LT3579IUFD-1-PBF View Datasheet(PDF) - Linear Technology
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Description
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LT3579IUFD-1-PBF Datasheet PDF : 40 Pages
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LT3579/LT3579-1
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power stage of the IC, inductor and diode have been replaced
by a combination of the equivalent transconductance
amplifier gmp and the current controlled current source
(which converts IVIN to
ηVIN
VOUT
IVIN ).
Gmp
acts
as
a
current
–
gmp
+
IVIN
H • VIN
VOUT
•
IVIN
1.215V
REFERENCE
VOUT
RESR RL
COUT
CPL R1
gma
CF RC RO
CC
R2
FB
R2
3579 F18
CC: COMPENSATION CAPACITOR
COUT: OUTPUT CAPACITOR
CPL: PHASE LEAD CAPACITOR
CF: HIGH FREQUENCY FILTER CAPACITOR
gma: TRANSCONDUCTOR AMPLIFIER INSIDE IC
gmp: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
RC: COMPENSATION RESISTOR
RL: OUTPUT RESISTANCE DEFINED AS VOUT/ILOADMAX
RO: OUTPUT RESISTANCE OF gma
R1, R2; FEEDBACK RESISTOR DIVIDER NETWORK
RESR: OUTPUT CAPACITOR ESR
H: CONVERTER EFFICIENCY (~90% AT HIGHER CURRENTS)
Figure 18. Boost Converter Equivalent Model
source where the peak input current, IVIN, is proportional
to the VC voltage.
Note that the maximum output currents of gmp and gma
are finite. The output of the gmp stage is limited by the
minimum switch current limit (see Electrical Specifications)
and gma is nominally limited to about ±12μA.
From Figure 18, the DC gain, poles and zeros can be
calculated as follows:
DC Gain:
ADC
=
gma
•RO
•gmp
•η•
VIN
VOUT
• RL
2
• 0.5R2
R1+ 0.5R2
Output Pole: P1=
2
2•π•RL •COUT
( ) Error Amp Pole: P2 =
1
2•π• RO +RC •CC
Error Amp Zero: Z1=
1
2•π•RC •CC
ESR Zero: Z2 =
1
2•π•RESR •COUT
RHP
Zero:
Z3
=
2•
VIN2 •RL
π • VOUT2
•L
High Frequency Pole: P3> fS
3
Phase Lead Zero: Z4 =
1
2• π •R1•CPL
Phase Lead Pole: P4 =
1
2•
π•
R1•0.5R2
R1+ 0.5R2
•CPL
Error Amp Filter Pole:
P5
=
2•π•
1
RC •RO
RC +RO
CF
,
CF
<
CC
10
35791f
29
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