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L6610

5

AC SENSE (MAINS UNDERVOLTAGE WARNING)

The device monitors the primary bulk voltage and warns the system when the power is about to be lost pulling

down the PW_OK output.

The ACsns pin is typically connected to one of the windings of the main transformer (see fig. 23). Through a

to clamp voltage spikes present.

The fault signal is generated by means of AC_GOOD, the output of an internal comparator; this comparator is

internally referred to a trimmable 2.5V reference and indicates an AC fault if the voltage applied at its externally

available (non-inverting) input is below the internal reference, as shown in fig. 23.

This comparator is provided with current hysteresis instead of a more usual voltage hysteresis: an internal 50µA

current generator is ON if the voltage is below 2.5V and is turned off when the voltage applied at the non-invert-

ing input exceeds 2.5V.

This approach provides an additional degree of freedom: it is possible to set the ON threshold and the OFF

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 23. 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 5 standard outputs (3.3V, ±5V, ±12V).

At the secondary side, the housekeeping circuitry governed by the L6610 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. 24 a detailed circuit for the sec-

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

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

L6610

ACsns

VB

AC_GOOD

V

ACsns

VB(on)

VB(off)

∆=50µA*R

1

∆

∆

PW_OK

ON