參數(shù)資料
型號: LTC3112EDHD#PBF
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: 12 A SWITCHING REGULATOR, 825 kHz SWITCHING FREQ-MAX, PDSO16
封裝: 5 X 4 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-229WJGD-2, DFN-16
文件頁數(shù): 11/32頁
文件大?。?/td> 651K
代理商: LTC3112EDHD#PBF
LTC3112
19
3112f
APPLICATIONS INFORMATION
Boost Mode Small Signal Model
When stepping up from a lower input voltage to a higher
output voltage, the buck-boost converter will operate in
boost mode where the small signal transfer function from
control voltage, VCOMP, to the output voltage is given by
the following expression.
V
O
V
COMP BOOST MODE
= G
BOOST
1
+
s
2
πf
Z
1–
s
2
πf
RHPZ
1
+
s
2
πf
O Q
+
s
2
πf
O
2
In boost mode operation, the transfer function is charac-
terized by a pair of resonant poles and a zero generated
by the ESR of the output capacitor as in buck mode.
However, in addition there is a right half plane zero which
generates increasing gain and decreasing phase at higher
frequencies. As a result, the crossover frequency in boost
mode operation generally must be set lower than in buck
mode in order to maintain sufcient phase margin.
The boost mode gain, GBOOST, is comprised of two com-
ponents: the pulse width modulator and the power stage.
The gain of the power stage in boost mode is given by the
following equation.
G
POWER
V
OUT
2
1– t
LOW f
()V
IN
By combining the individual terms, the total gain in boost
mode can be reduced to the following expression. Notice
that unlike in buck mode, the gain in boost mode is a
function of both the input and output voltage.
G
BOOST
2
V
OUT
2
V
IN
In boost mode operation, the frequency of the right half
plane zero, fZ, is given by the following expression. The
frequency of the right half plane zero decreases at higher
loads and with larger inductors.
f
RHPZ =
R1– t
LOW f
()2 V
IN
2
πLV
OUT
2
In boost mode, the resonant frequency of the power stage
has a dependence on the input and output voltage as shown
by the following equation.
f
O =
1
2
π
R
S +
RV
IN
2
V
OUT
2
LC
O R + RC
()
1
2
π
V
IN
V
OUT
1
LC
Finally, the magnitude of the quality factor of the power
stage in boost mode operation is given by the following
expression.
Q
=
LC
ORRS +
RV
IN
2
V
OUT
2
L
+ C
ORSR
Compensation of the Voltage Loop
The small signal models of the LTC3112 reveal that the
transfer function from the error amplier output, VCOMP,
to the output voltage is characterized by a set of resonant
poles and a possible zero generated by the ESR of the output
capacitor as shown in the Bode plot of Figure 4. In boost
mode operation, there is an additional right half plane zero
that produces phase lag and increasing gain at higher freq-
uencies. Typically, the compensation network is designed to
ensure that the loop crossover frequency is low enough that
the phase loss from the right half plane zero is minimized.
The low frequency gain in buck mode is a constant, but
varies with both VIN and VOUT in boost mode.
Figure 4. Buck-Boost Converter Bode Plot
GAIN
PHASE
BOOST MODE
BUCK MODE
–20dB/DEC
–40dB/DEC
fO
f
3112 F06
fRHPZ
–90°
–180°
–270°
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