L6611 Просмотр технического описания (PDF) - STMicroelectronics

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L6611

DIGITALLY PROGRAMMABLE SECONDARY HOUSEKEEPING CONTROLLER

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L6611

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 kΩ) 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

contributions or even eliminate one of them by connecting external resistors of much lower value (RL,

RH1 and/or RH2 in fig. 15) that bypass the internal ones appropriately.

For example using RL=2.4K, RH1=3.9K and RH2=24K, then the ratio between +5V and +12V output

weight will be equal to 6:4.

By simply making RH1 = RL (for example 2.4K) with no RH2, only the +5V output is kept under feed-

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

VDD

to MAIN

control

RB

+12V output

+5V output

5V

12V

Aout

L6611

168K

_

600K

+

80.6K

8ms SS

+2.5V

GND

Zc

RH1

Ainv

optional, to change

feedback weight

RH2

RL

– 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-

nally available (Binv, pin#2) and is connected to the output divider (RH and RL); the output pin (Bout,

18/28

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