LTC3728_15 Datasheet, PDF (24/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time, t_ON(MIN), is the smallest time duration
that the LTC3728 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:
t_ON(MIN)
<
V_OUT
V_IN(f)
If the duty cycle falls below what can be accommodated
by the minimum on-time, the LTC3728 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The typical tested minimum on-time of the LTC3728 is
100ns under an ideal condition without switching noise.
However, the minimum on-time can be affected by PCB
switching noise in the voltage and current loops. With
reasonably good PCB layout, minimum 30% inductor
current ripple and about 15mV sensing ripple voltage,
200ns minimum on-time is a conservative number to
start with.
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 flows 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
sufficient 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
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, V_SEC, is normally set (as
shown in Figure 6a) by the turns ratio N of the trans-
former:
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):
V_SEC(MIN)


0.8V


1+
R6
R5


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 0.18μA
internal current source pulling the pin high. Include this
current when choosing resistor values R5 and R6.
Table 1 summarizes the possible states available on the
FCB pin:
Table 1
FCB Pin
0V to 0.75V
0.85V < V_FCB < 4.0V
Feedback Resistors
>4.8V
Condition
Forced Continuous Both Controllers
(Current Reversal Allowed—Burst
Inhibited)
Minimum Peak Current Induces
Burst Mode Operation
No Current Reversal Allowed
Regulating a Secondary Winding
Burst Mode Operation Disabled
Constant-Frequency Mode Enabled
No Current Reversal Allowed
No Minimum Peak Current
3728fg
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