LTC3707 Linear Technology, LTC3707 Datasheet - Page 22

• Manufacturer Part Number: LTC3707
• Description: High Efficiency / 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3707
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 flows 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
synchronous switch may not be on for a sufficient amount
of time to transfer power from the output capacitor to the
secondary load. Forced continuous operation will support
secondary windings providing there is sufficient synchro-
nous 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
shown in Figure 6a by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
V
If V
temporary continuous switching operation until V
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 0.18 A
22
SEC
V
V
SEC
SEC
SEC MIN
to the FCB pin sets a minimum voltage V
SEC
(
drops below this level, the FCB voltage forces
(N + 1) V
will droop. An external resistive divider from
)
. 0 8
OUT
V
U
1
R
R
U
6
5
IN
Figure 8. Active Voltage Positioning Applied to the LTC3707
/V
SEC
W
OUT
is normally set as
ratio is low, the
INTV
U
SEC(MIN)
CC
R
R
T2
T1
SEC
R
:
C
C
is
C
I
TH
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
0.85V < V
Feedback Resistors
= V
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
specifications. Voltage positioning can easily be added to
the LTC3707 by loading the I
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage of the error amplifier, or 1.2V
(see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifier. 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-tech.com)
0V to 0.75V
LTC3707
INTVCC
FCB
3707 F08
< V
INTVCC
– 2V
Condition
Forced Continuous (Current Reversal
Minimum Peak Current Induces
Burst Mode Operation
No Current Reversal Allowed
Regulating a Secondary Winding
Burst Mode Operation Disabled
Constant Frequency Mode Enabled
No Current Reversal Allowed
No Minimum Peak Current
Allowed—Burst Inhibited)
TH
pin with a resistive divider
3707f