LT3471 Просмотр технического описания (PDF) - Linear Technology
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LT3471 Datasheet PDF : 16 Pages
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LT3471
APPLICATIONS INFORMATION
DIODE SELECTION
A Schottky diode is recommended for use with the
LT3471. For high efficiency, a diode with good thermal
characteristics at high currents should be used such as
the On 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
forward 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 care-
ful attention to board layout. You will not get advertised
performance with careless layout. Figure 5 shows the
recommended component placement.
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 con-
verter. Because of the fast current control loop, the power
stage of the IC, inductor and diode have been replaced by
the equivalent transconductance amplifier gmp. gmp acts as
a current source where the output current is proportional
to the VC voltage. Note that the maximum output current
of gmp is finite due to the current limit in the IC.
CONTROL 1
GND
RSS1
CSS1
GND
CSS2
RSS2
CONTROL 2
GND
C1
VOUT1 D1
C3
GND
L1
VCC
L2
•
C5
SW1
SW2
10
9
8
7
6
SHDN/SS1
SHDN/SS2
C4
VOUT2
L3
•
D2
GND
LT3471
PIN 11 GND
–
gmp
+
VOUT
CPL
RESR RL
VC
RC
RO
1.00V
REFERENCE
gma
COUT
R1
CC
R2
3471 F06
CC: COMPENSATION CAPACITOR
COUT: OUTPUT CAPACITOR
CPL: PHASE LEAD CAPACITOR
gma: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
gmp: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
RC: COMPENSATION RESISTOR
RL: OUTPUT RESISTANCE DEFINED AS VOUT DIVIDED BY ILOAD(MAX)
RO: OUTPUT RESISTANCE OF gma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
RESR: OUTPUT CAPACITOR ESR
FB1N FB1P VREF FB2P FB2N
1
2
3
4
5
R4 R3
VOUT1
R2 R1
VOUT2
C2
Figure 6. Boost Converter Equivalent Model
3471 F05
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)
3471fb
10
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