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參數(shù)資料
| 型號： | LTC3727IG-1 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING CONTROLLER, 580 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封裝： | 5.30 MM, PLASTIC, SSOP-28 |
| 文件頁數(shù)： | 15/32頁 |
| 文件大小： | 366K |
| 代理商： | LTC3727IG-1 |
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22
LTC3727/LTC3727-1
3727fc
loading conditions. The open-loop DC gain of the control
loop is reduced depending upon the maximum load step
specifications. Voltage positioning can easily be added to
the LTC3727 by loading the ITH pin with a resistive divider
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.com).
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 VSEC is normally set as
shown in Figure 6 by the turns ratio N of the transformer:
VSEC (N + 1) VOUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then VSEC will droop. An external resistive divider from
VSEC to the FCB pin sets a minimum voltage VSEC(MIN):
VV
R
SEC MIN
()
.
+
08
1
6
5
where R5 and R6 are shown in Figure 2.
If VSEC drops below this level, the FCB voltage forces
temporary continuous switching operation until VSEC 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 0.18
μA
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
APPLICATIO S I FOR ATIO
WU
UU
Figure 8. Active Voltage Positioning Applied to the LTC3727
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:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
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 LTC3727 circuits: 1) LTC3727 VIN current (in-
cluding loading on the 3.3V internal regulator), 2) INTVCC
regulator current, 3) I2R losses, 4) Topside MOSFET
transition losses.
1. The VIN current has two components: the first is the DC
supply current given in the Electrical Characteristics table,
ITH
RC
RT1
INTVCC
CC
3727 F08
LTC3727
RT2
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