LT3467/LT3467A
10
3467afe
APPLICATIONS INFORMATION
Compensation—Theory
Like all other current mode switching regulators, the
LT3467/LT3467A needs to be compensated for stable
and effi cient operation. Two feedback loops are used in
the LT3467/LT3467A: a fast current loop which does not
require compensation, and a slower voltage loop which
does. Standard Bode plot analysis can be used to under-
stand and adjust the voltage feedback loop.
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure 6 shows the key equivalent elements of a boost
converter. Because of the fast current control loop, the
power stage of the IC, inductor and diode have been re-
placed by the equivalent transconductance amplifi er g
mp
.
g
mp
acts as a current source where the output current is
proportional to the V
C
voltage. Note that the maximum
output current of gmp is fi nite due to the current limit
in the IC.
From Figure 6, the DC gain, poles and zeroes can be
calculated as follows:
Output Pole: P1=
2
2•π •R
L
•C
OUT
Error Amp Pole: P2=
1
2•π •R
O
•C
C
Error Amp Zero: Z1=
1
2•π •R
C
•C
C
DC GAIN: A=
1.255
V
OUT
2
•V
IN
•g
ma
•R
O
•g
mp
•R
L
•
1
2
ESR Zero: Z2 =
1
2•π •R
ESR
•C
OUT
RHP Zero: Z3=
V
IN
2
•R
L
2•π •V
OUT
2
•L
High Frequency Pole: P3>
f
S
3
Phase Lead Zero: Z4 =
1
2•π •R1•C
PL
Phase Lead Pole: P4 =
1
2•π •C
PL
•
R1• R2
R1+R2
The current mode zero is a right-half plane zero which can
be an issue in feedback control design, but is manageable
with proper external component selection.
–
+
–
+
g
ma
R
C
R
O
R2
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
C
PL
: PHASE LEAD CAPACITOR
g
ma
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
g
mp
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
C
: COMPENSATION RESISTOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
DIVIDED BY I
LOAD(MAX)
R
O
: OUTPUT RESISTANCE OF g
ma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
R
ESR
: OUTPUT CAPACITOR ESR
3467 F06
R1
C
OUT
C
PL
R
L
R
ESR
V
OUT
V
C
C
C
g
mp
1.255V
REFERENCE
Figure 6. Boost Converter Equivalent Model
