LTC3536_15 Datasheet, PDF (16/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Applications Information
increasing gain and decreasing phase at higher frequen-
cies. As a result, the crossover frequency in boost mode
operation generally must be set lower than in buck mode
in order to maintain sufficient phase margin.
G=
V_IN
• ^RLOAD
R_S
•
1–
1+
R_S
R_LOAD
R_LOAD
R_S
•


•


V_OUT
V_IN
V_IN
V_OUT
²

²

ω_O =
R_S
+
R_LOAD


V_IN
V_OUT


2
( ) ^LC_OUT ^R_LOAD ^{+ R}_C
In boost mode operation, the frequency of the right-half
plane zero, f_Z, is given by the following expression. The
frequency of the right half plane zero decreases at higher
loads and with larger inductors.
ω_Z
=


V_IN
V_OUT


2
R_LOAD
L
– R_S
,
f_Z
=


V_IN
V_OUT


2
R_LOAD
2πL
– R_S
Finally, the magnitude of the quality factor of the power
stage in boost mode operation is given by the following
expression:
( ) _{( ) Q}
=
L
+
LC_OUT R_LOAD
C_OUTR_LOADR_C


+ R_C
V_IN
V_OUT


R_S + R_LOAD
2
+ R_SC_OUT
 _VIN
 ^V_OUT
R_LOAD
²

+ R_C
Buck-Boost Mode
When the converter operates in buck-boost mode and the
small-signal transfer function from control voltage, V_C, to
the output voltage is given by the following expression:
V_OUT
V_C
(s)
Buck-Boost
Mode
=
( ) _{17.62 •G}
1+ sR_CC_OUT


1–
s
ω_Z


1+
s
ω_OQ
+


s
ω_O


2
Also in buck-boost mode operation, the transfer function
is characterized by a pair of resonant poles and a zero
generated by the ESR of the output capacitor as in buck
mode and a right half plane zero.
( _{( )} ) _G
=
0.15
•
V_OUT R_LOAD •
ε •(1.85 – ε)
ε² • 1.85 – R_S
• R_S + R_LOAD
• (1.85
• ε²
–
ε
)
where the variable ε is defined:
ε = ^{VIN • 1.85}
V_OUT + V_IN
( ) ^{ωO =}
R_S + R_LOAD • ε²
LC_OUT R_LOAD + R_C
In buck-boost mode operation, the frequency of the right-
half plane zero, f_Z, is given by the following expression.
The frequency of the right-half plane zero decreases at
higher loads and with larger inductors.
ω
Z
=
1.85
•
ε²R_LOAD – R_S
L •(1.85 –
• (1.85
ε)
–
ε
)
3536fa
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