3mm × 3mm Monolithic DC/DC Boost/Inverting Converters with 65V Power Switches

3mm × 3mm Monolithic DC/DC Boost/Inverting Converters
with 65V Power Switches
Joshua Moore
The vast array of power supply rails required by modern electronics has popularized the
use of compact, easy-to-use monolithic DC/DC converters, such as the LT3580 boost/
inverting converter. The LT8580, LT8570, and LT8570-1 build on the success of the LT3580,
increasing the switch voltage to 65V and the input voltage to 40V, while retaining features
and pin compatibility. The LT8580 includes a 65V, 1A power switch, whereas the LT8570 and
LT8570-1 step the switch current limit down to 0.5A and 0.25A, respectively. Various current
options enable application optimization—a monolithic converter sized for specific demands
can be smaller and more efficient than one designed for greater load currents. Optimized
sizing for current limit helps limit input and output current in the event of a short or failure.
In addition to the new options, all
devices in the family—LT®3580, LT8580,
LT8570, and LT8570-1—are pin compatible. With a few simple component
changes, the same PCB layout can be
used for a range of applications, allowing fast turnaround design changes and
reuse. The LT8580, LT8570, and LT8570-1
retain the LT3580’s features such as single
resistor feedback, for both positive and
negative output voltages, overtemperature
protection, frequency foldback, and an
external clock input pin. And, like the
LT3580, many features are user adjustable, including oscillator frequency,
soft-start, UVLO and output voltage. All
are available in thermally enhanced 8-pin
3mm × 3mm DFN or 8-pin MSE packages.
65V POWER SWITCH
The LT8580/LT8570/LT8570-1 incorporate
an internal 65V power switch, for applications with high input and output voltages.
Furthermore, VIN is capable of handling
up to 40V. This can greatly simplify
applications. For example, Figure 1 shows
the necessary circuitry to create a 48V
output with the LT3580; the LT8570’s 65V
switch simplifies the circuit in Figure 2.
Table 1. Feature comparison of monolithic pin-compatible boost/inverting DC/DC converters
LT3580
LT8570
LT8570-1
LT8580
Input Range
2.5V to 32V
2.55V to 40V
2.55V to 40V
2.55V to 40V
Max Switch Voltage
42V
65V
65V
65V
Max Switch Current
2A
1A
0.5A
0.25A
Integrated Power Switch
L
L
L
L
Frequency Foldback
L
L
L
L
External Clock Input
L
L
L
L
Overtemperature Protection
L
L
L
L
Positive And Negative Output Voltages
L
L
L
L
Single Resistor Feedback
L
L
L
L
Packages
8-Pin MSE
8-Pin MSE
8-Pin MSE
8-Pin MSE
14 | July 2015 : LT Journal of Analog Innovation
design features
All devices in the family—LT3580, LT8580, LT8570, and
LT8570-1—are pin compatible. With a few simple component
changes, the same PCB layout can be used for a wide variety of
applications, allowing fast turnaround design changes and reuse.
D3
D2
R1
L1
VIN
12V
VOUT
48V
COUT2
D1
C1
L1
VIN
12V
RSHDN
VOUT
48V
RSHDN
VIN
SW
SHDN
FB
VIN
RFB
COUT1
LT3580
CIN
SYNC
GND
SYNC
RC
RT
CF
CC
CSS
The LT8580, LT8570 and LT8570-1 include
a number of configuration options. The
oscillator frequency can be adjusted from
200kHz to 1.5MHz. While lower switching frequencies tend to be more efficient,
higher switching frequencies offer smaller
solution sizes. Also, choice of oscillator
frequency may be useful for avoiding
interference with sensitive RF circuitry.
15µH
FBX
VIN
56.2k
SYNC
6.04k
47pF
0.22µF
FBX
3.3nF
Figure 3. LT8580 configured as 5V input to 12V
output boost converter
130k
1µF
LT8570-1
0.47µF
SS
VOUT
12V
50mA
SW
SHDN
4.7µF
VC
GND
A final configuration option is softstart. By varying the soft-start capacitor, the user can adjust the rate of
increase of the inductor current. The
faster the inductor current increases, the
faster the output rises during start-up.
However, allowing the inductor current
to increase slowly reduces output voltage overshoot and avoids large input
transient currents during start-up.
10k
130k
LT8580
SYNC
CF
CC
CSS
47µH
VIN
5V
SW
SHDN
2.2µF
Another configuration option is undervoltage lockout, which, for most applications, is configurable with just one
resistor from VIN to SHDN. This allows
the parts to be used in situations where
source impedance may be high, the
source may ramp slowly or where it is
desirable that the part not discharge
the source below some threshold.
VOUT
12V
200mA
10k
VIN
RC
SS
GND
Figure 2. LT8570 configured for 48V output
USER CONFIGURABILITY
VIN
5V
COUT
VC
RT
Figure 1. LT3580 configured for 48V output
RFBX
FBX
LT8570
CIN
SS
RT
SW
SHDN
VC
RT
RT
D1
RT
VC
GND
56.2k
6.04k
SS
47pF
0.22µF
3.3nF
Figure 4. LT8570-1 configured as a 5V input to 12V output
boost converter
July 2015 : LT Journal of Analog Innovation | 15
VIN
9V TO 16V
UP TO 40V
TRANSIENT
•
C1
1µF
L1
22µH
D1
L2
22µH
487k
SW
SHDN
COUT
4.7µF
VC
GND
84.5k
CIN
4.7µF
16.2k
SS
0.22µF
BOOST CONVERTER
COUT
4.7µF
VC
GND
13.7k
SS
47pF
0.22µF
10nF
Figure 6. LT8580 configured as 5V–40V In to −15V out
dual inductor inverting converter
LT8580 required 0805 size capacitors, the
LT8570-1 can use 0603 size capacitors.
SEPIC CONVERTERS
The SEPIC topology creates a positive
voltage where the input voltage may
be less than or greater than the output
voltage. Due to a lack of DC path from
input to output, it also offers output
disconnect, so that there is no output
voltage if the converter is shut down.
Output disconnect makes the converter
resistant to damage in case of output
shorts. The application in Figure 5 shows
the LT8580 configured to produce 12V
from an input range of 9V to 16V, and
able to survive 40V transients on VIN .
90
640
80
560
70
480
60
400
50
320
40
240
160
30
EFFICIENCY
POWER LOSS
20
10
0
50
100
150
LOAD CURRENT (mA)
200
POWER LOSS (mW)
The boost topology creates an output
voltage greater than the input voltage.
Since the boost converter is the simplest
topology for LT8580, LT8570, and LT8570-1,
it can clearly illustrate how the different
current limits affect solution size. Figure 3
shows the LT8580 in a 12V out boost converter and Figure 4 shows the LT8570-1
in the same converter. Note that the
only significant circuit changes required
between the two are the inductor, the
input capacitor, and the output capacitor.
Both applications use similar inductors in
the Würth WE-LQS family, but the LT8580
requires an inductor that is 5mm × 5mm,
while LT8570-1 can use an inductor
that is only 3mm × 3mm. This reduces
the inductor footprint from 25mm2
to 9mm2. At the same time, the height
drops from 4mm to 1.5mm. Also, where
182k
L1, L2: COILCRAFT 22µH MSD7342-223
D1: CENTRAL SEMI CMMSH1-60
CIN: 4.7µF, 50V, 1206, X5R
COUT : 4.7µF, 25V, 1206, X7R
C1: 1µF, 100V, 0805, X7S
EFFICIENCY (%)
The FBX pin on LT8580/LT8570/LT8570-1
makes setting output voltage easy for both
inverting and noninverting topologies. In
both cases, only a single resistor from VOUT
to FBX is needed to set the output voltage—the converter topology determines
whether the output is positive or negative.
16 | July 2015 : LT Journal of Analog Innovation
SYNC
113k
Figure 5. LT8580 configured as 9V–16V In to 12V
output SEPIC converter
SIMPLE AND EASY OUTPUT VOLTAGE
CONFIGURATION
FBX
LT8580
1nF
L1, L2: WÜRTH 22µH WE-DD 744877220
D1: DIODES INC. DFLS1100
CIN: 4.7µF, 50V, 1206, X7R
COUT : 4.7µF, 25V, 1206, X7R
C1: 1µF, 100V, 0805, X7S
VOUT
–15V
90mA (VIN = 5V)
210mA (VIN = 12V)
420mA (VIN = 40V)
•
SW
SHDN
RT
22pF
L2
22µH
D1
VIN
LT8580
RT
C1
1µF
L1
22µH
10k
130k
FBX
SYNC
•
VIN
5V TO 40V
•
VIN
CIN
4.7µF
VOUT
12V
240mA
80
0
Figure 7. Efficiency and power loss for Figure 6 with
VIN = 12V
DUAL INDUCTOR INVERTING
CONVERTER
The dual inductor inverting topology
creates a negative voltage from a positive
input voltage, which may be greater than
or less than the magnitude of the output
voltage. This topology, like the SEPIC,
has output disconnect. In addition, this
topology tends to have a quieter output
than the boost or SEPIC, since L2 is in
series with the output. The converter in
Figure 6 shows the LT8580 configured as
a dual inductor inverting converter with
a −15V output, and Figure 7 shows the
efficiency and power loss versus load.
CONCLUSION
The popular LT3580 monolithic boost/
inverting converter has been joined by
the pin-compatible LT8580, LT8570, and
LT8570-1 converters, which add current
options and higher voltages, while
retaining the features of the LT3580.
These new options provide an additional
means of optimizing a power supply
for a given application. Depending on
the intended load, solution size and
part counts can be reduced. By retaining pin compatibility, transition within
the LT3580, LT8580, LT8570 and LT8570-1
family of parts is easy, allowing simple
design changes and PCB reuse. n
Similar pages