DC549A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
LT3431
DESCRIPTION
Demonstration circuit 549 is a monolithic step-down DC/DC switching converter using the LT3431. The board is
optimized for 5V output at up to 2A
load current for a steady state input
voltage range of 7.5V to 20V. The
LT3431 is capable of withstanding input transients to 60V and runs at
500kHz with steady state input voltages that require duty cycles between
25% and 75%. For duty cycles below
25% the part may pulse skip to effectively average the lower duty cycle
and still provide minimum on-time
switch cycles. With its wide input
voltage range, 3A internal power
switch, high 500kHz switching frequency and thermally enhanced package, the LT3431 is a very versatile
and powerful IC for DC/DC converters
that require compact size, high efficiency and tolerance to high input
voltage transients.
The LT3431 500kHz switching frequency
allows all of the components to be
small, surface mount devices. Synchronization with an external clock
of up to 700kHz is possible. The current-mode control topology creates
fast transient response and good loop
stability with a minimum number of
external compensation components. In
particular, the current-mode architecture allows the use of ceramic input and output capacitors for increased reliability, extremely low
output ripple voltage and small component size. The low resistance internal power switch (0.1Ω) maintains
high efficiencies of up to 87%. The
SHDN pin can be used to program undervoltage lockout or place the part
in micropower shutdown, reducing supply current to 30µA by driving the
pin low.
The LT3431 datasheet gives a complete
description of the part, operation
and applications information. The datasheet must be read in conjunction
with this Quick Start Guide for demonstration circuit 549. In particular, the datasheet section on “Thermal Calculations” is important for
estimating whether a given application’s combination of input voltage,
load current and frequency will cause
the LT3431 to exceed it’s absolute
maximum rated junction temperature.
The part is assembled in a small 16pin thermally enhanced package with
exposed pad where proper board layout
is essential for maximum thermal performance. See the datasheet section
“Layout Considerations”.
NOTE: Do not hot-plug the input voltage terminal VIN. The absolute maximum voltage on VIN is 60V and hotplugging a power supply through wire
leads to the demonstration card can
cause the voltage on the extremely
low-ESR ceramic input capacitor to
ring to twice its DC value. This is
due to high currents instantaneously
generated in the inductive supply
leads from an input voltage step on
the low-ESR ceramic input capacitor.
An over-voltage can result if care is
not taken to ensure that this does
not happen. A bulky higher-ESR capacitor, and an additional inductive
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
filter between the bulk capacitor and
the input capacitor can be added to
the circuit to dampen hot-plug transient ringing. See Application Note
88 for more details.
Design files for this circuit board
are available. Call the LTC factory.
Table 1. Typical Performance Summary
PARAMETER
CONDITION
Steady State Input Voltage Range
VOUT = 5V, IOUT ≤ 2A, TA = 25°C 7.5–20V
Maximum Transient Input Voltage
VOUT
VALUE
60V
IOUT ≤ 2A
5V ±3%
Maximum Output Current
2A
Typical Switching Frequency
500kHz
Typical Efficiency
VIN = 12V, VOUT = 5V, IOUT = 2A 83%
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
QUICK START PROCEDURE
Demonstration circuit 549 is easy to
set up to evaluate the performance of
the LT3431. Refer to Figure 1 for
proper measurement equipment setup
and follow the procedure below:
NOTE: Make sure that the input voltage
does not exceed 60V.
NOTE: The synchronization and shutdown
can be left floating (disconnected)
if their functions are not being
used.
1. Connect the power supply, load, and
meters shown in Figure 1.
2. After
all connections are made,
turn on input power and verify that
the output voltage is 5.0V.
functions are optional and their pins
+
+
–
–
BENCH
SUPPLY
+
–
+
LOAD
–
–
+
+
–
–
+
OPTIONAL
Figure 1. Proper Measurement Equipment Setup
CUSTOMIZING THE DEMONSTRATION CIRCUIT
CUSTOMIZING THE OUTPUT VOLTAGE
The components assembled on the board
are optimized for a wide input voltage range and a 5V output. The feedback resistors (R2, R3) can be
changed to adjust the output voltage
according to the following equation:
VOUT = 1.22 (1 + R2/R3)
The Thevenin resistance seen at the
FB pin should be less than 3.8k to
maximize frequency foldback
start-up and short-circuit.
during
For output voltages below 3V, the
boost diode should be moved from D2
to D3 to provide the minimum boost
voltage required for the internal
power switch. Make sure that the
boost capacitor (C4) has a voltage
rating greater than the output voltage for applications where the boost
diode is placed in D2 and greater
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
than the input voltage when the boost
diode is placed in position D3.
For output voltages greater than 5V,
the optional ‘blocking’ zener diode
D4 can be used to reduce the boost
voltage across C4 from the output
voltage to some lower voltage between
3V and 5V. This diode transfers power
dissipation from inside the IC to the
discrete component outside the IC,
allowing higher ambient temperature
operation for the part. It is recommended that an SMAZ7V5 zener diode is
used in D4 when VOUT = 12V. To properly install D4, the small trace
shorting the anode to the cathode of
D4 on the board must be opened (an
Exacto knife works well) before D4 is
soldered to the board. In the ‘Thermal Calculations’ section of Applications Information in the datasheet,
the boost power consumption in the IC
must account for the zener blocking
diode VZ, so the boost voltage
changes from VOUT to (VOUT – VZ).
COMPENSATION
Demonstration Circuit 549 has a frequency compensation network that is
optimized for the ceramic output capacitor C5, the wide input voltage
range 7.5V to 20V, and 5V output. Improved loop bandwidth can be achieved
for various output voltages, output
capacitors, and input voltage ranges
by adjusting R1, C1, and C2. Additional optional component locations
for feedforward capacitor (C8) and
resistor (R4 for short circuit feedback pin protection when feedforward
capacitor is used) are located in
parallel with R2. For more information, see the ‘Frequency Compensation’ section in the Applications Information in the datasheet, Application Notes 19 or Application Note 76.
SOFT START
When the ratio of the input voltage
divided by the output voltage (plus
the forward voltage of the catch diode) is greater than four
VIN/(VOUT + VF) > 4
the soft start circuit made up of
components C9, R5, R6, and Q1 should
be used to control output voltage
rise
time
and
overshoot
during
startup. The soft start circuit is
covered in detail in ‘Buck Converter
with Adjustable Soft Start’ in the
Applications Information in the datasheet.
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
100%
90%
80%
EFFICIENCY (%)
70%
60%
50%
8.0 V in
40%
12.0 Vin
30%
20.0 Vin
20%
10%
0%
0
500
1000
1500
2000
LO AD CURRENT (mA)
Figure 2. Efficiency
2800
Tj = 125C
Maximum Load Current (mA)
2600
2A Load, Tj < 125C
2400
2200
2000
1800
1600
1400
1200
7
9
11 13 15 17 19 21 23 25
VI N ( V )
Figure 3. Maximum Load Current Capabilities
(TA = 25°C)
5
5
D3
MMSD914T1
(Opt.)
2
1
E1
+ C10
(Opt.)
E2
3
4
6
Vin
Vin
BOOST
15
SHDN
SW
2
14
SYNC
SW
5
E4
C
7
NC
13
NC
1
Gnd
8
Gnd
BIAS
Gnd
Vc
9
16
11
R4
(Opt.)
Gnd
C
D1
30BQ060
SMC
R6
(Opt.)
3
C1
220pF
1
2
C2
.015uF
Q1
MMBT2222A
(Opt.)
NOTE: UNLESS OTHERWISE SPECIFIED
R5
(Opt.)
B
LINEAR TECHNOLOGY CORPORATION
For All Optional Components On Boards:
Q1,R5,R6,C9 Show Ref. Des. And Outline
D3,D4 Show Ref. Des. With No Outline
C6,C7,C8,C10,R4 Show No Ref. Des. And No Outline
1630 McCARTHY BLVD.
MILPITAS, CA 95035
PHONE (408) 432-1900
FAX (408) 434-0507
* High Vin Possible ( Up to 60V)
With Thermal Constraints. See Quick Start Guide
Title
Size
4
3
A
3A, 500kHz High Voltage Buck Converter
Document Number
DC549A
Date: Tuesday, September 03, 2002
5
E6
Vout
[email protected]
C5
47uF
6.3V
Cer
C8
(Opt.)
12
R1
1.5K
A
C6
(Opt.)
0805
10
R3
4.99k
1%
B
E5
2
C9
(Opt.)
R2
15.4k
1%
Gnd
D
L1
10uH
UP2B-100
1
U1
LT3431EFE
FB
1
D4
SMAZ7V5
(Opt.)
2
1
C4
0.22uF
16V
E3
SYNC
D2
MMSD914T1
SOD123
1
2
C3
2.2uF
100V
Gnd
SHDN
2
1
Vin
7.5V-20V*
3
2
D
4
2
LT3431EFE
Sheet
1
of
1
Rev
1
Bill Of Material
Demo Bd. # 549A
Linear Technology Corporation
LT3431EFE
6/17/2005
1:38 PM
Item
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Qty Reference
1
1
1
1
1
0
0
0
1
1
0
0
6
1
0
0
1
1
1
0
1
4
4
C1
C2
C3
C4
C5
C6
C8,C9
C10
D1
D2
D3
D4
E1-E6
L1
L1
Q1
R1
R2
R3
R4,R5,R6
U1
4 CORNERS
Part Description
CAP, NPO 220pF 25V
CAP, X7R 0.015uF 50V
CAP, Y5U
2.2uF 100V
CAP, X7R 0.22uF 16V
CAP, CER
47uF 6.3V
CAP, 0805
( Opt. )
CAP, 0603
( Opt. )
CAP, C-CASE ( Opt. )
DIODE SCHOTTKY,3A, 60V,SMA
DIODE, 200mA, 100V
DIODE,
( Opt. )
DIODE,
( Opt. )
TP, TURRET, .094"
INDUCTOR, SMT, 10uH, L-COIL-UP2B
INDUCTOR,
( Opt. )
TRANS,
( Opt. )
RES, 1.5K 1%, 0603
RES, 15.4k 1%, 0603
RES, 4.99k 1%, 0603
RES,
( Opt. )
IC, LT3431EFE
STANDOFF, NYLON HEX #4-40 x 1/4"
SCREW, #4-40 x 1/4
Page 1 - of - 1
Manufacture / Part #
AVX, 06033A221KAT1A
0603
AVX, 06035C153KAT1A
0603
UNITED CHEMI-CON, THCS50E2A225ZT
TAIYO YUDEN, EMK212BJ224MG
0805
TDK, C4532X5R0J476M
1812
IR, 30BQ060
SMC
DIODES INC. 1N4148W-7
SOD-123
ON SEMI., MMSD914T1-7
SOD-123
DIODES INC., SMAZ7V5
SMA
MILL-MAX, 2501-2
COOPER ELECT. UP2B-100
SUMIDA, CDRH8D28
DIODES INC., MMBT2222A-7 SOT-23
AAC, CR16-1501FM
AAC, CR16-1542FM
AAC, CR16-4991FM
LINEAR TECH., LT3431EFE
TSSOPEF16
MICRO PLASTICS, #14HTSP101
ANY
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