L6611

18/28

2

ERROR AMPLIFIERS AND REFERENCE VOLTAGES

Three error amplifiers are implemented on the IC to achieve regulation of the output voltages: a brief description

follows for each section.

– Main section: error amplifier A and Soft-Start.

The circuit is designed to directly control the Main primary PWM through an optocoupler, providing

very good regulation and galvanic isolation from the primary side. Typical solutions require a shunt

regulator, like the TL431, as a reference and feedback amplifier to sense the output voltage and gen-

erate a corresponding error voltage; this voltage is then converted in a current transferred to the pri-

mary side through the optocoupler.

The feedback E/A amplifier is integrated in the IC: its non-inverting input is connected to an internally gen-

erated voltage reference, whose default value is typically 2.5V. It can however be trimmed to obtain a better

precision (see "On board trimming and mode operating" section). Then, no TL431 is needed.

The E/A inverting input (Ainv, pin#5) and the E/A output (Aout, pin#4) are externally available and the

frequency compensation network (Zc) will be connected between them (see fig. 15).

The high impedance (in the hundred k

Ω) internal divider from 12V and 5V UV/OV sense pins elimi-

nates the need for an external one in most applications, allowing a further reduction in the number of

external component.

Under closed loop condition, the two upper branches, connected to 12V and 5V pins, supply equally

the current flowing through R3= 80.6K (equal to 2.5V/R3).

In order to avoid high current peaks in the primary circuit and output voltage overshoots at start-up,

the IC provides an on-board 8ms soft-start, a quasi-monotonic ramp from 0V to 2.5V for the A error

amplifier reference voltage,. In fact, if this reference gets the nominal value as soon as the power-up

occurs, the A E/A will go out of regulation and tend to sink much more current, thus forcing PWM to

work with the maximum duty-cycle.

– E/A and references voltage

Being the inverting input of E/A externally available, it is possible to change the "weight" of the two

weight will be equal to 6:4.

back because the contribution of +12V branch (through the internal 600K resistor) will be negligible.

The pin #24 (12V) has to be connected to +12V output to guarantee the OV/UV monitoring.

Figure 15. Main feedback section

– 3.3V section, error amplifier B.

It is the error amplifier used to set the magamp core through an external circuitry (see a typical sche-

matic in figure 16).

The non-inverting input of the error amplifier is connected to a trimmable 1.25V internal voltage ref-

erence (see "On board trimming and mode operating" paragraph). The E/A inverting input is exter-

V

DD

to MAIN

control

Zc

Aout

R

B

12V

5V

+2.5V

600K

168K

80.6K

8ms SS

L6611

GND

_

+

+12V output

+5V output

Ainv

R

L

R

H1

R

H2

optional, to change

feedback weight

V

DD

to MAIN

control

Zc

Aout

R

B

12V

5V

+2.5V

600K

168K

80.6K

8ms SS

L6611

GND

_

+

+12V output

+5V output

Ainv

R

L

R

H1

R

H2

optional, to change

feedback weight