LINEAR TECHNOLOGY LT3580 Technical data
Single Resistor Sets Positive or Negative Output
for DC/DC Converter – Design Note 435
Jesus Rosales
Introduction
Many electronic subsystems, such as VFD (vacuum
fl ourecent display), TFT-LCD, GPS or DSL applications,
require more than just a simple step-down or step-up
DC/DC conver ter. They may r equire inverting, noninver ting
converters or both. Designers usually resort to different
regulator ICs to control various polarity outputs, thus
®
increasing the inventory list. The LT
3580 solves this
problem by controlling either positive or negative outputs
using the same feedback confi guration. It contains an integrated 2 A, 42V switch and packs ma ny popular features
such as soft-start, adjustable frequency, synchronization
and a wide input range into a small footprint. The LT3580
comes in an 8-pin 3mm × 3mm DFN or MSOP packages
and can be used in multiple confi gurations such as boost,
SEPIC, fl yback and Cuk topologies.
Sensing Output Voltage Has Never Been Easier
The LT3580 has a novel FB pin architecture that simplifi es the design of inverting and noninverting topologies.
Namely, there are two int ernal error amplifi ers; one s enses
positive outputs and the other negative. Additionally, the
LT358 0 has integrated the groun d side feedback r esistor
to minimize component count. To illustrate the benefi ts,
notice how the schematics in Figures 1, 3 and 5 need
only one feedback resistor.
A single sense resistor simply connects to the FB pin
on one side and to the output on the other regardless of
the output polarity, eliminating the confusion associated
with positive or negative output sensing and simplifying
the board layout. A user decides the output polarity he
needs, the topology he wants to use and the LT3580
does the rest.
Adjustable/Synchronizable Switching Frequency
It is often necessary to operate a converter at a particular
f r e q u e n c y , e s p e c i a l l y i f t h e c o n v e r t e r i s u s e d i n a n R F c o m munications product that is sensitive to spectral noise in
certain frequency bands. Also, if the area available for a
convert er is limited, operating at higher f requencies allows
the use of tiny component sizes, reducing the real estate
required and the output ripple. If power loss is a concern,
switching at a lower frequency reduces switching losses,
improving effi ciency. The switching frequency can be set
from 200kHz to 2.5MHz via a single resistor from the RT
pin to ground. The device can also be synchronized to an
external clock via the SYNC pin.
Soft-Start and Undervoltage Lockout
To alleviate high inrush current levels during start-up, the
LT3580 includes a soft-start feature which controls the
ramp rate of the switch current by the use of a capacitor
from SS to ground.
The SHDN pin in the LT3580 serves two purposes. Tying
it high or low turns the converter on or off. In situations
where the input suppl y is current limited, has a high source
impedance or ramps up/down slowly, the SHDN pin can
be confi gured to provide undervoltage lockout through
a simple resistor divider from V
to ground.
IN
Boost Converters
A boost converter, shown in Figure 1, produces a positive output voltage always higher than its input. Figure 2
shows the effi ciency graph for the boost converter in
Figure 1 at a 4.2V input.
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All other trademarks are the property of their respective owners.
L1
6.8μH
C2
4.7μF
16V
0.1μF
CDRH5D16-6R8
34
V
IN
5
SHDN
8
SYNC
SS
796
C6
LT3580
RT GND
R6
47.5k
V
3V TO
10V
IN
Figure 1. 3V-10V to 12V, 300mA Boost Converter
SW
D1
DFLS220L
1
FB
2
V
C
R2
130k
R5
20k
C7
1nF
DN435 F01
V
OUT
12V
300mA AT 3V
500mA AT 4.2V
C4
10μF
16V
IN
IN
02/08/435
90
80
EFFICIENCY (%)
70
60
100 200 300 400
OUTPUT CURRENT (mA)
Figure 2. Effi ciency for the Figure 1 Converter at 4.2V
C1
L1
SW
RT GND
R6
47.5k
1μF
50V
1
FB
2
V
C
5V TO
24V
4.7μF
V
25V
8.2μH
0.1μF
5
SHDN
8
SYNC
SS
C6
SD25-8R2
34
V
IN
LT3580
796
IN
C2
SD14-150
D2
DFLS140L
R5
20k
C7
2.2nF
L2
15μH
DN435 F02
500500 150 250 350 450
IN
V
OUT
–12V
360mA
R2
143k
C4
10μF
16V
DN435 F03
Figure 3. 5V-24V to –12V, 350mA Cuk Converter
SEPIC Converters
Figure 5 shows a SEPIC converter. A SEPIC converter is
similar to the Cuk in that it can step up or step down the
i n p u t ; i t o f f e r s o u t p u t d i s c o n n e c t a n d s h o r t - c i r c u i t p r o t e c tion but produces a positive output. Figure 6 shows the
switch waveform of the SEPIC converter during an output
short-circuit event. Notice how the switching frequency
folds back to one-fourth of the regular frequency as soon
as the output voltage is shorted to ground. This feature
enhances short-circuit performance for both Cuk and
SEPIC converters.
C1
L1
1μF
10μH
SW
RT GND
R6
56.2k
35V
FB
V
C
9V TO
24V
2.2μF
V
25V
0.1μF
5
SHDN
8
SYNC
SS
C6
SD25-100
34
V
IN
LT3580
796
IN
C2
Figure 5. 9V-24V to 10.5V, 600mA SEPIC Converter
1
2
DFLS140
L2
10μH
SD14-100
R5
20k
C7
2.2nF
D2
V
OUT
10.5V
600mA
R2
110k
C4
10μF
16V
DN435 F05
90
80
EFFICIENCY (%)
70
60
80 160 240 320
400 120 200 280 360
OUTPUT CURRENT (mA)
Figure 4. Effi ciency for the Figure 3 Converter at 5V
DN435 F04
IN
Cuk Converter
Figure 3 shows a schematic for a Cuk converter, which
produces a negative output with no DC path to the
source. The output can be either higher or lower in
amplitude than the input. The Cuk converter has output
short-circuit protection, which is made more robust by
the frequency foldback feature in the LT3580. Figure 4
shows the effi ciency graph for the Cuk converter in
Figure 3 at a 5V input.
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V
OUT
5V/DIV
V
SW
10V/DIV
2μs/DIV
DN435 F06
Figure 6. Short-Circuit Event for the
Figure 5 Converter at 24V
IN
Conclusion
The LT3580 features a unique feedback architecture that
allows i t to be confi gured as an inver ting or noninver ting
converter. Now, the same device can be used to produce
regulated voltages of either polarity, allowing for a reducti on in inve nt or y co un t. It s ma ny ad dit io na l fe at ure s s uc h
as soft-start, adjustable switching frequency, shutdown,
synchronizing capability, confi gurable undervoltage
lockout, frequency foldback, external compensation and
wide input range simplify the design of inverting and
noninverting converters.
For applications help,
call (408) 432-1900, Ext. 3231
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
dn435f LT/TP 0208 387K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2008
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