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PDF LT1615ES5-1 Data sheet ( Hoja de datos )
| Número de pieza | LT1615ES5-1 | |
| Descripción | Micropower Step-Up DC/DC Convertersin SOT-23 | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s Low Quiescent Current:
20µA in Active Mode
<1µA in Shutdown Mode
s Operates with VIN as Low as 1V
s Low VCESAT Switch: 250mV at 300mA
s Tiny 5-Lead SOT-23 Package
s Uses Small Surface Mount Components
s High Output Voltage: Up to 34V
U
APPLICATIO S
s LCD Bias
s Handheld Computers
s Battery Backup
s Digital Cameras
LT1615/LT1615-1
Micropower Step-Up
DC/DC Converters
in SOT-23
DESCRIPTIO
The LT®1615/LT1615-1 are micropower step-up DC/DC
converters in a 5-lead SOT-23 package. The LT1615 is
designed for higher power systems with a 350mA current
limit and an input voltage range of 1.2V to 15V, whereas
the LT1615-1 is intended for lower power and single-cell
applications with a 100mA current limit and an extended
input voltage range of 1V to 15V. Otherwise, the two
devices are functionally equivalent. Both devices feature a
quiescent current of only 20µA at no load, which further
reduces to 0.5µA in shutdown. A current limited, fixed off-
time control scheme conserves operating current, result-
ing in high efficiency over a broad range of load current.
The 36V switch allows high voltage outputs up to 34V to
be easily generated in a simple boost topology without the
use of costly transformers. The LT1615’s low off-time of
400ns permits the use of tiny, low profile inductors and
capacitors to minimize footprint and cost in space-con-
scious portable applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
1-Cell Li-Ion to 20V Converter for LCD Bias
VIN
2.5V TO 4.2V
L1
10µH
D1
C1
4.7µF
VIN SW
LT1615
SHDN
FB
GND
C1: TAIYO YUDEN LMK316BJ475
C2: TAIYO YUDEN TMK316BJ105
D1: MOTOROLA MBR0530
L1: MURATA LQH3C100K24
R1
2M
R2
130k
20V
12mA
C2
1µF
1615/-1 TA01
Efficiency
85
80
VIN = 4.2V
75
70 VIN = 2.5V
VIN = 3.3V
65
60
55
50
0.1
0.3 1 3 10 30
LOAD CURRENT (mA)
1615/-1 TA01a
1
1 page  
LT1615/LT1615-1
APPLICATIO S I FOR ATIO
Choosing an Inductor
Several recommended inductors that work well with the
LT1615 and LT1615-1 are listed in Table 1, although there
are many other manufacturers and devices that can be
used. Consult each manufacturer for more detailed infor-
mation and for their entire selection of related parts. Many
different sizes and shapes are available. Use the equations
and recommendations in the next few sections to find the
correct inductance value for your design.
Table 1. Recommended Inductors
PART
VALUE (µH) MAX DCR (Ω)
LQH3C4R7 4.7 0.26
LQH3C100 10 0.30
LQH3C220 22 0.92
CD43-4R7
CD43-100
CDRH4D18-4R7
CDRH4D18-100
4.7
10
4.7
10
0.11
0.18
0.16
0.20
DO1608-472
DO1608-103
DO1608-223
4.7
10
22
0.09
0.16
0.37
VENDOR
Murata
(714) 852-2001
www.murata.com
Sumida
(847) 956-0666
www.sumida.com
Coilcraft
(847) 639-6400
www.coilcraft.com
output voltages below 7V, a 4.7µH inductor is the best
choice, even though the equation above might specify a
smaller value. This is due to the inductor current over-
shoot that occurs when very small inductor values are
used (see Current Limit Overshoot section).
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD Bias application), a 21µH inductor is called for with
the above equation, but a 10µH inductor could be used
without excessive reduction in maximum output current.
Inductor Selection—SEPIC Regulator
The formula below calculates the approximate inductor
value to be used for a SEPIC regulator using the LT1615.
As for the boost inductor selection, a larger or smaller
value can be used.
L
=
2
VOUT +
ILIM
VD
tOFF
Inductor Selection—Boost Regulator
The formula below calculates the appropriate inductor
value to be used for a boost regulator using the LT1615 or
LT1615-1 (or at least provides a good starting point). This
value provides a good tradeoff in inductor size and system
performance. Pick a standard inductor close to this value.
A larger value can be used to slightly increase the available
output current, but limit it to around twice the value
calculated below, as too large of an inductance will in-
crease the output voltage ripple without providing much
additional output current. A smaller value can be used
(especially for systems with output voltages greater than
12V) to give a smaller physical size. Inductance can be
calculated as:
( )VOUT − VIN MIN + VD
L = ILIM tOFF
where VD = 0.4V (Schottky diode voltage), ILIM = 350mA or
100mA, and tOFF = 400ns; for designs with varying VIN
such as battery powered applications, use the minimum
VIN value in the above equation. For most systems with
Current Limit Overshoot
For the constant off-time control scheme of the LT1615,
the power switch is turned off only after the 350mA (or
100mA) current limit is reached. There is a 100ns delay
between the time when the current limit is reached and
when the switch actually turns off. During this delay, the
inductor current exceeds the current limit by a small
amount. The peak inductor current can be calculated by:
IPEAK
=
ILIM
+
VIN(MAX)
L
−
VSAT
100ns
Where VSAT = 0.25V (switch saturation voltage). The
current overshoot will be most evident for systems with
high input voltages and for systems where smaller induc-
tor values are used. This overshoot can be beneficial as it
helps increase the amount of available output current for
smaller inductor values. This will be the peak current seen
by the inductor (and the diode) during normal operation.
For designs using small inductance values (especially at
input voltages greater than 5V), the current limit over-
shoot can be quite high. Although it is internally current
5
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