Dec 2003 High Input Voltage Monolithic Switcher Steps Up and Down Using Single Inductor

DESIGN FEATURES
High Input Voltage Monolithic
Switcher Steps Up and Down
Using Single Inductor
Introduction
Ultra-wide input voltage requirements
are a common design problem for DC/
DC converter applications. When that
range includes voltages both above
and below the output voltage, a converter must perform both step-up
and step-down functions, making the
design problem even more complex.
A high-voltage input range makes
the problem even tougher. These
design issues are commonplace in
the automotive environment, where
battery powered electronics have to
handle everything from cold crank
to load dump, a range that can span
from 4V to 60V. The requirements
of an automotive battery-powered
design are somewhat unique, as the
operational input voltage is typically
relatively well controlled, but an ultrawide input range must be considered
since short-duration events can create
extreme input voltage shifts.
There are several common solutions
to the step-up/step-down problem.
One solution is to cascade multiple
circuits, such as a boost converter followed by a buck converter or an LDO,
where the boost converter prevents the
output of the step-down converter from
dropping out at low input voltages.
With high input voltages, the upper
bound of the input voltage is directly
imposed on the step-down stage,
which makes use of an LDO impractical. Cascading a boost converter and
high-voltage buck converter will work,
but such a topology incurs penalties
VBIAS
1.25V
BURST
CONTROL
CIRCUITS
BIAS
BURST_EN
VIN
SENSE
AMPLIFIER
VBST
COMPARATOR
SLOPE
COMP
OSCILLATOR 200kHz
FREQUENCY
CONTROL
MODE
CONTROL
BOOSTED
DRIVER
SW_H
12
5
4
2
3
SWITCH
CONTROL
LOGIC
SW_L
15
DRIVER
GND
VFB
ERROR
AMPLIFIER
30%
LOAD
by Jay Celani
14
7
1.231V
VC
+
Burst Mode
CONTROL
SHDN
6
11
SHUTDOWN
–
15%
LOAD
5µA
0.7V
SS
VOUT
SGND
1, 8, 9,16
10
13
3433 BD
VOUT
+
Figure 1. LT3433 block diagram
24
Linear Technology Magazine • December 2003
DESIGN FEATURES
of complexity, reduced efficiency, and
cost. A SEPIC converter is a popular
topology for step-up/step-down applications, but a SEPIC converter
has inherent drawbacks such as
the expense of two inductors, low
efficiencies, high switch voltage and
current stresses, and high output
ripple currents.
Enter the LT3433, a high-voltage
monolithic DC/DC converter that
incorporates two switch elements,
allowing for a unique topology that
accommodates both step-down and
step-up conversion using a single
inductor. When the input voltage is
significantly higher then the output,
the LT3433 operates in a modified
non-synchronous buck configuration
using a boosted-drive high-side switch.
When the input voltage approaches
or falls below the output voltage, the
LT3433 activates a ground-referred
switch, which creates a bridged
switching configuration, or a buck/
boost converter, allowing for very low
dropout and/or step-up conversion.
The LT3433 is primarily intended
for use in step-down applications
that have transient low voltage input
requirements, such as 12V automotive applications that must support
a cold-crank condition, where the
battery-rail can briefly drop down to
4V. The LT3433 could also be used
to significant benefit in certain applications where a SEPIC topology is
currently the best option.
VIN
SW
L
CIN
VOUT
COUT
D
Step-Down (VIN > VOUT)
D
L
VIN
CIN
SW
VOUT
COUT
Step-Up (VIN < VOUT)
VIN
SW
D
L
CIN
D
SW
VOUT
COUT
3433 F01
Step-Up/Step-Down (VIN > VOUT or VIN < VOUT)
Figure 2. Merging the elements of a buck
DC/DC converter topology and a boost DC/DC
converter topology form the LT3433 bridged
network, enabling buck/boost conversion
using a single inductor
tion, minimizing maintenance power
for battery powered applications. Burst
Mode operation can be disabled by
shorting the BURST_EN pin to either
the VOUT pin or the VBIAS pin, or by biasing the pin using an external supply.
Shorting BURST_EN to ground enables
Burst Mode. The LT3433 also has a
low-current shutdown mode, reducing
quiescent current to ~10µA when the
SHDN pin is pulled below 0.4V.
The LT3433 uses both current limit
and frequency foldback to help control inductor current runaway during
startup and short-circuit conditions.
A soft-start feature is also included to
reduce output overshoot and inrush
currents during startup. Soft-start
duration is controlled by a capacitor placed between the SS pin and
ground. The LT3433 does not suffer
from current limit reduction typically
associated with slope-compensation,
so switch current limit is unaffected
by duty-cycle.
Switching between buck and buck/
boost modes of operation is controlled
automatically by the LT3433. While
in buck mode, if the converter input
voltage becomes close enough to the
output voltage to require a duty-cycle
greater than 75%, the LT3433 enables
a second switch during the “switch on”
time, which pulls the output side of the
inductor to ground. This bridged-configuration switching operation allows
voltage conversion to continue when
VIN approaches or is less than VOUT.
Bridged Topology
In the simplest terms, a buck DC/
DC converter switches the VIN side of
the inductor, while a boost converter
switches the VOUT side of the inductor.
The LT3433 bridged topology merges
the elements of buck and boost topologies, providing switches on both sides
of the inductor, as shown in Figure 2.
Operating both switches simultaneously achieves both step-up and
step-down functionality.
Maximum duty-cycle capability
(DCMAX) gates the dropout capabilities
of a buck converter. As VIN – VOUT is
What’s Inside?
The LT3433 incorporates a 200kHz
constant frequency, current mode
architecture and can operate with
input voltages from 4V to 60V. The
LT3433 is packaged in a 16-pin fused
TSSOP exposed pad leadframe package, which provides a small footprint
and excellent thermal characteristics.
An internal 1% voltage reference allows programming of precision output
voltages up to 20V using an external
resistor divider. A block diagram of the
LT3433 is shown in Figure 1.
Burst Mode operation improves efficiencies during light-load conditions.
The LT3433 quiescent current drops
to ~100µA while in Burst Mode operaLinear Technology Magazine • December 2003
VOUT
5V
4V ≤ VIN ≤ 8.5V: 125mA
8.5V ≤ VIN ≤ 60V: 350mA
L1
100µH
CoEv DU1352-101M
B160A
B120A
1N4148
0.1µF
VIN
4V TO 60V
2.2µF
VBST
PWRGND
SW_H
VIN
+
(BURST)
1nF100pF
68k
100k
0.5%
LT3433
VOUT
BURST_EN
VBIAS
VC
SHDN
VFB
305k
0.5%
47µF
SW_L
1N4148
0.1µF
SS
SGND
(NO BURST)
0.01µF
Figure 3. 4V–60V to 5V converter
25
DESIGN FEATURES
90
500
80
MAXIMUM OUTPUT CURRENT (mA)
VIN = 13.8V
EFFICIENCY (%)
70
VIN = 13.8V
BURST
60
VIN = 4V
50
VIN = 4V
(BURST)
40
30
20
0.1
1
10
100
OUTPUT CURRENT (mA)
VOUT = 5V
BUCK
400
300
200
BRIDGED
100
0
1000
0
10
20
30
VIN (V)
40
50
60
Figure 4. Conversion efficiency of the 4V–60V
to 5V converter shown in Figure 3
Figure 5. Maximum output current vs VIN for
the 4V–60V to 5V converter shown in Figure 3
reduced, the required duty cycle increases toward DCMAX until the limit
of the converter is reached, and the
converter loses regulation. With a
second switch bridging the switched
inductor between VIN and ground, the
entire input voltage is imposed across
the inductor, which reduces the duty
cycle required to maintain regulation. Using this topology, regulation
is maintained as VIN approaches or
drops below VOUT.
operation is near 8V, so the converter
operates primarily in buck mode, except during a cold crank condition.
This converter accommodates loads
up to 400mA and produces efficiencies greater than 83% when operating
with a nominal 13.8V input. Conversion efficiencies with VIN = 4V and
VIN = 13.8V in both burst-enabled
and burst-disabled configurations
are shown in Figure 4.
During a cold-crank condition,
where the battery line voltage drops
below 8V, the converter switches into
buck/boost mode to maintain output voltage regulation. Because the
LT3433 switch current limit is fixed,
converter load capability is reduced
while operating as a buck/boost converter. Output current capability vs.
input voltage is shown in Figure 5.
A 4V–60V to 5V converter is shown
in Figure 3. This design is well suited
to 12V automotive applications where
output regulation is required with battery line voltages from 4V cold crank
through 60V load dump. The input
voltage threshold for bridged mode
D2
1N4148
VBATT
(SWITCHED)
C5
1µF
10V
R4
20k
VBATT
4V TO 60V
DZ1
20V
C4
2.2µF
100V
R3
100k
C3 100pF
C2
1nF R1
68k
R2
100k
SHDN
VOUT
5V
4V < VIN < 8.5V: 125mA
8.5V < VIN < 60V: 350mA
L1
100µH
COEV DU1352-101M
DS1
B160A
DS2
B120A
VBST
SW_L
SW_H PWRGND
LT3433
VIN
VOUT
BURST_EN
VBIAS
VC
SHDN
C7
47µF
10V
VOUT
0.1V/DIV
1ms/DIV
Figure 7. 4V–60V to 5V converter output
characteristic (AC coupled) during a 13.8V to
4V 1ms VIN transition with 125mA output load
24
20
200mA
D1
1N4148
C6
0.1µF 10V
16
175mA
12
125mA
VFB
SS
SGND
150mA
8
C1
0.01µF
R5
305k
4
MODE SWITCH:
VIN H-L: 7.9V
VIN L-H: 8.3V
Figure 6. 4V–60V to 5V converter with switched Burst Mode enable and shutdown
26
VIN
5V/DIV
VIN(MIN) (V)
4V–60V Input to 5V Output
Automotive Converter
With an input of 4V, the converter accommodates loads up to 125mA.
It is important to use low-V F
Schottky diodes in a LT3433 converter design. Minimizing the forward
voltages of the external catch and
forward diodes directly reduces operational duty cycle, which increases
output current capability, especially
during bridged switching. Reduced
Schottky forward voltages also increase operational efficiency, which
further increases available output
current capability. The B120/160
diodes used are a good compromise
between size and low VF. An inductor
with low series resistance also helps
to maximize converter efficiency and
performance.
In maintenance applications, reduced Q-current operation is desired
for light-load and no-load conditions.
This is easily accomplished by shorting
the BURST_EN pin to SGND to enable
Burst Mode. In certain low-current applications, however, the IC could enter
burst operation during normal load
conditions. If the additional output
4
8
12
16
20
VOUT (V)
Figure 8. Typical LT3433 converter minimum
input voltages vs VOUT for various maximum
load currents
Linear Technology Magazine • December 2003
DESIGN FEATURES
L1
200µH
TDK SLF12565T-221M1R0
DS1
B160A
D2
1N4148
DS2
B120A
C7
0.47µF
20V
VIN
8V TO 60V
C6
2.2µF
100V
C3 100pF
VBST
(BURST)
R1 68k
C2 1nF
R2
20k
BURST_EN
VBIAS
VC
SHDN
VFB
SS
SGND
R3
176k
C5
47µF
10V
SW_L
SW_H PWRGND
LT3433
VIN
VOUT
VOUT
12V
8V < VIN < 18V: 125mA
18V < VIN < 60V: 380mA
D1
1N4148
C6
0.1µF
20V
C4
0.01µF
MODE SWITCH:
VIN H-L: 16.6V
VIN L-H: 17V
(NO BURST)
Figure 9. 8V–60V to 12V converter
ripple and noise generated by Burst
Mode operation is not desired for
normal operation, BURST_EN can be
biased using an external supply that
is disabled during a no-load condition.
This prevents entering Burst Mode
operation during normal operation,
and enables Burst Mode operation
only when it is required. The 4V–60V
to 5V automotive converter shown in
Figure 6 incorporates a dynamic Burst
Mode function controlled by a switched
battery input, and also accommodates
a user-enabled shutdown feature.
Not only does this LT3433 converter operate across a large range of
DC input voltages, 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, as shown in Figure 7.
input voltage where output regulation
can be maintained. Figure 8 shows
typical minimum input voltages as a
function of converter output voltage
and required load current.
burst-enabled and burst-disabled configurations are shown in Figure 10.
Output current capability vs. input
voltage is shown in Figure 11.
8V–60V to 12V Converter
The LT3433 simplifies the design of
ultra-wide input range DC/DC voltage
converters, and is particularly suited
for step-down applications that require
short-duration step-up conversion.
Automatic transitioning between buck
and buck/boost modes of operation
provides seamless output regulation
over wide input voltage ranges and
during input voltage transients. The
outstanding thermal characteristics of
the TSSOP package make the LT3433
usable in harsh environments, and
the small-footprint package, use of
a single inductor, and few external
components provide board space efficient solutions.
The 8V–60V to 12V converter shown
in Figure 9 can provide output current
up to 125mA with inputs as low as
8V. This is suitable for 12V automotive applications without cold-crank
requirements, as well as many other
applications such as those powered
by inexpensive wall adapters. This
converter typically switches operational modes at 17V and operates in
buck mode with higher input voltages.
This LT3433 converter accommodates
loads up to 435mA and produces efficiencies above 89% when operating
with a 20V input. Conversion efficiencies with VIN = 8V and VIN = 20V in both
Conclusion
Increased Output Voltages
Linear Technology Magazine • December 2003
500
100
VIN = 20V
90
VIN = 20V
(BURST)
VIN = 8V
60
50
40
VIN = 8V
(BURST)
1
10
100
OUTPUT CURRENT (mA)
300
200
BUCK
100
30
20
0.1
IOUT(MAX) (mA)
70
BRIDGED
400
80
EFFICIENCY (%)
The LT3433 can be used in converter
applications with output voltages from
3.3V through 20V, but as converter
output voltages increase, output current and duty cycle limitations can
prevent operation with VIN at the
extreme low end of the LT3433 operational range. When a converter
operates as a buck/boost, the output
current becomes discontinuous, which
reduces output current capability by
roughly a factor of (1 – DC), where DC
= duty cycle. As such, the output current requirement dictates a minimum
1000
Figure 10. Conversion efficiency of the 8V–60V
to 12V converter
0
0
10
20
30
VIN (V)
40
50
60
Figure 11. Maximum output current vs VIN
for the 8V–60V to 12V converter
27
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