Datasheet
LT3580
11
3580fg
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power stage of the IC, inductor and diode have been replaced
by a combination of the equivalent transconductance
amplifier g
mp
and the current controlled current source
(which converts I
VIN
to ηV
IN
/V
OUT
• I
VIN
). g
mp
acts as
a current source where the peak input current, I
VIN
, is
proportional to the VC voltage. η is the efficiency of the
switching regulator, and is typically about 88%.
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.
From Figure 4, the DC gain, poles and zeros can be cal-
culated as follows:
Output Pole: P1=
2
2•π •R
L
•C
OUT
Error Amp Pole: P2 =
1
2•π •R
O
+R
C
⎡
⎣
⎤
⎦
•C
C
Error Amp Zero: Z1=
1
2•π •R
C
•C
C
DC Gain:
(Breaking Loop at FB Pin)
A
DC
= A
OL
(0) =
∂V
C
∂V
FB
•
∂I
VIN
∂V
C
•
∂V
OUT
∂I
VIN
•
∂V
FB
∂V
OUT
=
g
ma
•R
0
(
)
•g
mp
• η•
V
IN
V
OUT
•
R
L
2
⎛
⎝
⎜
⎞
⎠
⎟
•
0.5R2
R1+ 0.5R2
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 •
R1•
R2
2
R1+
R2
2
•C
PL
Error Amp Filter Pole:
P5 =
1
2•π•
R
C
•R
O
R
C
+R
O
•C
F
,C
F
<
C
C
10
The current mode zero (Z3) is a right-half plane zero
which can be an issue in feedback control design, but is
manageable with proper external component selection.
Figure 4. Boost Converter Equivalent Model
–
+
–
+
g
ma
R
C
R
O
R2
R2
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
C
PL
: PHASE LEAD CAPACITOR
C
F
: HIGH FREQUENCY FILTER 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
3580 F04
R1
FB
C
OUT
C
PL
R
L
R
ESR
V
OUT
I
VIN
V
C
C
C
C
F
g
mp
1.215V
REFERENCE
H•V
IN
V
OUT
•I
VIN
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