l6611 STMicroelectronics, l6611 Datasheet - Page 19

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

Figure 16. Magamp control feedback section
– Auxiliary section, error amplifier C.
Figure 17. Auxiliary feedback section
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.
pin#3) drives the external circuitry that biases the magamp core. Between these pins it is connected
the compensation network (Z
improve response time.
The feedback control circuit determines the magamp "off" time, converting the voltage at the output
of error amplifier into a current I
value, V(B
age across R
PWM waveform across D
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.
out
) decreases; this causes a higher voltage across R
E
and a larger reset current I
to AUX
to AUX
control
control
magamp
magamp
2
to get narrower. This pulls the output voltage back to the desired level and
D
D
1
1
C
). The maximum positive output voltage is clamped at about 3.5V to
R
, which resets the magamp. If the output voltage exceeds its preset
I
I
R
R
R
R
D
D
B
B
2
2
Cout
Cout
R
R
Q
Q
E
E
V
V
1
1
D2
D2
R
Zc(aux)
Zc(aux)
GND
GND
(V
R
R
BE
R
R
DMON
DMON
L
L
C
C
S
S
OCP bounce
OCP bounce
of Q
1
Bout
Bout
_
_
+
+
L6611
L6611
C
C
is supposed constant). A larger I
R
R
+1.25V
+1.25V
H
H
Zc
Zc
Cinv
Cinv
+
+
_
_
L6611
L6611
+1.25V
+1.25V
C
R
R
V
V
which, in turn, implies a larger volt-
L
L
AUX
AUX
Binv
Binv
R
R
H
H
+3.3V
+3.3V
R
R
L
L
R
causes the
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