LT3471 Просмотр технического описания (PDF) - Linear Technology
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LT3471 Datasheet PDF : 16 Pages
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LT3471
APPLICATIONS INFORMATION
From Figure 6, the DC gain, poles and zeroes can be
calculated as follows:
Output Pole: P1=
2
2 • π • RL • COUT
Error Amp Pole: P2=
1
2 • π • RO • CC
Error Amp Zero: Z1=
1
2 • π • RC • CC
DC
GAIN:
A=
VREF
VOUT
• gma
• RO
• gmp
• RL
•
1
2
ESR
Zero:
Z2
=
2
•
π
•
1
RESR
•
COUT
RHP
Zero:
Z3=
2
•
VIN2 • RL
π • VOUT2
•L
High Frequency Pole: P3> fS
3
Phase
Lead
Zero
:
Z4
=
2
•
π
•
1
R1•
CPL
Phase
Lead
Pole
:
P4
=
2
•
π
•
1
CPL •
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
RL
20
Ω
COUT
4.7
μF
RESR
10
mΩ
RO
0.9
MΩ
CC
90
pF
CPL
33
pF
RC
55
kΩ
R1
90.9
kΩ
R2
15
kΩ
VOUT
7
V
VIN
3.3
V
gma
50
μmho
gmp
9.3
mho
L
2.2
μH
fS
1.2
MHz
Comment
Application Specific
Application Specific
Application Specific
Not Adjustable
Not Adjustable
Adjustable
Not Adjustable
Adjustable
Adjustable
Application Specific
Application Specific
Not Adjustable
Not Adjustable
Application Specific
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.
70
60
50
40
30
20
10
0
–10
–20
–30
100
0
–50
–100
–150
–200
–250
–300
GAIN
PHASE
1k
10k
100k
FREQUENCY (Hz)
–350
–400
1M
3471 F07
Figure 7. Bode Plot of 3.3V to 7V Application
3471fb
11
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