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LTC3728L-1

3728l1fc

APPLICATIONS INFORMATION

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 Both Controllers

(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

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 either or both controllers by loading the ITH pin with

a resistive divider having a Thevenin equivalent voltage

source equal to the midpoint operating voltage range 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)

Efﬁciency Considerations

The percent efﬁciency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efﬁciency and which change would

produce the most improvement. Percent efﬁciency can

be expressed as:

%Efﬁciency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC3728L-1 circuits: 1) IC VIN current (including

loading on the 3.3V internal regulator), 2) INTVCC regula-

tor current, 3) I2R losses, 4) Topside MOSFET transition

losses.

1. The VIN current has two components: the ﬁrst is the DC

supply current given in the Electrical Characteristics table,

which excludes MOSFET driver and control currents; the

second is the current drawn from the 3.3V linear regulator

output. VIN current typically results in a small (<0.1%)

loss.

Figure 8. Active Voltage Positioning

Applied to the LTC3728L-1

ITH

RT1

INTVCC

3728L1 F08

LTC3728L-1

RT2

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