LT3581 LINER [Linear Technology], LT3581 Datasheet - Page 27

• Manufacturer Part Number: LT3581
• Description: 3.3A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

appenDix
From Figure 16, the DC gain, poles and zeros can be
calculated 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.
DC Gain
(Breaking loop at FB pin)
Error Amp Pole P
Error Amp Zero Z
ESR Zero Z
RHP Zero Z
High Frequency Pole P
Phase
Phase Lead Pole P
Error Amp Filter Pole
P
(
O O utput Pole P
A
g
5
DC
ma
=
=
•
2 2
L L ead Zero Z
R R
A
• •
:
O
OL
π
:
)
:
•
( )
0
:
2
R
3
R
g
mp
C
=
C
=
=
1
1
:
+
•
:
=
2
:
2
:
∂
•
∂ ∂
R
R
V
• •
2
• •
V
2
1

 
O
4
O
FB
4
C
π
π
• •
=
η
=
V
:
=
=
:
•
IN
π
•
•
2
2
C
R
2
3
2
V
2
∂
V
• •
• •
F
∂
ESR
OUT
V
•
• • •
R
>
I
OUT
• •
π
1
π
V
VIN
IN
,
2
R R
π
L
π
C
C
f
3
S
L
•
2
F
•
R
1
 
•
•
C
•
R
C
R C
<
R
R
C
1
R
OUT
L
∂
OUT
1
1
O
1
∂
2
•
C
10
V
L
•
+
1 1
•
1
I
C
+
C
OUT
VIN

 
R
R
C C
R
2
PL
2
•
2
2
C
R
•
•
  •
1
C
∂
0 5
+
∂
.
PL
V
C
V
0 5
OUT
C
R
FB
.
2
R
=
2
Using the circuit in Figure 18 as an example, Table 8 shows
the parameters used to generate the Bode plot shown in
Figure 17.
Table 8. Bode Plot Parameters
PARAMETER
R
C
R
R
C
C
C
R
R1
R2
V
V
V
g
g
L
f
From Figure 17, the phase is –130° when the gain reaches
0dB giving a phase margin of 50°. The crossover frequency
is 17kHz, which is more than three times lower than the
frequency of the RHP zero Z3 to achieve adequate phase
margin.
OSC
ma
mp
L
OUT
ESR
O
C
F
PL
C
REF
OUT
IN
Figure 17. Bode Plot for Example Boost Converter
170
150
130
110
–10
–30
90
50
10
70
30
10
VALUE
1.215
1000
14.5
10.5
14.6
15.1
305
130
270
9.4
1.5
56
12
1
0
5
2
100
GAIN
FREQUENCY (Hz)
UNITS
µmho
mho
MHz
1k
mΩ
kΩ
kΩ
kΩ
kΩ
µH
µF
pF
pF
pF
Ω
V
V
V
PHASE
10k
100k
Application Specific
Application Specific
Application Specific
Application Specific
Application Specific
Application Specific
Optional/Adjustable
Optional/Adjustable
Not Adjustable
Not Adjustable
Not Adjustable
Not Adjustable
Not Adjustable
COMMENT
3851 F17
Adjustable
Adjustable
Adjustable
Adjustable
1M
LT3581
–40
–80
–120
–160
–180
–200
–240
–280
–320
–360
0

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