LT1615/LT1615-1
Micropower Step-Up
DC/DC Converters
in ThinSOT
FEATURES
s
DESCRIPTIO
s
s
s
s
s
Low Quiescent Current:
20
µ
A in Active Mode
<1
µ
A in Shutdown Mode
Operates with V
IN
as Low as 1V
Low V
CESAT
Switch: 250mV at 300mA
Uses Small Surface Mount Components
High Output Voltage: Up to 34V
Low Profile (1mm) ThinSOT
TM
Package
APPLICATIO S
s
s
s
s
LCD Bias
Handheld Computers
Battery Backup
Digital Cameras
The LT
®
1615/LT1615-1 are micropower step-up DC/DC
converters in a 5-lead low profile (1mm) ThinSOT pack-
age. 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, resulting 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-conscious portable applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
L1
10µH
1-Cell Li-Ion to 20V Converter for LCD Bias
85
D1
20V
12mA
SW
LT1615
SHDN
C1
4.7µF
GND
FB
R2
130k
1615/-1 TA01
V
IN
2.5V TO 4.2V
V
IN
80
V
IN
= 4.2V
75
EFFICIENCY (%)
R1
2M
C2
1µF
70
65
60
55
50
0.1
C1: TAIYO YUDEN LMK316BJ475
C2: TAIYO YUDEN TMK316BJ105
D1: MOTOROLA MBR0530
L1: MURATA LQH3C100K24
0.3
U
Efficiency
V
IN
= 2.5V
V
IN
= 3.3V
1
3
10
LOAD CURRENT (mA)
30
1615/-1 TA01a
U
U
sn16151 16151fas
1
LT1615/LT1615-1
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
SW 1
GND 2
FB 3
4 SHDN
5 V
IN
V
IN
, SHDN Voltage ................................................... 15V
SW Voltage .............................................................. 36V
FB Voltage .................................................................V
IN
Current into FB Pin ................................................. 1mA
Junction Temperature ........................................... 125°C
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
LT1615ES5
LT1615ES5-1
LT1615IS5
LT1615IS5-1
S5 PART MARKING
LTIZ
LTKH
LTXZ
LTBHT
S5 PACKAGE
5-LEAD PLASTIC SOT-23
T
JMAX
= 125°C,
θ
JA
= 256°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
Quiescent Current
FB Comparator Trip Point
FB Comparator Hysteresis
Output Voltage Line Regulation
FB Pin Bias Current (Note 3)
Switch Off Time
Switch V
CESAT
Switch Current Limit
SHDN Pin Current
SHDN Input Voltage High
SHDN Input Voltage Low
Switch Leakage Current
Switch Off, V
SW
= 5V
1.2V < V
IN
< 12V
V
FB
= 1.23V
V
FB
> 1V
V
FB
< 0.6V
CONDITIONS
LT1615-1
LT1615
Not Switching
V
SHDN
= 0V
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.2V, V
SHDN
= 1.2V unless otherwise noted.
MIN
TYP
MAX
1.0
1.2
20
q
UNITS
V
V
µA
µA
V
mV
%/V
nA
ns
µs
30
1
1.255
0.1
80
1.205
1.23
8
0.05
q
30
400
1.5
85
250
75
300
100
350
2
8
0.9
I
SW
= 70mA (LT1615-1)
I
SW
= 300mA (LT1615)
LT1615-1
LT1615
V
SHDN
= 1.2V
V
SHDN
= 5V
120
350
125
400
3
12
0.25
0.01
5
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1615E and LT1615E-1 are guaranteed to meet performance
specifications from 0°C to 70°C. Specifications over the
– 40°C to 85°C operating temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT1615I/LT1615I-1 is guaranteed to meet performance specifications over
the –40°C to 85°C operating temperature range.
Note 3:
Bias current flows into the FB pin.
sn16151 16151fas
2
U
mV
mV
mA
mA
µA
µA
V
V
µA
W
U
U
W W
W
LT1615/LT1615-1
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage
(V
CESAT
)
0.60
0.55
QUIESCENT CURRENT (µA)
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1.20
–50
–25
0
25
50
TEMPERATURE (°C)
75
0
100
15
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
FEEDBACK VOLTAGE (V)
SWITCH VOLTAGE (V)
I
SWITCH
= 500mA
I
SWITCH
= 300mA
Switch Off Time
550
500
400
350
SWITCH OFF TIME (ns)
450
400
350
300
250
–50
V
IN
= 1.2V
V
IN
= 12V
PEAK CURRENT (mA)
300
250
200
150
LT1615
V
IN
= 1.2V
SHUTDOWN PIN CURRENT (µA)
–25
0
25
50
TEMPERATURE (°C)
PI FU CTIO S
SW (Pin 1):
Switch Pin. This is the collector of the internal
NPN power switch. Minimize the metal trace area con-
nected to this pin to minimize EMI.
GND (Pin 2):
Ground. Tie this pin directly to the local
ground plane.
FB (Pin 3):
Feedback Pin. Set the output voltage by
selecting values for R1 and R2 (see Figure 1):
SHDN (Pin 4):
Shutdown Pin. Tie this pin to 0.9V or higher
to enable the device. Tie below 0.25V to turn off the device.
V
IN
(Pin 5):
Input Supply Pin. Bypass this pin with a
capacitor as close to the device as possible.
V
OUT
R1
=
R2
−
1
1.23
sn16151 16151fas
U W
75
Feedback Pin Voltage and
Bias Current
1.25
50
25
Quiescent Current
V
FB
= 1.23V
NOT SWITCHING
1.24
VOLTAGE
1.23
40
BIAS CURRENT (nA)
23
30
21
V
IN
= 12V
19
V
IN
= 1.2V
17
1.22
CURRENT
20
1.21
10
1615/-1 G01
1615/-1 G02
1615/-1 G03
Switch Current Limit
V
IN
= 12V
25
Shutdown Pin Current
20
15
25°C
10
100°C
5
LT1615-1
100
V
IN
= 12V
V
IN
= 1.2V
50
100
0
–50
0
–25
0
25
50
TEMPERATURE (°C)
75
100
0
5
10
SHUTDOWN PIN VOLTAGE (V)
15
1615/-1 G03
1615/-1 G04
1615/-1 G05
U
U
U
3
LT1615/LT1615-1
BLOCK DIAGRA
V
IN
V
OUT
R1
(EXTERNAL)
R2
(EXTERNAL)
OPERATIO
The LT1615 uses a constant off-time control scheme to
provide high efficiencies over a wide range of output
current. Operation can be best understood by referring to
the block diagram in Figure 1. Q1 and Q2 along with R3 and
R4 form a bandgap reference used to regulate the output
voltage. When the voltage at the FB pin is slightly above
1.23V, comparator A1 disables most of the internal cir-
cuitry. Output current is then provided by capacitor C2,
which slowly discharges until the voltage at the FB pin
drops below the lower hysteresis point of A1 (typical
hysteresis at the FB pin is 8mV). A1 then enables the
internal circuitry, turns on power switch Q3, and the
current in inductor L1 begins ramping up. Once the switch
current reaches 350mA, comparator A2 resets the one-
shot, which turns off Q3 for 400ns. L1 then delivers
current to the output through diode D1 as the inductor
current ramps down. Q3 turns on again and the inductor
4
W
L1
C1
5
D1
V
OUT
V
IN
4
SHDN
1
SW
C2
R5
40k
R6
40k
+
–
FB
3
Q1
Q2
X10
R3
30k
R4
140k
A1
ENABLE
400ns
ONE-SHOT
DRIVER
RESET
Q3
+
0.12Ω
A2
–
42mV*
2
GND
1615/-1 BD
* 12mV FOR LT1615-1
Figure 1. LT1615 Block Diagram
U
current ramps back up to 350mA, then A2 resets the one-
shot, again allowing L1 to deliver current to the output.
This switching action continues until the output voltage is
charged up (until the FB pin reaches 1.23V), then A1 turns
off the internal circuitry and the cycle repeats. The LT1615
contains additional circuitry to provide protection during
start-up and under short-circuit conditions. When the FB
pin voltage is less than approximately 600mV, the switch
off-time is increased to 1.5µs and the current limit is
reduced to around 250mA (70% of its normal value). This
reduces the average inductor current and helps minimize
the power dissipation in the LT1615 power switch and in
the external inductor and diode. The LT1615-1 operates in
the same manner, except the switch current is limited to
100mA (the A2 reference voltage is 12mV instead of
42mV).
sn16151 16151fas
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 (
µ
H)
MAX DCR (
Ω
)
LQH3C4R7
LQH3C100
LQH3C220
CD43-4R7
CD43-100
CDRH4D18-4R7
CDRH4D18-100
DO1608-472
DO1608-103
DO1608-223
4.7
10
22
4.7
10
4.7
10
4.7
10
22
0.26
0.30
0.92
0.11
0.18
0.16
0.20
0.09
0.16
0.37
VENDOR
Murata
(814) 237-1431
www.murata.com
Sumida
(847) 956-0666
www.sumida.com
Coilcraft
(847) 639-6400
www.coilcraft.com
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:
L
=
V
OUT
−
V
IN
(
MIN
)
+
V
D
I
LIM
t
OFF
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA or
100mA, and t
OFF
= 400ns; for designs with varying V
IN
such as battery powered applications, use the minimum
U
V
IN
value in the above equation. For most systems with
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.
W
U
U
V
+
V
L
=
2
OUT D
I
LIM
t
OFF
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:
I
PEAK
V
IN(MAX)
−
V
SAT
=
I
LIM
+
100ns
L
Where V
SAT
= 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
sn16151 16151fas
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