LTC3707IGN datasheet(22/32 Pages) LINER

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LTC3707

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 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

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 VSECisnormallysetasshown

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) ≈ 0.8V 1+

⎛

⎝⎜

⎞

⎠⎟

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

Condition

0V to 0.75V

Forced Continuous (Current Reversal

Allowed—Burst Inhibited)

0.85V < VFCB < VINTVCC – 2V

Minimum Peak Current Induces

Burst Mode Operation

No Current Reversal Allowed

Feedback Resistors

Regulating a Secondary Winding

= VINTVCC

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 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 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)

ITH

RT1

INTVCC

3707 F08

LTC3707

RT2

Figure 8. Active Voltage Positioning Applied to the LTC3707

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