LTC3728EG#TRPBF Linear Technology, LTC3728EG#TRPBF Datasheet - Page 24

• Manufacturer Part Number: LTC3728EG#TRPBF
• Description: IC SW REG SYNC STP-DN DUAL28SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728EG#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 5.5 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 3.5 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
LTC3728
Minimum On-Time Considerations
Minimum on-time, t
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:
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 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
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
24
t
ON(MIN)
<
V
V
IN
OUT
(f)
ON(MIN)
, is the smallest time duration
IN
/V
OUT
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 trans-
former:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current, then
V
the FCB pin sets a minimum voltage V
where R5 and R6 are shown in Figure 2.
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 0.18μA
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
Feedback Resistors
>4.8V
SEC
V
V
SEC
SEC
SEC(MIN)
will droop. An external resistive divider from V
FCB
≅ (N + 1) V
drops below this level, the FCB voltage forces
< 4.0V
0.8V 1+
OUT
R6
R5
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
SEC
, is normally set (as
SEC(MIN)
:
SEC
SEC
3728fg
to
is