Datasheet
LT3581
27
3581fa
For more information www.linear.com/LT3581
From Figure 16, the DC gain, poles and zeros can be
calculated as follows:
DC Gain
AA
DC OL
:
()
(Breaking loop at FB pin)
==0
∂∂
∂
•
∂
∂
•
∂
∂
•
∂
∂
=
•
V
V
I
V
V
I
V
V
g
C
FB
VIN
C
OUT
VIN
FB
OUT
ma
RRg
V
V
RR
RR
Omp
IN
OUT
L
()
••••
•
+
η
2
05
05
2
12
.
.
OOutput Pole P
RC
ErrorAmp Pole P
L OUT
:
:
1
2
2
2
=
•• •
=
π
11
2
1
1
2
•• +
•
=
•• •
π
π
RR C
ErrorAmp Zero Z
RC
OC C
C
:
CC
ESR OUT
IN
ESRZeroZ
RC
RHP Zero Z
V
:
:
2
1
2
3
2
=
•• •
=
•
π
RR
VL
High FrequencyPoleP
f
Phase
L
OUT
S
2
3
3
2
•• •
>
π
:
LLead Zero Z
RC
Phase Lead Pole P
PL
:
:
4
1
21
4
1
2
=
•• •
=
π
•••
•
+
•
=
π
R
R
R
R
C
ErrorAmp Filter Pole
P
PL
1
2
2
1
2
2
5
1
:
22
10
••
•
+
•
π
RR
RR
C
C
C
CO
CO
F
F
C
,
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.
appenDix
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 VALUE UNITS COMMENT
R
L
14.5 Ω Application Specific
C
OUT
9.4 µF Application Specific
R
ESR
1 mΩ Application Specific
R
O
305 kΩ Not Adjustable
C
C
1000 pF Adjustable
C
F
56 pF Optional/Adjustable
C
PL
0 pF Optional/Adjustable
R
C
10.5 kΩ Adjustable
R1 130 kΩ Adjustable
R2 14.6 kΩ Not Adjustable
V
REF
1.215 V Not Adjustable
V
OUT
12 V Application Specific
V
IN
5 V Application Specific
g
ma
270 µmho Not Adjustable
g
mp
15.1 mho Not Adjustable
L 1.5 µH Application Specific
f
OSC
2 MHz Adjustable
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.
Figure 17. Bode Plot for Example Boost Converter
FREQUENCY (Hz)
10
50
GAIN (dB)
PHASE (DEG)
70
90
110
130
100 1k 10k 100k 1M
3851 F17
30
10
–10
–30
150
170
–120
–80
–40
–240
–280
–160
–180
–360
–320
–200
0
PHASE
GAIN