LT3580
APPLICATIONS INFORMATION
Compensation—Theory
Like all other current mode switching regulators, the
LT3580 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT3580—
a fast current loop which does not require compensation,
and a slower voltage loop which does. Standard bode plot
analysis can be used to understand 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 4 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 g
mp
.
g
mp
acts as a current source where the output current is
proportional to the VC voltage.
–
From Figure 4, the DC gain, poles and zeros can be cal-
culated as follows:
Output Pole: P1=
2
2 • • R
L
• C
OUT
1
2 • • R
O
• C
C
1
2 • • R
C
• C
C
1
4
Error Amp Pole: P2
=
Error Amp Zero: Z1=
DC Gain:
A
=
V
REF
V
OUT 2
• V
IN
• g
ma
• R
O
• g
mp
• R
L
•
1
2 • • R
ESR
• C
OUT
V
IN2
• R
L
2 • • V
OUT 2
• L
f
S
3
ESR Zero: Z2
=
RHP Zero: Z3
=
g
mp
V
OUT
C
PL
R
ESR
C
OUT
R1
R
L
V
C
R
C
C
C
R
O
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
Figure 4. Boost Converter Equivalent Model
Note that the maximum output currents of g
mp
and g
ma
are
finite. The limits for g
mp
are in the Electrical Characteristics
section (switch current limit), and g
ma
is nominally limited
to about ±12μA.
12
+
+
g
ma
1.215V
REFERENCE
High Frequency Pole: P3
>
Phase Lead Zero: Z4
=
Phase Lead Pole: P4
=
–
R2
3580 F04
1
2 • • R1• C
PL
1
2 • • C
PL
•
R1• R2
R1+R2
The current mode zero (Z3) is a right-half plane zero which
can be an issue in feedback control design, but is manage-
able with proper external component selection.
3580fc
LINER [ LINEAR TECHNOLOGY ]
