l6611 STMicroelectronics, l6611 Datasheet - Page 18

• Manufacturer Part Number: l6611
• Description: Digitally Programmable Secondary Housekeeping Controller
• Manufacturer: STMicroelectronics

L6611
2
Three error amplifiers are implemented on the IC to achieve regulation of the output voltages: a brief description
follows for each section.
Figure 15. Main feedback section
18/28
– Main section: error amplifier A and Soft-Start.
– E/A and references voltage
– 3.3V section, error amplifier B.
ERROR AMPLIFIERS AND REFERENCE VOLTAGES
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 ) 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.
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 (R
R
For example using R
weight will be equal to 6:4.
By simply making R
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.
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 (R
H1
and/or R
H2
in fig. 15) that bypass the internal ones appropriately.
to MAIN
to MAIN
control
control
H1
L
=2.4K, R
= R
V
V
DD
DD
R
R
B
B
L
(for example 2.4K) with no R
H1
=3.9K and R
L6611
L6611
Aout
Aout
+12V output
+12V output
+5V output
+5V output
+2.5V
+2.5V
_
_
+
+
Zc
Zc
168K
168K
8ms SS
8ms SS
H2
5V
5V
=24K, then the ratio between +5V and +12V output
80.6K
80.6K
GND
GND
600K
600K
12V
12V
H2
, only the +5V output is kept under feed-
Ainv
Ainv
R
R
H1
H1
R
R
L
L
R
R
H2
H2
H
optional, to change
optional, to change
feedback weight
feedback weight
and R
L
); the output pin (Bout,
L
,