Datasheet

LT3471
11
3471fb
APPLICATIONS INFORMATION
Figure 7. Bode Plot of 3.3V to 7V Application
From Figure 6, the DC gain, poles and zeroes can be
calculated as follows:
Output Pole: P1=
2
2•π •R
L
•C
OUT
Error Amp Pole: P2=
1
2•π •R
O
•C
C
Error Amp Zero: Z1=
1
2•π •R
C
•C
C
DC GAIN: A=
V
REF
V
OUT
•g
ma
•R
O
•g
mp
•R
L
•
1
2
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 •C
PL
•
R1• R2
R1+R2
The Current Mode zero is a right half plane zero which can
be an issue in feedback control design, but is manageable
with proper external component selection.
Using the circuit of Figure 2 as an example, Table 3 shows
the parameters used to generate the Bode plot shown in
Figure 7.
Table 3. Bode Plot Parameters
Parameter Value Units Comment
R
L
20 Ω Application Specifi c
C
OUT
4.7 μF Application Specifi c
R
ESR
10 mΩ Application Specifi c
R
O
0.9 MΩ Not Adjustable
C
C
90 pF Not Adjustable
C
PL
33 pF Adjustable
R
C
55 kΩ Not Adjustable
R1 90.9 kΩ Adjustable
R2 15 kΩ Adjustable
V
OUT
7 V Application Specifi c
V
IN
3.3 V Application Specifi c
g
ma
50 μmho Not Adjustable
g
mp
9.3 mho Not Adjustable
L 2.2 μH Application Specifi c
f
S
1.2 MHz Not Adjustable
From Figure 7, the phase is –115° when the gain reaches
0dB giving a phase margin of 65°. This is more than
adequate. The crossover frequency is 50kHz.
FREQUENCY (Hz)
0
GAIN (dB)
PHASE (DEG)
60
70
–10
–20
50
20
40
30
10
100 10k 100k 1M
3471 F07
–30
–350
–50
0
–100
–250
–150
–200
–300
–400
1k
GAIN
PHASE