Datasheet

LTC3707
22
3707fb
APPLICATIONS INFORMATION
FCB Pin Operation
The FCB pin can be used to regulate a secondary winding
or as a logic level input. Continuous operation is forced
when the FCB pin drops below 0.8V. During continuous
mode, current fl ows continuously in the transformer pri-
mary. The secondary winding(s) draw current only when
the bottom, synchronous switch is on. When primary
load currents are low and/or the V
IN
/V
OUT
ratio is low,
the synchronous switch may not be on for a suffi cient
amount of time to transfer power from the output capacitor
to the secondary load. Forced continuous operation will
support secondary windings providing there is suffi cient
synchronous switch duty factor. Thus, the FCB input pin
removes the requirement that power must be drawn from
the inductor primary in order to extract power from the
auxiliary windings. With the loop in continuous mode, the
auxiliary outputs may nominally be loaded without regard
to the primary output load.
The secondary output voltage V
SEC
is normally set as shown
in Figure 6a by the turns ratio N of the transformer:
V
SEC
≅ (N + 1) V
OUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current, then
V
SEC
will droop. An external resistive divider from V
SEC
to
the FCB pin sets a minimum voltage V
SEC(MIN)
:
V
SEC(MIN)
≈0.8V 1+
R6
R5
⎛
⎝
⎜
⎞
⎠
⎟
If V
SEC
drops below this level, the FCB voltage forces
temporary continuous switching operation until V
SEC
is
again above its minimum.
In order to prevent erratic operation if no external connec-
tions are made to the FCB pin, the FCB pin has a 0.18µA
internal current source pulling the pin high. Include this
current when choosing resistor values R5 and R6.
The following table summarizes the possible states avail-
able on the FCB pin:
Table 1
FCB Pin Condition
0V to 0.75V
Forced Continuous (Current Reversal
Allowed—Burst Inhibited)
0.85V < V
FCB
< V
INTVCC
– 2V Minimum Peak Current Induces
Burst Mode Operation
No Current Reversal Allowed
Feedback Resistors Regulating a Secondary Winding
= V
INTVCC
Burst Mode Operation Disabled
Constant Frequency Mode Enabled
No Current Reversal Allowed
No Minimum Peak Current
Voltage Positioning
Voltage positioning can be used to minimize peak-to-peak
output voltage excursions under worst-case transient
loading conditions. The open-loop DC gain of the control
loop is reduced depending upon the maximum load step
specifi cations. Voltage positioning can easily be added to
the LTC3707 by loading the I
TH
pin with a resistive divider
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage of the error amplifi er, or 1.2V
(see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifi er.
The maximum output voltage deviation can theoretically
be reduced to half or alternatively the amount of output
capacitance can be reduced for a particular application.
A complete explanation is included in Design Solutions
10. (See www.linear.com)
I
TH
R
C
R
T1
INTV
CC
C
C
3707 F08
LTC3707
R
T2
Figure 8. Active Voltage Positioning Applied to the LTC3707