lt3471edd-trpbf Linear Technology Corporation, lt3471edd-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt3471edd-trpbf
• Description: Dual 1.3a, 1.2mhz Boost/inverter In 3mm ? 3mm Dfn
• Manufacturer: Linear Technology Corporation

APPLICATIONS
Semiconductor MBRM120. This is a 20V diode. Where the
switch voltage exceeds 20V, use the MBRM140, a 40V
diode. These diodes are rated to handle an average for-
ward current of 1.0A. In applications where the average
forward current of the diode is less than 0.5A, use the
Philips PMEG 2005, 3005, or 4005 (a 20V, 30V or 40V
diode, respectively).
LAYOUT HINTS
The high speed operation of the LT3471 demands careful
attention to board layout. You will not get advertised
performance with careless layout. Figure 5 shows the
recommended component placement.
Figure 5. Suggested Layout Showing a Boost on SW1 and an
Inverter on SW2. Note the Separate Ground Returns for All High
Current Paths (Using a Multilayer Board)
CONTROL 1
V
GND
OUT1
GND
C3
D1
R4
SW1
R
FB1N
L1
SS1
10
1
R3
V
U
SHDN/SS1
OUT1
FB1P
9
2
C
SS1
PIN 11 GND
INFORMATION
U
LT3471
GND
V
V
REF
8
3
CC
R2
SHDN/SS2
C1
C2
FB2P
7
4
R1
V
W
OUT2
FB2N
C
R
SW2
SS2
SS2
6
5
•
L2
C5
CONTROL 2
U
GND
GND
L3
•
V
OUT2
D2
C4
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Compensation—Theory
Like all other current mode switching regulators, the
LT3471 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT3471: a
fast current loop which does not require compensation,
and a slower voltage loop which does. Standard Bode plot
analysis can be used to understand and adjust the voltage
feedback loop.
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure 6 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power stage of the IC, inductor and diode have been
replaced by the equivalent transconductance amplifier
g
is proportional to the V
output current of g
IC.
From Figure 6, the DC gain, poles and zeroes can be
calculated as follows:
mp
Error Amp Pole: P2 =
Error Amp Zero: Z1=
DC GAIN: A =
ESR Zero:
RHP Zero: Z3 =
High Frequency Pole: P3 >
Phase Lead Zero Z
Phase Lead Pole P
Output Pole: P1=
. g
mp
acts as a current source where the output current
Z
2
=
V
V
mp
OUT
REF
:
:
2
2
is finite due to the current limit in the
• •
• •
2 • • R
4
4
C
π
π
V
•
=
=
IN
voltage. Note that the maximum
π
g
2 • • R
2 • • R
2
R
2
2
ma
V
OUT
ESR
• •
•
•
π
1
π
R
π
2
π π
•
L
R
L
2
f
•
•
3
•
1
S
1
O
C
R C
C
•
1
C
C
O
PL
1
OUT
L
•
OUT
•
1
•
g
•
C
•
mp
C
C
PL
R
C
R R
1
1
•
+
•
R
R
L
LT3471
2
2
•
2
1
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