參數(shù)資料
型號: LTC3536EDD#PBF
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: 4 A SWITCHING REGULATOR, 1440 kHz SWITCHING FREQ-MAX, PDSO10
封裝: 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10
文件頁數(shù): 7/28頁
文件大?。?/td> 1088K
代理商: LTC3536EDD#PBF
LTC3536
15
3536f
Figure 4. Small-Signal Model
variety of output voltages, switching frequencies, and
external component values supported by the LTC3536.
VIN is the input supply voltage, VOUT the programmed
output voltage, L is the external buck-boost inductor, COUT
the output capacitor, RS the series resistance in the power
path (it can be approximated as twice the average power
switch resistance plus the DC resistance of the inductor)
and RC is the output capacitor ESR.
Buck Mode
The small-signal transfer function of the buck-boost
converter is different in the buck and boost modes of op-
eration and care must be taken to ensure stability in both
operating regions. When stepping down from a higher
input voltage to a lower output voltage, the converter
will operate in buck mode and the small-signal transfer
function from the error amplifier output, VC, to the con-
verter output voltage is given by the following equation:
VOUT
VC
s
( )
Buck Mode
= 2.64 VIN
1
+ sRCCOUT
1
+
s
ωOQ
+
s
ωO
2
This transfer function has a single zero created by the
output capacitor ESR and a resonant pair of poles. In most
applications, an output capacitor with a very low ESR is
utilized in order to reduce the output voltage ripple to ac-
ceptable levels. Such low values of capacitor ESR result
in a very high frequency zero and as a result the zero is
commonly too high in frequency to significantly impact
compensation of the feedback loop.
The denominator of the buck mode transfer function
exhibits a pair of resonant poles generated by the LCOUT
filtering of the power stage. The resonant frequency of
the power stage, fO, is given by the following expression
where L is the value of the inductor in henries.
ωO =
1
LCOUT
, fO =
1
2
π LCOUT
Thequalityfactor,Q,hasasignificantimpactoncompensa-
tion of the voltage loop since a higher Q factor produces
a sharper loss of phase near the resonant frequency. The
qualityfactorisinverselyrelatedtotheamountofdamping
in the power stage and is substantially influenced by the
average series resistance of the power stage, RS. Lower
values of RS will increase the Q and result in a sharper
loss of phase near the resonant frequency and will require
more phase boost or lower bandwidth to maintain an
adequate phase margin.
Q
=
LCOUT
COUT RC +RS
(
)+ L
RLOAD
Boost Mode
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, VC, to the output voltage is given by the
following expression:
VOUT
VC
s
( )
Boost Mode
= 2.64G
1
+ sRCCOUT
(
) 1– s
ωZ
1
+
s
ωOQ
+
s
ωO
2
Inboostmodeoperation,thetransferfunctionischaracter-
ized 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
APPLICATIONS INFORMATION
PWM
VC
VIN
VOUT
COUT
RC
RLOAD
3536 F04
A
D
B
C
L
RS
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