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LT3471EDD#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT3471
10
3471fb
APPLICATIONS INFORMATION
DIODE SELECTION
A Schottky diode is recommended for use with the
LT3471. For high effi ciency, 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 effi cient
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stage of the IC, inductor and diode have been replaced by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 amplifi er g
mp
. g
mp
acts as
a current source where the output current is proportional
to the V
C
voltage. Note that the maximum output current
of g
mp
is fi nite due to the current limit in the IC.
–
+
–
+
g
ma
R
C
R
O
R2
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
C
PL
: PHASE LEAD CAPACITOR
g
ma
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
g
mp
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
C
: COMPENSATION RESISTOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
DIVIDED BY I
LOAD(MAX)
R
O
: OUTPUT RESISTANCE OF g
ma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
R
ESR
: OUTPUT CAPACITOR ESR
3471 F06
R1
C
OUT
C
PL
R
L
R
ESR
V
OUT
V
C
C
C
g
mp
1.00V
REFERENCE
Figure 6. Boost Converter Equivalent Model
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)
10
GND
GND
SHDN/SS1
9
8 7
SHDN/SS2
FB1N
R4
R2
R3
FB1P
V
OUT1
R1
C2
3471 F05
C3
C
SS1
C
SS2
R
SS1
R
SS2
C1
•
•
C4
D2
V
OUT1
V
OUT2
SW1 SW2
C5
D1
L1
L2
L3
V
OUT2
FB2P FB2N
V
REF
6
1 2 3 4 5
LT3471
PIN 11 GND
V
CC
GND
GND
GND
CONTROL 1
CONTROL 2
LT3471
11
3471fb
APPLICATIONS INFORMATION
Figure 7. Bode Plot of 3.3V to 7V Application
From Figure 6, the DC gain, poles and zeroes can be
calculated as follows:
Output Pole: P1=
2
2•π •R
L
•C
OUT
Error Amp Pole: P2=
1
2•π •R
O
•C
C
Error Amp Zero: Z1=
1
2•π •R
C
•C
C
DC GAIN: A=
V
REF
V
OUT
•g
ma
•R
O
•g
mp
•R
L
•
1
2
ESR Zero: Z2=
1
2•π •R
ESR
•C
OUT
RHP Zero: Z3=
V
IN
2
•R
L
2•π •V
OUT
2
•L
High Frequency Pole: P3>
f
S
3
Phase Lead Zero: Z4 =
1
2•π •R1•C
PL
Phase Lead Pole: P4 =
1
2•π •C
PL
•
R1• R2
R1+R2
The Current Mode zero is a right half plane zero which can
be an issue in feedback control design, but is manageable
with proper external component selection.
Using the circuit of Figure 2 as an example, Table 3 shows
the parameters used to generate the Bode plot shown in
Figure 7.
Table 3. Bode Plot Parameters
Parameter Value Units Comment
R
L
20 Ω Application Specifi c
C
OUT
4.7 μF Application Specifi c
R
ESR
10 mΩ Application Specifi c
R
O
0.9 MΩ Not Adjustable
C
C
90 pF Not Adjustable
C
PL
33 pF Adjustable
R
C
55 kΩ Not Adjustable
R1 90.9 kΩ Adjustable
R2 15 kΩ Adjustable
V
OUT
7 V Application Specifi c
V
IN
3.3 V Application Specifi c
g
ma
50 μmho Not Adjustable
g
mp
9.3 mho Not Adjustable
L 2.2 μH Application Specifi c
f
S
1.2 MHz Not Adjustable
From Figure 7, the phase is –115° when the gain reaches
0dB giving a phase margin of 65°. This is more than
adequate. The crossover frequency is 50kHz.
FREQUENCY (Hz)
0
GAIN (dB)
PHASE (DEG)
60
70
–10
–20
50
20
40
30
10
100 10k 100k 1M
3471 F07
–30
–350
–50
0
–100
–250
–150
–200
–300
–400
1k
GAIN
PHASE
LT3471
12
3471fb
TYPICAL APPLICATIONS
R
SS1
4.7k
R
SS2
4.7k
CONTROL 1
3471 TA02
C
SS1
0.33μF
C
SS2
0.33μF
C1
10μF
V
IN
2.6V TO 4.2V
Li-Ion
SHDN/SS1 FB1N
9
0V
1.8V
CONTROL 2
0V
1.8V
8
7
SW1
SW2
LT3471
L2
15μH
L3
15μH
GND
FB1P
V
IN
FB2P
FB2N
V
REF
C2
0.1μF
C3
4.7μF
C6
33pF
V
OUT1
7V
500mA WHEN V
IN
= 4.2V
350mA WHEN V
IN
= 3.3V
250mA WHEN V
IN
= 2.6V
V
OUT2
–7V TO –4V
–7V WHEN V
CONTROL
= 0V
–4V WHEN V
CONTROL
= 1
–7V, 300mA WHEN V
IN
= 4.2V
–7V, 250mA WHEN V
IN
= 3.3V
–7V, 200mA WHEN V
IN
= 2.6V
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 10V
C5: XR5 OR X7R 16V
C6: OPTIONAL
D1, D2: ON SEMICONDUCTOR MBRM-120
L1: SUMIDA CR43-2R2
L2: SUMIDA CDRH4D18-100
L3: SUMIDA CDRH4D18-150
C6
75pF
R2
15k
R5
20k
R6
10k
R3
90.9k
R4
15k
R1
105k
C5
1μF
SHDN/SS2
V
IN
10
1
2
3
5
4
11 6
V
IN
C4
10μF
D2
L1
2.2μH
D1
V
CONTROL
0V TO 1V
I
OUT
(mA)
0
50
EFFICIENCY (%)
55
65
70
75
200
400
500
95
3471 TA02b
60
100 300
80
85
90
V
OUT
= 7V
V
IN
= 4.2V
V
IN
= 4.2V
V
IN
= 3.3V
V
IN
= 3.3V
V
IN
= 2.6V
V
IN
= 2.6V
V
OUT
= –7V
Li-Ion OLED Driver Effi ciency
Li-Ion OLED Driver
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT3471EDD#TRPBF

Mfr. #:
Buy LT3471EDD#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 2x 1.3A, 1.2MHz Boost/Inverter in 3 3
Lifecycle:
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