November 2004 - 1.2MHz, 2A, Monolithic Boost Regulator Delivers High Power in Small Spaces

DESIGN IDEAS
1.2MHz, 2A, Monolithic
Boost Regulator Delivers
High Power in Small Spaces
Introduction
1.3
TA = 25°C
VOUT = 5V
COUT = 22µF
1.1 L = 2.2µH
IOUT(MAX) (A)
Even as cell phones, computers
and PDAs shrink, they require an
increasing number of power supply
voltages. The challenge, of course,
is how to squeeze more voltage converter circuits into less space—without
sacrificing power or efficiency. Boost
converters, in particular, are becoming
more prevalent, as main supply voltages are lowered to accommodate core
logic circuits, while many components
require a higher supply voltage. The
LTC3426 boost converter meets the
challenge with converter-shrinking
features, including a low RDS(ON) monolithic switch, internal compensation
and a 3mm × 3mm × 1mm ThinSOT
package. The LTC3426 operates at
high frequency and therefore works
with small, low cost inductors and
tiny ceramic capacitors.
The LTC3426 incorporates a
constant frequency current mode
architecture, which is low noise and
provides fast transient response. With
VOUT
500mV/DIV
IOUT
250mA/DIV
0.9
0.7
IL
500mA/DIV
0.5
0.3
1.8
2.2
2.6
3
VIN (V)
3.4
3.8
VIN = 1.8V
VOUT = 3.3V
COUT = 22µF
L = 2.5µH
4.2
Figure 1. High current outputs are attainable
with minimum 2A switch limit.
3-Phase Buck Controller for
Intel VRM9/VRM10 with
Active Voltage Positioning ........... 23
a minimum peak current level of 2A,
the LTC3426 delivers up to 900mA of
output current. Figure 1 shows the
converters output current capability
at 5V as a function of VIN with peak
inductor current at 2A. An input supply range of 1.6V to 4.3V makes the
LTC3426 ideal for local supplies ranging from 2.5V to 5V. Efficiencies above
90% are made possible by its low 0.11Ω
(typ.) RDS(ON) internal switch.
There is no need for an external
compensation network because the
LTC3426 has a built-in loop compensation network. This reduces
size, lowers overall cost and greatly
simplifies the design process. Figure 2
shows the VOUT response to a 250mAto-500mA load step in a 1.8V to 3.3V
application.
Redundant 2-Wire Bus for
High Reliability Systems ............. 25
VIN
2.5V
DESIGN IDEAS
1.2MHz, 2A, Monolithic Boost
Regulator Delivers High Power
in Small Spaces........................... 22
Kevin Ohlson
by Xiaoyong Zhang
John Ziegler
–48V Backplane Impedance
Analyzer Takes the Guesswork Out
of Sizing Clippers and Snubbers .. 27
Mitchell Lee
Compact Power Supply Drives
TFT-LCD and LED Backlight ......... 31
Dongyan Zhou
Tiny, Low Noise Boost and
Inverter Solutions ........................ 33
Eric Young
22
by Kevin Ohlson
L1
2.5µH
D1
SW
VOUT
VIN
C1
10µF
OFF ON
LTC3426
SHDN
GND
FB
R1
75k
1%
R2
44.2k
1%
C1: TDK C1608X5R0J106
C2: TAIYO YUDEN JMK316BJ266
D1: ON SEMICONDUCTOR MBRM120LT3
L1: SUMIDA CDRH5D28-2R5 2
Figure 3. Application circuit for
3.3V output delivers 800mA
VOUT
3.3V
800mA
C2
22µF
40µs/DIV
Figure 2. Fast transient response
to load step of 250mA to 500mA
The Shutdown input can be driven
with standard CMOS logic above either
VIN or VOUT (up to 6V maximum). Quiescent current in shutdown is less than
1µA. A simple resistive pull-up to VIN
configures the LTC3426 for continuous operation when VIN is present.
3.3V Output
800mA Converter
Some applications require local 3.3V
supplies which are utilized periodically
yet are required to deliver high currents. The LTC3426 is an ideal solution
which requires minimal board space
and, when in shutdown, draws less
than 1µA quiescent current. Figure 3
shows a circuit which delivers up to
800mA at 3.3V from a 2.5V input.
This circuit also works with VIN down
to 1.8V with 750mA output. The output voltage is easily programmed by
changing the feedback ratio of R1 and
R2 according to the formula:
R1 

VOUT = 1.22V •  1 + 
 R2 
Lithium-Ion 5V
Boost Converter
Some portable applications still require a 5V supply. Figure 4 shows a
circuit which operates from a single
Lithium-Ion battery and delivers at
continued on page 32
Linear Technology Magazine • November 2004
DESIGN IDEAS
at around 42V to protect the internal
power devices.
Proper layout is important to achieve
the best performance. Paths that
carry high switching current should
be kept short and wide to minimize
the parasitic inductance. In the boost
regulator, the switching loop includes
the internal power switch, the Schottky
diode (internal or external), and the
output capacitor. In the negative
output regulator, the switching loop
includes the internal power switch,
the flying capacitor between the SW2
and D2 pins, and the internal Schottky
diode.
Connect the output capacitors of
the AVDD and LED switchers directly
to the PGND14 pin before returning to
the ground plane. Connect the output
capacitor of the VON switcher to the
PGND23 pin before returning to the
ground plane. Also connect the bottom
feedback resistors to the AGND pin.
Connect the PGND14, PGND23 and
AGND pins to the top layer ground
pad underneath the exposed copper
ground on the backside of the IC.
The exposed copper helps to reduce
thermal resistance. Multiple vias into
ground layers can be placed on the
ground pad directly underneath the
part to conduct the heat away from
the part.
LTC3426, continued from page 22
Component Selection
current should be greater than 1A.
A low forward voltage Schottky diode
reduces power loss in the converter
circuit.
Layout Considerations
least 750mA from a VIN as low as 3V.
When fully charged to 4.2V, over 1A
can be supplied. The photograph of
a demonstration board in Figure 5
shows just how small the board area
is for this application, 10mm × 12mm.
Tiny ceramic bypass capacitors and
surface mount inductors keep the
design small.
Figure 6 shows efficiency exceeding
90% and remaining greater than 85%
over a load range from 10mA to 900mA
with a fully charged battery.
LTC3426
SHDN
FB
R1
95.3k
1%
R2
30.9k
1%
VOUT
5V
750mA AT 3V
C2
22µF
80
VIN = 3V
75
70
65
60
50
Figure 4. Compact application circuit for VOUT at 5V
further eases the burden of heavy
capacitive loads by providing strong
pull-up currents during rising edges to
reduce the rise time. Thanks to these
two features, the LTC4302 enables the
implementation of much larger 2-wire
bus systems than are possible with a
simple unbuffered multiplexer.
VIN = 4.2V
85
55
C1: TDK C1608X5R0J475M
C2: TAIYO YUDEN JMK316BJ226ML
D1: ON SEMICONDUCTOR MBR120VLSFT1
L1: SUMIDA CDRH4D28-2R2 2
LTC4302, continued from page 26
EFFICIENCY (%)
VOUT
GND
32
100
90
SW
OFF ON
The addition of the LTC3426 to Linear
Technology’s high performance boost
converter family allows the designer
to deliver high current levels with
minimal board space. An on chip
switch and internal loop compensation
reduces component count to provide
an inexpensive solution for spot regulation applications.
D1
VIN
C1
10µF
Conclusion
95
L1
2.2µH
VIN
3V TO 4.2V
The LTC3426 requires just a few external components to accomodate various
VIN and VOUT combinations. Selecting
the proper inductor is important to
optimize converter performance and
efficiency. An inductor with low DCR
increases efficiency and reduces selfheating. Since the inductor conducts
the DC output current plus half the
peak-to-peak switching current, select
an inductor with a minimum DC rating of 2A. To minimize VOUT ripple,
use low ESR X5R ceramic capacitors.
The average Schottky diode forward
current is equal to the DC output
current therefore the diode average
Figure 5. Photograph of demo
board of circuit in Figure
4—board area is 10mm × 12mm
For further information on any
of the devices mentioned in this
issue of Linear Technology, use
the reader service card or call the
LTC literature service number:
1-800-4-LINEAR
Ask for the pertinent data sheets
and Application Notes.
1
10
100
LOAD CURRENT (mA)
1000
Figure 6. Up to 92% efficiency in Lithium-Ion
battery to 5V output applications
Impedance Analyzer, continued from page 30
assume that either the inductance is
well damped, or it is shunted by large
value capacitances.
Notes
1. This subject is treated in some detail in the
LTC1647 data sheet, Figures 9, 10, and 11
inclusive.
2. An hp 5210A Frequency Meter or any common
counter gives adequate accuracy for most measurements.
Linear Technology Magazine • November 2004