LTC3727A-1
21
3727a1fa
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC3727A-1 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 the LTC3727A-1 is approximately
120ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases up to about
170ns. 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 signifi cant amount of cycle skipping can
occur with correspondingly larger inductor current and
output 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
on both controllers when the FCB pin drops below 0.8V.
During continuous mode, current fl ows continuously in
the transformer primary. The secondary winding(s) draw
current only when the bottom, synchronous switch is on.
When primary load currents are low and/or the V
IN
/V
OUT
ratio is low, the synchronous switch may not be on for a
suffi cient amount of time to transfer power from the output
capacitor to the secondary load. Forced continuous opera-
tion will support secondary windings providing there is
suffi cient 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 V
SEC
is normally set as shown
in Figure 6 by the turns ratio N of the transformer:
V
SEC
≅ (N + 1) V
OUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
SEC
will droop. An external resistive divider from
V
SEC
to the FCB pin sets a minimum voltage V
SEC(MIN)
:
VV
R
R
SEC MIN()
.≅+
⎛
⎝
⎜
⎞
⎠
⎟
08 1
6
5
APPLICATIONS INFORMATION
where R5 and R6 are shown in Figure 2.
If V
SEC
drops below this level, the FCB voltage forces
temporary continuous switching operation until V
SEC
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 0.18mA
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 < 6.8V Minimum Peak Current Induces
Burst Mode Operation
No Current Reversal Allowed
Feedback Resistors Regulating a Secondary Winding
>7.3V 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
specifi cations. Voltage positioning can easily be added
to the LTC3727A-1 by loading the I
TH
pin with a resistive
divider having a Thevenin equivalent voltage source
equal to the midpoint operating voltage range of the error
amplifi er, or 1.2V (see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifi er.
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).
