ltc1876eg-trpbf Linear Technology Corporation, ltc1876eg-trpbf Datasheet - Page 23

• Manufacturer Part Number: ltc1876eg-trpbf
• Description: High Efficiency, 2-phase, Dual Synchronous Step-down Switching Controller And Step-up Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
approximately 220kHz. The FREQSET pin can be grounded
to lower this frequency to approximately 140kHz or tied to
the INTV
pin may be driven with a voltage from 0 to INTV
modulate the oscillator frequency as shown in Figure 5.
Minimum On-Time Considerations
Minimum on-time t
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.
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 V
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
t
ON MIN
(
CC
)
pin to yield approximately 310kHz. The FREQSET
V
V f
IN
OUT
( )
U
ON(MIN)
U
is the smallest time duration
IN
/V
OUT
W
ratio is low, the
CC
U
to fix or
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
shown in Figure 6a by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
V
If V
temporary continuous switching operation until V
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 A
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
0V to 0.75V
0.85V < V
Feedback Resistors
>4.8V
Remember that both controllers are temporarily forced
into continuous mode when the FCB pin falls below 0.8V.
SEC
V
V
SEC
SEC
SEC MIN
to the FCB pin sets a minimum voltage V
SEC
FB
(
drops below this level, the FCB voltage forces
< 4.3V
(N + 1) V
will droop. An external resistive divider from
)
. 0 8 1
V
OUT
R
R
6
5
Condition
Forced Continuous (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
SEC
is normally set as
LTC1876
SEC(MIN)
23
SEC
1876fa
:
is