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LTC1876

1876fa

approximately 220kHz. The FREQSET pin can be grounded

to lower this frequency to approximately 140kHz or tied to

the INTVCCpintoyieldapproximately310kHz.TheFREQSET

pin may be driven with a voltage from 0 to INTVCC to fix or

modulate the oscillator frequency as shown in Figure 5.

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest time duration

that the step down controller is capable of turning on the

top MOSFET. It is determined by internal timing delays and

the gate charge required to turn on the top MOSFET. Low

duty cycle applications may approach this minimum on-

time limit and care should be taken to ensure that.

ON MIN

OUT

()

If the duty cycle falls below what can be accommodated by

the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for each controller is generally less

than 200ns. However, as the peak sense voltage decreases

the minimum on-time gradually increases up to about

300ns. This is of particular concern in forced continuous

applications with low ripple current at light loads. If the

duty cycle drops below the minimum on-time limit in this

situation, a significant amount of cycle skipping can occur

with correspondingly larger current and voltage ripple.

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 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 < VFB < 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

Remember that both controllers are temporarily forced

into continuous mode when the FCB pin falls below 0.8V.

APPLICATIO S I FOR ATIO

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