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pin#3) drives the external circuitry that biases the magamp core. Between these pins it is connected

improve response time.

The feedback control circuit determines the magamp "off" time, converting the voltage at the output

achieves regulation.

It is possible to use this section to drive a pass transistor to obtain 3.3V with a linear regulator; in the

"Application idea" section an example is showed to implement this solution.

Figure 18. Magamp control feedback section

– Auxiliary section, error amplifier C. This section (fig. 19) provides the feedback signal for the auxiliary

converter following the same operating principles as the Main section. The auxiliary output voltage

(Vaux) is often defined as "Standby voltage" because the converter remains alive during standby con-

dition (the Main converter is stopped) to supply the chip and all the ancillary circuits. Typical values for

its output voltage are 5V or 3.3V.

The inverting input (Cinv, pin#9) is connected to the output voltage through an external resistor divider

whereas the non-inverting one is connected to a 1.25V trimmable internal voltage reference (see "On

board trimming and mode operating" paragraph).

The compensation network Zc(aux) is placed between E/A inverting input and output pins.

When Dmon recognizes an undervoltage condition on the auxiliary output, an internal n-channel MOS

(in open drain configuration) grounds E/A output pin; the high current flowing through the optocoupler

is then transferred to the primary side causing a duty cycle as short as possible; this prevents a high

energy transfer from primary to secondary under short circuit conditions, thus reducing the thermal

stress on the power components.

Figure 19. Auxiliary feedback section

+3.3V

Zc

Bout

Binv

+1.25V

R

L

R

H

_

+

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magamp

R

C

R

S

R

E

D

1

I

R

D

2

V

D2

Q

1

L

C

+3.3V

Zc

Bout

Binv

+1.25V

R

L

R

H

_

+

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magamp

R

C

R

S

R

E

D

1

I

R

D

2

V

D2

Q

1

L

C

V

AUX

to AUX

control

Zc(aux)

Cout

Cinv

R

B

+1.25V

R

H

R

L

_

+

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OCP bounce

GND

DMON