LTC3707 Datasheet, PDF (22/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
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 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_IN/V_OUT ratio is low,
the 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
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
0V to 0.75V
0.85V < V_FCB < V_INTVCC – 2V
Feedback Resistors
= V_INTVCC
Condition
Forced Continuous (Current Reversal
Allowed—Burst Inhibited)
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
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_TH pin with a resistive divider
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.com)
22
INTV_CC
R_T2
R_T1
I_TH
R_C
LTC3707
C_C
3707 F08
Figure 8. Active Voltage Positioning Applied to the LTC3707
3707fb