lt3579iufd-trpbf Linear Technology Corporation, lt3579iufd-trpbf Datasheet - Page 29

• Manufacturer Part Number: lt3579iufd-trpbf
• Description: Lt3579/lt3579-1 6a Boost/inverting Dc/dc Converter With Fault Protection Features
• Manufacturer: Linear Technology Corporation

APPENDIX
feedback loop.
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure 18 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power stage of the IC, inductor and diode have been replaced
by a combination of the equivalent transconductance
amplifi er g
(which converts I
source where the peak input current, I
to the V
Note that the maximum output currents of g
C
F
R
C
C
C
C
Figure 18. Boost Converter Equivalent Model
voltage.
mp
R
–
+
O
C
C
C
C
g
g
R
R
R
R1, R2; FEEDBACK RESISTOR DIVIDER NETWORK
R
ma
mp
C
OUT
PL
F
C
L
O
ESR
: CONVERTER EFFICIENCY (~90% AT HIGHER CURRENTS)
and the current controlled current source
g
: HIGH FREQUENCY FILTER CAPACITOR
: COMPENSATION CAPACITOR
: COMPENSATION RESISTOR
: OUTPUT RESISTANCE DEFINED AS V
: OUTPUT RESISTANCE OF gma
mp
: PHASE LEAD CAPACITOR
: TRANSCONDUCTOR AMPLIFIER INSIDE IC
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
: OUTPUT CAPACITOR
: OUTPUT CAPACITOR ESR
VIN
g
ma
to
+
–
V OUT
ηV IN
I
VIN
REFERENCE
1.215V
I VIN
V
•
OUT
R2
R2
V
). G
IN
• I
FB
mp
VIN
VIN
OUT
acts as a current
, is proportional
/I
C
LOADMAX
PL
mp
R1
and g
V
R
C
OUT
ESR
OUT
3579 F18
ma
R
L
are fi nite. The output of the g
minimum switch current limit (see Electrical Specifi cations)
and g
From Figure 18, the DC gain, poles and zeros can be
calculated as follows:
DC Gain
Output Pole P
Error Amp Po
Error Amp Zero Z
ESR Zero Z
RHP Zero Z
High Frequency Pole P
P
Phase Lead Pole P
Error Amp Filter P
P
A
h h ase Lead Zero Z
5
DC
ma
=
=
2
is nominally limited to about ±12μA.
g
• •
ma
:
π
:
:
•
R
R R
:
R
2
3
C
C
1
O
l l e P
=
=
1
+
•
•
:
:
=
R
2
2
:
g
:
O
• •
O
mp
• •
o o le
2
2
1
4 4
π
π
4
C
• •
V
=
=
LT3579/LT3579-1
• •
I I N
:
=
F
π
=
:
R
η
2
2
V
,
2
2
3
ESR
2
• •
OUT
C
• •
R C
•
1
• •
>
• • •
π R R C
F
π
2
R
V
L
π
π
V
<
OUT
L
f
•
3
mp
2
S
•
IN
(
1
C
10
•
C
R
R
R C
R
C
OUT
1
C
L
OUT
O
stage is limited by the
1
1
1
•
•
1
+
•
+
R
2
0 5
0 5
R
C
1
L
PL
.
.
C
•
R
)
R
R
•
2
2
1
C
0 5
+ +
•
C
.
C
0 5
PL
R
.
2
R
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