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Boost Regulator Makes Low Profile SEPIC with Both Step-Up
and Step-Down Capability – Design Note 317
Keith Szolusha
3V to 20V Input, 5V Output, 3mm Maximum
Height SEPIC
Figure 1 shows a 3V to 20V input, 5V output 3mm maximum height SEPIC using the LT®1961, a 1.25MHz, current mode, monolithic, 1.5A peak switch current, boost
converter. The output current capability of this circuit
varies with input voltage (see Figure 3). At 3V input, the
converter can supply up to 410mA of load current and
as high as 830mA of load current at 20V input. The tiny
coupling capacitor used here is large enough to handle
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08/03/317_conv
VIN
3V TO 20V
IL1
CIN
2.2μF
25V
CERAMIC
X5R
CCOUP
1μF 25V
CERAMIC X5R
CATCH DIODE
UPS140
VOUT
5V
ISW
VIN
VSW
LT1961
SHDN FB
SYNC VC
GND GND
L2
CDRH4D28-100
ICOUP
31.6k
15nF
IL2
100pF
10k
1%
1k
COUT
10μF
6.3V
CERAMIC
X5R
DN317 F01
HIGH ΔI/Δt DISCONTINUOUS CURRENT PATH INDICATED IN BOLD FOR LAYOUT
Figure 1. LT1961 in a 3V to 20V Input to 5V Output
All Ceramic SEPIC (3mm Maximum Height)
100
90
80
EFFICIENCY (%)
One alternative to a transformer-based topology is to
use two low profile inductors and a SEPIC coupling
capacitor which transfers the energy between the two
inductors much like the core of a transformer. The
coupling capacitor provides a low impedance path
for the inductor currents to pass either from the input
(primary) inductor through the catch diode and to
the output, or from the output (secondary) inductor
back through the switch to ground. Both inductors act
continuously and independently, making their selection
easier than selecting the transformer for a flyback or a
typical SEPIC circuit. The inductors are not restricted
to having the same inductance and can be individually
picked for peak currents and allowable ripple.
L1
CDRH4D28-100
70
60
50
40
VOUT = 5V
5VIN
8VIN
12VIN
20VIN
30
20
10
0
200
0
600
800
400
LOAD CURRENT (mA)
1000
DN317 F02
Figure 2. Efficiency of the Circuit in Figure 1
1200
VOUT = 5V
IL2 PEAK CURRENT
1000
CURRENT (mA)
Introduction
Automotive, distributed power and battery-powered
applications often operate at a voltage that is derived
from a widely variable bus voltage. Frequently the operating voltage falls somewhere in the middle of the bus
voltage range, such as a 12V automotive operating voltage, from a 4V to 18V bus. These applications require a
DC/DC converter that can step up or step down, depending on the voltage present on the bus. Flyback and SEPIC
designs are commonly used single-switch solutions for
this problem, but both of these solutions typically use
a transformer which poses layout and height problems
for applications where space is at a premium.
800
MAXIMUM
LOAD
CURRENT
600
IL1 PEAK CURRENT
400
200
0
0
2
4
6 8 10 12 14 16 18 20
DN317 F03
INPUT VOLTAGE (V)
Figure 3. The Peak Inductor Currents in L1 and L2 Sum to
1.5A, the Peak Switch Current. Maximum Output Current
Is the Average Current in L2 at Peak Switch Current
4V to 18V Input, 12V Output, 3mm Maximum
Height SEPIC
12V buses are often derived from sources with a wide
input voltage range. For instance, automotive solutions
can have steady-state operating voltages as high as 18V
and as low as 4V for cold-crank conditions. Figure 4
shows a simple, low cost and low profile (≤3mm) solution that avoids the high cost of using both a boost
and buck converter and maintains 12V system power
during cold-crank conditions.
Efficiency, as shown in Figure 5, is typically greater than
75% and as high as 80%. This is better than average
for 12V SEPICs and not much less than a similarly
priced and sized 12V buck converter solution which is
limited to greater than 14V input. Maximum load current increases with input voltage, as shown in Figure 6.
500mA load current is possible at 12V input and up to
600mA at 18V. The maximum switch current of the
LT1961 is 1.5A and is the sum of the peak current in
L1 and L2. Higher output voltage raises the current in
the input inductor.
100
VOUT = 12V
90
5VIN
80
15VIN
70
EFFICIENCY (%)
the RMS ripple current transferring between the primary
and secondary sides of the circuit, and to maintain a
voltage equal to the input voltage in order to provide good
regulation and maximum output power. The current
mode control topology of the LT1961 and the small 10μF
ceramic output capacitor provide excellent transient
response over the wide input voltage range.
12VIN
60
50
40
30
VIN
VSW
LT1961
SHDN FB
SYNC VC
GND GND
CIN
2.2μF
25V
CERAMIC
X5R
20
10
VOUT
12V
L2
CDRH5D28-150
47pF
90.9k
6.8nF
100pF
10k
10k
1%
COUT
10μF
16V
CERAMIC
X5R
0
0
100
300
400
200
LOAD CURRENT (mA)
1200
Figure 4. LT1961 in a 4V to 18V Input to 12V
Output 3mm Maximum Height All Ceramic SEPIC
The catch diode has a 40V reverse breakdown voltage
rating in order to handle the voltage induced across it
during the switch off-time which is equal to the output
voltage plus the input voltage. The 35V maximum switch
voltage rating of the LT1961 allows the input voltage to
go up as high as 18V. With a DC voltage equal to the
input voltage, the coupling capacitor raises the voltage
at the switch node to a level equal to the input voltage
plus the output voltage. Tiny voltage spiking present
on the switch node of any switching converter requires
a few volts of headroom between the maximum switch
voltage rating and the sum of the input and output voltages. The switching spikes are reduced to a minimum
by keeping the high ΔI/Δt discontinuous current path
(indicated in bold in Figures 1 and 4) as short as possible. The placement of the two power inductors is not
crucial which makes it easier to create a power supply
layout that fits confined spaces.
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600
DN317 F05
VOUT = 12V
DN317 F04
HIGH ΔI/Δt DISCONTINUOUS CURRENT PATH INDICATED IN BOLD FOR LAYOUT
500
Figure 5. Efficiency of the Circuit in Figure 4
IL1 PEAK CURRENT
1000
CURRENT (mA)
VIN
4V TO 18V
L1
CDRH5D28-150
CCOUP
1μF 25V CATCH DIODE
CERAMIC X5R UPS140
800
IL2 PEAK CURRENT
600
MAX LOAD CURRENT
400
200
0
2
4
6
8 10 12 14 16
INPUT VOLTAGE (V)
18
20
DN317 F06
Figure 6. The Peak Inductor Currents and Maximum Load
Current of the Circuit in Figure 4
Conclusion
The LT1961 fits into SEPIC solutions for applications
with wide input voltage ranges. The solutions are small,
simple and low profile. All ceramic capacitors and tiny
components help keep power supply costs to a minimum. The 2-inductor SEPICs shown here eliminate the
use of a tall transformer and offer layout flexibility to
fit tight design constraints.
For applications help,
call (408) 432-1900
dn317f_conv LT/TP 0803 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 2003