L6611 Просмотр технического описания (PDF) - STMicroelectronics

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L6611

DIGITALLY PROGRAMMABLE SECONDARY HOUSEKEEPING CONTROLLER

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L6611

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

the compensation network (ZC). The maximum positive output voltage is clamped at about 3.5V to

improve response time.

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

of error amplifier into a current IR, which resets the magamp. If the output voltage exceeds its preset

value, V(Bout) decreases; this causes a higher voltage across RC which, in turn, implies a larger volt-

age across RE and a larger reset current IR (VBE of Q1 is supposed constant). A larger IR causes the

PWM waveform across D2 to get narrower. This pulls the output voltage back to the desired level and

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 16. Magamp control feedback section

magamp

D1

L

D2

VD2

IR

RE

RC

Q1

RS

+3.3V

C

RH

Zc

Bout

Binv

_

+

RL

+1.25V

L6611

– Auxiliary section, error amplifier C.

This section (fig. 17) provides the feedback signal for the auxiliary converter following the same oper-

ating principles as the Main section. The auxiliary output voltage (Vaux) is often defined as "Standby

voltage" because the converter remains alive during standby condition (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#7) 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 17. Auxiliary feedback section

to AUX

control

RB

Zc(aux)

VAUX

RH

RL

DMON

_

Cinv

Cout

+

OCP bounce +1.25V

GND

L6611

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