LTC3401 Просмотр технического описания (PDF) - Linear Technology
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LTC3401 Datasheet PDF : 16 Pages
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LTC3401
APPLICATIO S I FOR ATIO
A logic level feed forward signal, VFF, is coupled through
components C5 and R6. The amount of feed forward
signal is attenuated with resistor R6 and is given by the
following relationship:
R6
≈
⎛
⎝⎜
VFVFO•URT5••ΔVIINOU•T1.5⎞⎠⎟
–
R5
where ΔIOUT = load current change.
VIN
LTC3401
3
4
VIN
SW
10
SHDN
7
VOUT
2
8
MODE/SYNC FB
6
PGOOD
9
VC
1
5
C3
Rt
GND
R5
LOAD FEED
FORWARD
SIGNAL
C5
R6 3.3nF
VFF
Figure 3
VOUT
3404 F03
Closing the Feedback Loop
The LTC3401 uses current mode control with internal
adaptive slope compensation. Current mode control elimi-
nates the 2nd order filter due to the inductor and output
capacitor exhibited in voltage mode controllers, and sim-
plifies it to a single-pole filter response. The product of the
modulator control to output DC gain plus the error amp
open-loop gain equals the DC gain of the system.
GDC = GCONTROLOUTPUT • GEA
GCONTROL
=
2 • VIN
IOUT
,
GEA
≈
2000
The output filter pole is given by:
fFILTERPOLE
=
π
•
IOUT
VOUT • COUT
Hz
where COUT is the output filter capacitor.
The output filter zero is given by:
fFILTERZERO
=
2
•
π
•
1
RESR
•
COUT
Hz
where RESR is the capacitor equivalent series resistance.
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP) and is given by:
fRHPZ
=
VIN2 •RO
2 • π • L • VO2
Hz
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
rolled off before the RHP zero frequency.
The typical error amp compensation is shown in Figure 4.
The equations for the loop dynamics are as follows:
fPOLE1
≈
2
•
π
•
20
1
• 106
•
Hz
CC1
which is extremelyclose to DC
fZERO1
=
2
•
π
•
1
RZ
•
CC1
Hz
fPOLE2
≈
2
•
π
•
1
RZ
•
CC2
Hz
ERROR
AMP
1.25V
FB
8
VC
9
CC1
RZ
VOUT
R1
R2
CC2
3401 F04
Figure 4
Refer to Application Note AN-76 for more closed loop
examples.
3401fb
11
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