1

∆

∆

PW_OK

ON