DN287 - High Voltage Buck Regualtors provide High Current, Low Profile Power Solutions for FireWire Peripherals

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High Voltage Buck Regulators Provide High Current, Low
Profile Power Solutions for FireWire Peripherals – Design Note 287
Keith Szolusha
Introduction
Faster data transfer requirements between a personal
computer and an increasing number of peripheral devices have boosted the popularity of the IEEE 1394 High
Performance Serial Bus (FireWire technology), which
enables speeds up to 400Mbps. FireWire technology
is finding its way into real time data transfer and data
intensive devices such as DVD players, digital video
recorders, zip drives and CDRW drives. There is a
growing need for cost effective, low profile, low ripple
DC/DC power supplies that provide 5V and 3.3V peripheral power directly from the FireWire cable at high
currents (1A or greater). The LT®1766 and LT3430 current mode regulators will satisfy these requirements.
Circuit Descriptions
FireWire power sources, such as personal computers,
provide 8V to 40V to the FireWire serial bus at currents
of up to 1.5A and can power up to 16 peripherals while
transferring data synchronously or asynchronously.
Power sink peripherals, which do not supply power
to the serial bus, receive power from the serial bus
via the two power pins of the 6-pin FireWire port.
Each peripheral must include an internal DC/DC
converter to convert the bus’s 8V to 40V input to a
5V or 3.3V output. Since many of these devices are
VIN
8V TO 40V
R1
4.7k
C2
15nF
C1
220pF
VIN BOOST
SYNC
SW
LT1766EFE
SHDN
BIAS
FB
VC
R2
8.66k
GND
C3: TDK C4532X7R1H225K
C5: TAIYO YUDEN JMK432BJ476M
L1: SUMIDA CDRH8D28-220
D1
10MQ060N
R3
4.99k
C5
47μF
CER
DN287 F01
Figure 1. This Low Component Count, 8V to 40V IN, 3.3V
at 1A OUT DC/DC Converter Uses All Ceramic Capacitors
(Max Height of 3.0mm)
06/02/287_conv
The LT1766/LT3430 high voltage buck regulators used
in these solutions provide high current for FireWire
peripherals with several key advantages, such as 60V
input capabilities, 200kHz operating frequency, high
power internal 1.5A/3A power switch current limits,
shutdown capability and thermally enhanced 16-pin
TSSOP exposed lead-frame packages.
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.
VOUT
3.3V AT 1A
C4
L1
0.33μF 22μH
VOUT
Figures 1 and 2 show two FireWire DC/DC converter
solutions, both using ceramic input and output capacitors. Figure 1 shows a 3.3V, 1A output solution and
Figure 2 shows a 5V, 2A output solution. Both have a
3.0mm maximum component height. The efficiencies
and output voltage ripple for these two circuits are
shown in Figures 3, 4 and 5. The output voltage ripple
is extremely low, less than 15mVP-P for both solutions
(see Figure 5). Higher output voltage yields overall
higher efficiency since the forward voltage drop of the
catch diode remains constant and is less significant.
VIN
8V TO 40V
D2
MMSD914
C3
2.2μF
CER
portable consumer electronics, power consumption
requirements are increasing while available space is
decreasing. One way to decrease the total size and cost
of a solution while keeping voltage ripple extremely low
is to use ceramic input and output capacitors.
D2
MMSD914
C3
4.7μF
CER
R1
3.3k
C2
22nF
C1
220pF
VIN BOOST
SYNC
SW
LT3430EFE
BIAS
SHDN
FB
VC
GND
C3: TDK C5750X7R1H475K
C5: TDK C4532X5R0J107M
L1: SUMIDA CEI-122 220
VOUT
5V AT 2A
C4
L1
0.68μF 22μH
R2
15.4k
VOUT
D1
30BQ060
R3
4.99k
C5
100μF
CER
DN287 F02
Figure 2. This Low Component Count, 8V to 40V IN, 5V
at 2A OUT DC/DC Converter Uses All Ceramic Capacitors
(Max Height of 3.0mm)
The external location of both the feedback and compensation components, combined with the current mode
architecture of the LT1766 and LT3430 makes it possible
to provide simple compensation solutions using ceramic
output capacitors. The circuits in Figures 1 and 2 use a
3-element compensation scheme. Note that these designs are not limited to the input and output voltages
and currents shown here. The transient response can
be optimized for other input and output values by simple
adjustments to the compensation network.
at the feedback node—a necessary evil for voltage mode
controllers. Nonceramic, higher ESR capacitors supply
the necessary increase in ripple voltage at the feedback
node, but also have the undesirable side effect of increasing ripple at the output as well. Some manufacturers
promote internal feedback or complicated compensation schemes that seem reasonable on the surface, but
severely limit flexibility in choosing output capacitor
sizes and types, input voltage ranges, and optimization
of transient response and loop stability.
Current mode regulators have major advantages over
voltage mode regulators which require higher ESR output capacitors or are difficult to design and have higher
parts count compensation schemes. With a voltage
mode buck regulator, it can be difficult to get proper
phase and gain margins using ceramic output capacitors
due to the capacitors’ low ESR and lack of ripple voltage
Conclusion
The LT1766 and LT3430 regulators provide simple, low
profile power supply solutions for FireWire peripherals.
They allow the use of ceramic input and output capacitors which minimizes the size, cost and ripple voltage
of the supply while maintaining high efficiency and
simplicity of design.
85
VIN = 12V
75
VIN = 8V
VIN = 24V
LT3430 EFFICIENCY (%)
80
LT1766 EFFICIENCY (%)
95
VIN = 8V
VIN = 40V
70
65
90
VIN = 12V
85
VIN = 24V
VIN = 40V
80
60
VOUT = 3.3V
55
100 200 300 400 500 600 700 800 900 1000
LOAD CURRENT (mA)
VOUT = 5V
75
200
600
1400
1000
LOAD CURRENT (mA)
DN287 F03
1800
DN287 F04
Figure 3. The Efficiency of the Circuit in Figure 1 is as
High as 84% and Increases as Input Voltage Decreases
Figure 4. The Efficiency of the Circuit in Figure 2 is as
High as 90%
R1
10mV/DIV
R2
2μs/DIV
DN287 F05
Figure 5. Trace R1 Output Voltage Ripple of Figure 1 with 24V IN, 3.3V OUT, 1A OUT.
Trace R2 Output Voltage Ripple of Figure 2 with 24V IN, 5V OUT, 2A OUT
Data Sheet Download
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