參數(shù)資料
型號(hào)： LTC1708-PG
廠商： Linear Technology Corporation
英文描述： Dual Adjustable 5-Bit VID High Efficiency, 2-Phase Current Mode Synchronous Buck Regulator Controller
中文描述： 可調(diào)式雙5位VID高效率，2相電流模式同步降壓穩(wěn)壓控制器
文件頁(yè)數(shù)： 23/32頁(yè)
文件大小： 358K
代理商： LTC1708-PG
23
LTC1708-PG
APPLICATIU
W
U
U
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 LTC1708 circuits: 1) LTC1708 V
IN
current (in-
cluding loading on the 3.3V internal regulator), 2) INTV
CC
regulator current, 3) I
2
R losses, 4) Topside MOSFET
transition losses.
1. The V
IN
current has two components: the first is the DC
supply current given in the Electrical Characteristics table,
which excludes MOSFET driver and control currents; the
second is the current drawn from the 3.3V linear regulator
output. V
IN
current typically results in a small (<0.1%) loss.
2. INTV
CC
current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from INTV
CC
to
ground. The resulting dQ/dt is a current out of INTV
CC
that
is typically much larger than the control circuit current. In
continuous mode, I
GATECHG
=f(Q
T
+Q
B
), where Q
T
and Q
B
are the gate charges of the topside and bottom side
MOSFETs.
Supplying INTV
CC
power through the EXTV
CC
switch input
from an output-derived source will scale the V
IN
current
V
V
R
R
SEC MIN
(
)
.
≈
+
0 8
1
6
5
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 0.18
μ
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
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
0.85V < V
FCB
< 4.3V
Feedback Resistors
>4.8V
Voltage Positioning
Voltage positioning can be used to minimize peak-to-peak
output voltage excursion under worst-case transient load-
ing conditions. The open-loop DC gain of the control loop
is reduced depending upon the maximum load step speci-
fications. Voltage positioning can easily be added to the
LTC1708 by loading the I
TH
pin with a resistive divider
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage 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
worst-case peak-to-peak output voltage deviation due to
transient loading can theoretically be reduced to half or
alternatively the amount of output capacitance can be
reduced for a particular application. A complete explana-
tion is included in Design Solutions 10 or the LTC1736
data sheet. (See www.linear-tech.com)
I
TH
EAIN
R
C
R
T1
INTV
CC
C
C
1708 F08
LTC1708-PG
R
T2
R1
V
OUT
R2
Figure 8. Active Voltage Positioning Applied to the LTC1708
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