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LTC1628/LTC1628-PG

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):

SEC MIN

()

≈+









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 0.18

µA

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 < VFCB < 4.3V

Minimum Peak Current Induces

Burst Mode Operation

No Current Reversal Allowed

Feedback Resistors

Regulating a Secondary Winding

>4.8V

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

ITH

RT1

INTVCC

1628 F08

LTC1628

RT2

Figure 8. Active Voltage Positioning Applied to the LTC1628

APPLICATIO S I FOR ATIO

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