LTC3401 Linear Technology, LTC3401 Datasheet - Page 11

• Manufacturer Part Number: LTC3401
• Description: 1A/ 3MHz Micropower Synchronous Boost Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
A logic level feed forward signal, V
components C5 and R6. The amount of feed forward
signal is attenuated with resistor R6 and is given by the
following relationship:
where I
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.
The output filter pole is given by:
G
R
G
f
FILTERPOLE
DC
6
CONTROL
V
IN
= G
LOAD FEED
FORWARD
OUT
V
10
SIGNAL
CONTROLOUTPUT
3
2
6
1
FF
V
V
SHDN
MODE/SYNC
PGOOD
R
= load current change.
OUT
IN
t
•
R
LTC3401
2 •
5
I
•
OUT
•
U
V
V
•
IN
V
IN
I
GND
V
OUT
OUT
SW
V
FB
V
OUT
FF
C
• .
, G
I
OUT
1 5
4
7
8
9
5
Figure 3
U
EA
• G
•
R6
C
–
EA
OUT
3.3nF
2000
C5
R
R5
C3
5
Hz
FF
W
, is coupled through
3404 F03
U
V
OUT
Refer to Application Note AN-76 for more closed loop
examples.
where C
The output filter zero is given by:
where R
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP) and is given by:
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:
f
f
f
which is extremelyclose to DC
f
f
FILTERZERO
RHPZ
POLE
POLE
ZERO
OUT
ESR
1
2
1
2
2
is the output filter capacitor.
is the capacitor equivalent series resistance.
2
2
• • •
V
• •
• •
• •
ERROR
IN
AMP
2
2 • •
L V
•
20 10
R
R
1
+
–
1
R
Z
Z
O
1
•
O
•
•
C
C
2
R
1.25V
Figure 4
C
C
FB
V
ESR
Hz
6
8
9
1
2
C
1
•
Hz
Hz
C
•
C
C
1
R
Hz
Z
OUT
C
C1
Hz
C
LTC3401
C2
V
OUT
R1
R2
3401 F04
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