LT3467/LT3467A
APPLICATIONS INFORMATION
Compensation—Theory
From Figure 6, the DC gain, poles and zeroes can be
calculated as follows:
Like all other current mode switching regulators, the
LT3467/LT3467A needs to be compensated for stable
and efficient 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.
2
Output Pole: P1=
2• π •RL •COUT
1
Error Amp Pole: P2=
2• π •RO •CC
1
Error Amp Zero: Z1=
2• π •RC •CC
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 amplifier gmp.
gmp acts as a current source where the output current is
proportional to the VC voltage. Note that the maximum
output current of gmp is finite due to the current limit
in the IC.
1.255
1
2
DC GAIN: A=
• V •gma •R •gmp •RL •
IN
O
2
VOUT
1
ESR Zero: Z2=
2• π •RESR •COUT
V
2 •RL
IN
RHP Zero: Z3=
2• π • VOUT2 •L
fS
3
High Frequency Pole: P3>
–
1
g
Phase Lead Zero: Z4 =
mp
V
OUT
2• π •R1•CPL
+
C
R
R
L
PL
ESR
1
C
OUT
1.255V
REFERENCE
Phase LeadPole:P4 =
+
–
R1•R2
R1+R2
V
C
g
2• π •CPL •
ma
R1
R2
R
C
R
O
C
C
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.
3467 F06
C : COMPENSATION CAPACITOR
C
C
C
: OUTPUT CAPACITOR
: PHASE LEAD CAPACITOR
OUT
PL
ma
mp
g
g
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R : COMPENSATION RESISTOR
C
L
O
R : OUTPUT RESISTANCE DEFINED AS V
DIVIDED BY I
LOAD(MAX)
OUT
R : OUTPUT RESISTANCE OF g
ma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
: OUTPUT CAPACITOR ESR
R
ESR
Figure 6. Boost Converter Equivalent Model
3467afe
10
Linear Systems [ Linear Systems ]
