DN435 - Single Resistor Sets Positive or Negative Output for DC/DC Converter

Single Resistor Sets Positive or Negative Output
for DC/DC Converter – Design Note 435
Jesus Rosales
Introduction
Many electronic subsystems, such as VFD (vacuum
flourecent display), TFT-LCD, GPS or DSL applications,
require more than just a simple step-down or step-up
DC/DC converter. They may require inverting, noninverting
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 configuration. It contains an integrated 2A, 42V switch and packs many 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 configurations such as boost,
SEPIC, flyback and Cuk topologies.
Sensing Output Voltage Has Never Been Easier
The LT3580 has a novel FB pin architecture that simplifies the design of inverting and noninverting topologies.
Namely, there are two internal error amplifiers; one senses
positive outputs and the other negative. Additionally, the
LT3580 has integrated the ground side feedback resistor
to minimize component count. To illustrate the benefits,
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
frequency, especially if the converter is used in an RF communications product that is sensitive to spectral noise in
certain frequency bands. Also, if the area available for a
converter is limited, operating at higher frequencies allows
02/08/435
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 efficiency. 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 supply is current limited, has a high source
impedance or ramps up/down slowly, the SHDN pin can
be configured to provide undervoltage lockout through
a simple resistor divider from VIN to ground.
Boost Converters
A boost converter, shown in Figure 1, produces a positive output voltage always higher than its input. Figure 2
shows the efficiency graph for the boost converter in
Figure 1 at a 4.2V input.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
VIN
3V TO
10V
L1
D1
6.8μH
DFLS220L
CDRH5D16-6R8
C2
4.7μF
16V
5
8
3
4
VIN
SW
SHDN
R2
130k
FB
LT3580
SYNC
SS
7
C6
0.1μF
RT
VC
GND
6
9
R6
47.5k
1
VOUT
12V
300mA AT 3VIN
500mA AT 4.2VIN
C4
10μF
16V
2
R5
20k
C7
1nF
DN435 F01
Figure 1. 3V-10V to 12V, 300mA Boost 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
input; it offers output disconnect and short-circuit protection 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.
EFFICIENCY (%)
90
80
70
60
0
50 100 150 200 250 300 350 400 450 500
OUTPUT CURRENT (mA)
DN435 F02
Figure 2. Efficiency for the Figure 1 Converter at 4.2VIN
L1
8.2μH
SD25-8R2
VIN
5V TO
24V
3
C2
4.7μF
25V
5
8
C1
1μF
50V
D2
DFLS140L
4
SW
VIN
SHDN
FB
LT3580
SYNC
SS
7
C6
0.1μF
RT
L2
15μH
SD14-150
VC
GND
6
9
R6
47.5k
VOUT
–12V
360mA
R2
143k
1
VIN
9V TO
24V
3
C2
2.2μF
25V
5
8
C4
10μF
16V
2
C1
1μF
35V
L1
10μH
SD25-100
SHDN
FB
LT3580
SYNC
SS
R5
20k
C7
2.2nF
L2
10μH
SD14-100
4
SW
VIN
7
C6
0.1μF
VC
RT
D2
DFLS140
GND
6
9
R6
56.2k
VOUT
10.5V
600mA
R2
110k
1
C4
10μF
16V
2
R5
20k
C7
2.2nF
DN435 F05
DN435 F03
Figure 3. 5V-24V to –12V, 350mA Cuk Converter
Figure 5. 9V-24V to 10.5V, 600mA SEPIC Converter
90
EFFICIENCY (%)
VOUT
5V/DIV
80
VSW
10V/DIV
70
2μs/DIV
60
0
40
80 120 160 200 240 280 320 360
OUTPUT CURRENT (mA)
DN435 F06
Figure 6. Short-Circuit Event for the
Figure 5 Converter at 24VIN
DN435 F04
Figure 4. Efficiency for the Figure 3 Converter at 5VIN
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 efficiency graph for the Cuk converter in
Figure 3 at a 5V input.
Conclusion
The LT3580 features a unique feedback architecture that
allows it to be configured as an inverting or noninverting
converter. Now, the same device can be used to produce
regulated voltages of either polarity, allowing for a reduction in inventory count. Its many additional features such
as soft-start, adjustable switching frequency, shutdown,
synchronizing capability, configurable undervoltage
lockout, frequency foldback, external compensation and
wide input range simplify the design of inverting and
noninverting converters.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 3231
www.linear.com
Linear Technology Corporation
dn435f LT/TP 0208 387K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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