LTC1628I View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1628I' PDF : 32 Pages View PDF
LTC1628/LTC1628-PG
APPLICATIO S I FOR ATIO
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 VIN/VOUT 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 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 VSEC is normally set as
shown in Figure 6a by the turns ratio N of the transformer:
VSEC ≅ (N + 1) VOUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then VSEC will droop. An external resistive divider from
VSEC to the FCB pin sets a minimum voltage VSEC(MIN):
VSEC(MIN)
≈
0.8V1+
R6
R5
If VSEC drops below this level, the FCB voltage forces
temporary continuous switching operation until VSEC 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 < VFCB < 4.3V
Feedback Resistors
>4.8V
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
The FLTCPL pin determines whether only the first or both
controllers are temporarily forced into continuous mode
when the FCB pin falls below 0.8V. Tying the FLTCPL pin
to ground will send only the first controller into continuous
operation while tying the FLTCPL pin to INTVCC will send
both controllers into continuous operation.
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 LTC1628 by loading the ITH 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
INTVCC
RT2
RT1
ITH
RC
LTC1628
CC
1628 F08
Figure 8. Active Voltage Positioning Applied to the LTC1628
22
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