LT3580 LINER [Linear Technology], LT3580 Datasheet - Page 12

• Manufacturer Part Number: LT3580
• Description: Boost/Inverting DC/DC Converter with 2A Switch, Soft-Start, and Synchronization
• Manufacturer: LINER [Linear Technology]

LT3580
APPLICATIONS INFORMATION
Compensation—Theory
Like all other current mode switching regulators, the
LT3580 needs to be compensated for stable and effi cient
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 amplifi er g
g
proportional to the VC voltage.
Note that the maximum output currents of g
fi nite. The limits for g
section (switch current limit), and g
to about ±12μA.
12
mp
R
acts as a current source where the output current is
C
C
C
C
C
g
g
R
R
R
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
R
C
V
ma
mp
C
OUT
PL
C
L
O
ESR
C
: COMPENSATION CAPACITOR
: OUTPUT RESISTANCE DEFINED AS V
: COMPENSATION RESISTOR
: OUTPUT RESISTANCE OF g
: PHASE LEAD CAPACITOR
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
: OUTPUT CAPACITOR
: OUTPUT CAPACITOR ESR
Figure 4. Boost Converter Equivalent Model
R
O
–
+
g
mp
g
ma
mp
+
–
are in the Electrical Characteristics
ma
REFERENCE
1.215V
OUT
C
PL
ma
DIVIDED BY I
is nominally limited
3580 F04
R1
R2
mp
LOAD(MAX)
R
ESR
C
and g
OUT
R
ma
L
V
OUT
mp
are
.
From Figure 4, the DC gain, poles and zeros can be cal-
culated as follows:
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.
Output Pole: P1=
Error Amp Pole: P2 =
Error Amp Zero: Z1=
DC Gain:
ESR Zero: Z2 =
RHP Zero: Z3 =
High Frequency Pole: P3 >
Phase Lead Zero: Z4 =
Phase Lead Pole: P4 =
A =
V
V
OUT
REF
2
• V
IN
• g
2 • • R
2 • • V
ma
2 • • R
V
• R
IN
2 • • R
2 • • R
2
O
2 • • C
2 • • R1• C
ESR
OUT
• R
• g
1
2
L
f
L
mp
3
• C
2
S
• C
1
1
• L
C
1
O
OUT
PL
• R
OUT
• C
• C
1
L
•
C
C
R1+R2
PL
R1• R2
•
4
1
3580fc