LTC3728 Linear Technology, LTC3728 Datasheet - Page 22

• Manufacturer Part Number: LTC3728
• Description: 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3728
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
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.
The following table summarizes the possible states avail-
able on the FCB pin:
Table 1
FCB Pin
0.85V < V
Feedback Resistors
>4.8V
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
22
0V to 0.75V
SEC
V
V
SEC
SEC
SEC MIN
to the FCB pin sets a minimum voltage V
SEC
FCB
(
≅ (N + 1) V
drops below this level, the FCB voltage forces
< 4.0V
will droop. An external resistive divider from
)
≈
0 8 1
.
V
OUT
U
⎛
⎜
⎝
+
R
R
U
6
5
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
W
is normally set as
U
SEC(MIN)
SEC
:
is
loop is reduced depending upon the maximum load step
specifications. Voltage positioning can easily be added to
the LTC3728 by loading the I
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage range of the error amplifier, or
1.2V (see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifier. 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-tech.com)
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3728 circuits: 1) LTC3728 V
cluding loading on the 3.3V internal regulator), 2) INTV
regulator current, 3) I
transition losses.
1. The V
supply current given in the Electrical Characteristics table,
%Efficiency = 100% – (L1 + L2 + L3 + ...)
IN
current has two components: the first is the DC
Figure 8. Active Voltage Positioning
Applied to the LTC3728
INTV
CC
R
R
T2
T1
R
C
C
2
C
R losses, 4) Topside MOSFET
I
TH
TH
pin with a resistive divider
LTC3728
www.DataSheet4U.com
3728 F08
IN
current (in-
3728fb
CC