DN330 - High Input Voltage Monolithic Switcher Steps Up and Down Using a Single Inductor (Design Note 330A)

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High Input Voltage Monolithic Switcher Steps Up and Down
Using a Single Inductor – Design Note 330A
Jay Celani
Introduction
Ultrawide input voltage requirements are a common
design problem for DC/DC converter applications, but
when that range includes voltages both above and below
the output voltage, the converter must perform both
step-up and step-down functions. The LT®3433 is a high
voltage monolithic DC/DC converter that incorporates
two switch elements, allowing for a unique topology that
accommodates both step-up and step-down conversion
using a single inductor.
The LT3433 uses a 200kHz constant frequency, current
mode architecture and operates with input voltages from
4V to 60V. An internal 1% accurate voltage reference
allows programming of precision output voltages up
to 20V using an external resistor divider. Burst Mode®
operation improves efficiencies during light-load conditions, reducing the device’s quiescent current to 100μA
during no-load conditions. A soft-start feature reduces
output overshoot and inrush currents during start-up,
and both current limit foldback and frequency foldback
are employed to control inductor current runaway during start-up and short-circuit conditions. The LT3433 is
available in a 16-pin fused TSSOP exposed pad package
which provides a small footprint and excellent thermal
characteristics.
When the converter input voltage is significantly higher
than the output voltage, the LT3433 operates as a
modified buck converter using a boosted-drive high
side switch. If the converter input voltage becomes
close enough to the output voltage to require a duty
cycle greater than 75% in buck mode, the LT3433
automatically enables a second switch. This second
switch pulls the output side of the switched inductor to
ground during the “switch on” time, creating a bridged
switching configuration.
During bridged switching, the LT3433 merges the
elements of buck and boost DC/DC converters as
shown in Figure 1. In the simplest terms, a buck
DC/DC converter switches the VIN side of the inductor,
02/04/330a_conv
while a boost converter switches the VOUT side of the
inductor. Combining the elements of both topologies
achieves both step-up and step-down functionality using
a single inductor, so voltage conversion can continue
when VIN approaches or is less than VOUT.
4V-60V Input to 5V Output
DC/DC Automotive Converter
A 4V-60V to 5V DC/DC converter is shown in Figure 2.
This converter is well suited for 12V automotive battery
applications, maintaining output voltage regulation with
battery line voltages from 4V cold crank through 60V
load dump. The threshold for bridged mode operation
is about 8V, so the converter will operate primarily in
buck mode except during a cold crank condition. During buck operation, this converter can provide load
currents up to 350mA with input voltages up to 60V.
Operating with a nominal 13.8V input, this LT3433
converter accommodates loads of 400mA and produces
efficiencies up to 82%.
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respective owners.
VIN
SW
CIN
L
VOUT
D
COUT
(1a) Step-Down (VIN > VOUT)
L
VIN
CIN
D
SW
VOUT
COUT
(1b) Step-Up (VIN < VOUT)
VIN
SW
CIN
L
D
D
SW
VOUT
COUT
DN330A F01
(1c) Step-Up/Step-Down (VIN > VOUT or VIN < VOUT)
Figure 1. The LT3433 Merges the Elements of
Step-Up and Step-Down DC/DC Converters
VOUT
5V
4V ≤ VIN ≤ 8.5V: 125mA
8.5V ≤ VIN ≤ 60V: 350mA
100μH
CoEv DU1352-101M
B160A
1N4148
B120A
VBST
0.1μF
10V
VIN
4V TO 60V
+
SW_L
SW_H
B160A
1N4148
47μF
10V
B120A
VBST
0.47μF
50V
VIN
8V TO 60V
PWRGND
330pF
1nF
2.2μF
100V
CERAMIC
1N4148
(BURST)
BURST_EN
VBIAS
VC
SHDN
0.1μF
10V
68k
330pF
1nF
PWRGND
VIN
VOUT
BURST_EN
VBIAS
VC
SHDN
1N4148
(BURST)
0.1μF
10V
68k
20k
100k
SS
VFB
305k
47μF
25V
LT3433
VOUT
VIN
+
SW_L
SW_H
LT3433
2.2μF
100V
CERAMIC
VOUT
12V
8V ≤ VIN ≤ 18V: 125mA
18V ≤ VIN ≤ 60V: 380mA
220μH
TDK SLF12565T-221M1R0
(NO
BURST)
SS
VFB
SGND
0.01μF
MODE SWITCH
VIN H-L: 7.9V
VIN L-H: 8.3V
176k
(NO
BURST)
SGND
0.01μF
DN330A F02
MODE SWITCH
VIN H-L: 16.6V
VIN L-H: 17V
DN330A F04
Figure 2. 4V-60V to 5V DC/DC Converter
Figure 4. 8V-60V to 12V Converter
100
90
VIN = 13.8V
NO BURST
80
VIN = 20V
NO BURST
90
80
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 13.8V
BURST
70
60
VIN = 4V
NO BURST
50
VIN = 4V
BURST
40
70
60
VIN = 20V
BURST
VIN = 8V
NO BURST
50
40
VIN = 8V
BURST
30
30
20
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
20
0.1
10
100
1
OUTPUT CURRENT (mA)
1000
DN330A F05
DN330A F03
Figure 3. 4V-60V to 5V Conversion Efficiency
Figure 5. 8V-60V to 12V Conversion Efficiency
When the input voltage drops below 8V, the converter
switches into bridged operation to maintain output
voltage regulation. Because the LT3433 switch current
limit is fixed, converter load capability is reduced while
operating in bridged mode. With an input of 4V, the
converter accommodates loads up to 125mA. Not only
does this LT3433 converter operate across a large range
of DC input voltages, but it also maintains tight output
regulation during input transients. When subjected to
a 1ms 13.8V to 4V input transition to simulate a cold
crank condition, regulation is maintained to 1% with
a 125mA load.
without cold-crank requirements, as well as many other
applications such as those powered by inexpensive
wall adapters. This converter operates in buck mode
with input voltages above 17V, accommodating loads
up to 380mA. This converter accommodates loads
up to 435mA and produces efficiencies above 89%
at 20V input.
8V-60V Input to 12V Output DC/DC Converter
As converter output voltages increase, switch current
and duty-cycle limitations prevent operation with VIN at
the extreme low end of the LT3433 operational range.
The 12V output converter shown in Figure 4 can provide
load current up to 125mA with an input voltage as low
as 8V. This is suitable for 12V automotive applications
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Conclusion
The LT3433 simplifies ultrawide input range DC/DC
voltage conversion, enabling simple and inexpensive
solutions to a variety of design problems. Automatic
transitioning between buck and bridged modes of
operation provides seamless output regulation for
wide input voltage ranges and input voltage transients.
The use of a small footprint TSSOP package, a single
inductor and few external components reduce board
space requirements, increase efficiency and improve
thermal characteristics.
For applications help,
call (408) 432-1900
dn330af_conv LT/TP 0204 344K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004
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