DN301 - 60V/3A Step-Down DC/DC Converter Maintains High Efficiency over a Wide Input Range

advertisement
60V/3A Step-Down DC/DC Converter Maintains High
Efficiency Over a Wide Input Range – Design Note 301
Mark Marosek
Introduction
Today’s high voltage applications—such as automotive,
industrial and FireWire peripherals—place increasing
demands on power supplies. They must provide high
power, high efficiency and low noise, in a small space
and over an ever widening range of operational input
voltages. Many high voltage DC/DC converter solutions
can meet some of these conditions at high input voltages but they are unable to maintain high efficiencies
at lower input voltages. Many of these same converters
have frequency compensation schemes that require
bulky input and output capacitors, which not only increase the size of the overall solution but also result in
high output ripple voltage. The LT®3430 is designed to
alleviate all of these problems.
The LT3430 is a monolithic step-down DC/DC converter
which utilizes a 3A peak switch current limit and has the
ability to operate with a 60V input. The LT3430 runs at
a fixed frequency of 200kHz and is housed in a small
thermally enhanced 16-pin TSSOP package enabling it
to save space while optimizing thermal management.
Its 5.5V to 60V input range makes the LT3430 ideal
for FireWire peripherals (typically 8V to 40V input),
as well as automotive systems requiring 12V, 24V and
D2
MMSD914T1
C3
4.7μF
100V
CER
VIN BOOST
SYNC
SW
OFF ON
LT3430EFE
SHDN
BIAS
R3
2.2k
C5
0.022μF
R1
15.4k
VOUT
GND
D1
30BQ060
R2
4.99k
DN301 F01
C1: AVX D CASE 100μF 10V TPSD107M010R0100
C2: AVX 0.68μF X7R 16V 0805YC684KAT1A
C3: UNITED CHEMI-CON 4.7μF 100V TCCR70E2A475M
C4: AVX 220pF X7R 50V 08055A221KAT
C5: AVX 0.022μF X7R 16V 0805YC223KAT
D1: INTERNATIONAL RECTIFIER 60V 3A SCHOTTKY 30BQ060
L1: SUMIDA 22μH CDRH104R
Figure 1. 42 to 5V Step-Down Converter
12/02/301_conv
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
100
FB
VC
C4
220pF
VOUT
5V AT 2A
C2
L1
0.68μF 22μH
Efficiency
Monolithic step-down converters capable of operation at
high input voltages are usually optimized for efficiency
at high input-to-output voltage differentials, where
the duty cycle is low. At low duty cycles, DC switch
losses are small compared to the overall losses, so the
switch design is often neglected, resulting in a switch
resistance that can be as poor as 0.5Ω for some 3A
converters. Such converters give up efficiency at high
duty cycle operation and limit their minimum input
voltage operating capability.
C5
100μF
10V
SOLID
TANTALUM
VOUT = 5V
VIN = 12V
90
EFFICIENCY (%)
VIN
7.5V–60V
42V input voltages (with the ability to survive load
dump transients as high as 60V). Furthermore, it was
designed to maintain excellent efficiencies with both
high and low input-to-output voltage differentials. Its
current mode architecture adds flexible frequency
compensation allowing the use of a ceramic output
capacitor—resulting in small solutions with extremely
low output ripple voltage (see Figures 3 and 4). Other
features include a shutdown pin, which has an accurate 2.38V undervoltage lockout threshold and a 0.4V
threshold for micropower shutdown (drawing only
25μA), and a SYNC pin, which allows the LT3430 to
be synchronized up to 700kHz.
VIN = 42V
80
70
60
50
0
0.5
1
1.5
LOAD CURRENT (A)
2
2.5
Figure 2. LT3430 Efficiency vs Load Current
Figure 1 shows a 42V to 5V converter using the LT3430.
To achieve high efficiency at high input voltages, the
LT3430 provides fast output-switch edge rates. To
further improve efficiency, the LT3430 provides a BIAS
pin to allow internal control circuitry to be powered from
the regulated output—light loads at high input voltages
require minimal quiescent current to be drawn from the
input. The peak efficiency for a 42V to 5V conversion
is greater than 82%, as shown in Figure 2.
The LT3430 is also capable of excellent efficiencies at
lower input voltages. The peak efficiency for a 12V to 5V
conversion is greater than 90%, as shown in Figure 2.
One key to achieving high efficiency for low input-tooutput voltage conversions is to use a low resistance
saturating switch. A pre-biased capacitor, connected
between the BOOST and SW pins, generates a boost
voltage above the input supply during switching. Driving
the switch from this boost voltage allows the 100mΩ
power switch to fully saturate. An output voltage as low
as 3.3V is enough to generate the required boost supply.
Small Size, Low Output Ripple Voltage (High
Switching Frequency, All Ceramic Solution)
The high 200kHz switching frequency of the LT3430
keeps circuits small by minimizing the inductor value
required to keep inductor ripple current low. The current
mode architecture of the LT3430 allows for a small, low
ESR ceramic capacitor to be used at the output—thus
providing an extremely low output ripple voltage solution
in a small space. Figure 3 shows a 5V/2A low profile
VIN
8V TO 40V
Peak Switch Current (Not Your Average Current
Mode Converter)
Most current mode converters have a reduced peak
switch current limit at high duty cycles. This is a result
of slope compensation, which is added to the converter’s
current sensing loop to prevent subharmonic oscillations for duty cycles above 50%. However, the LT3430
is able to maintain its peak switch current limit over the
full duty cycle range. For applications that require high
duty cycles, this offers significant advantages—including a lower inductor value, lower minimum VIN and/or
higher output current capability—over typical current
mode converters with similar peak switch current limits.
Conclusion
The LT3430 features a 3A peak switch current limit,
100mΩ internal power switch and a 5.5V to 60V operating range, making it ideal for automotive, industrial
and FireWire peripheral applications. It is highly efficient
over the entire operating range and it includes important
features to save space and reduce output ripple—including a 200kHz fixed operating frequency, a current
mode architecture and availability in a small, thermally
enhanced 16-pin TSSOP package.
D2
MMSD914T1
C3
4.7μF
CER
50V
OFF ON
R1
3.3k
C2
22nF
(<3mm) solution for FireWire peripherals which uses
a ceramic output capacitor. The output ripple voltage
of this circuit is only 26mVP-P, much less than the
80mVP-P incurred when a tantalum capacitor with an
ESR of 80mΩ is used (see Figure 4).
C1
220pF
VIN BOOST
SYNC
SW
LT3430EFE
BIAS
SHDN
FB
VC
GND
VOUT
5V AT 2A
C4
L1
0.68μF 22μH
100μF, 0.08Ω
TANTALUM
OUTPUT CAPCITOR
R2
15.4k
VOUT
D1
30BQ060
R3
4.99k
C3: TDK C5750X7R1H475K
C5: TDK C4532X5R0J107M
L1: SUMIDA CEI-122 220
www.linear.com
Linear Technology Corporation
20mV/DIV
100μF CERAMIC
OUTPUT CAPCITOR
DN301 F03
Figure 3. Low Profile (Max Height of 3.0mm)
Low Output Ripple Voltage Solution
Data Sheet Download
C5
100μF
CER
2μsV/DIV
DC301 F04
Figure 4. Output Ripple Voltage Comparison for a
Tantalum vs Ceramic Output Capacitor in the Circuit
Shown in Figure 3 with VIN = 24V and IOUT = 2A
For applications help,
call (408) 432-1900
dn301f_conv LT/TP 1202 351.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2002
Similar pages