LT1617/LT1617-1
Micropower Inverting
DC/DC Converters
in SOT-23
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
Tiny 5-Lead SOT-23 Package
APPLICATIO S
s
s
s
s
LCD Bias
Handheld Computers
Battery Backup
Digital Cameras
The LT
®
1617/LT1617-1 are micropower inverting DC/DC
converters in a 5-lead SOT-23 package. The LT1617 is
designed for higher power systems with a 350mA current
limit and an input voltage range of 1.2V to 15V, whereas
the LT1617-1 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 without the use of costly transformers.
The LT1617’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.
TYPICAL APPLICATION
1-Cell Li-Ion to –15V Inverting Converter
L1
22µH
5
V
IN
LT1617
4
C1
4.7µF
SHDN
GND
2
NFB
3
24.9k
C3
0.22µF
1
SW
D1
267k
C2
4.7µF
L2
22µH
80
–15V
12mA
EFFICIENCY (%)
V
IN
2.5V TO 4.2V
75
70
65
60
55
C1: TAIYO YUDEN LMK316BJ475
C2: TAIYO YUDEN EMK316BJ475
C3: TAIYO YUDEN TMK316BJ224
L1, L2: MURATA LQH3C220K34
D1: MOTOROLA MBR0530
1617/-1 TA01
50
0.1
U
Efficiency
V
IN
= 4.2V
V
IN
= 2.5V
1
10
LOAD CURRENT (mA)
30
1617/-1 TA01a
U
U
1
LT1617/LT1617-1
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SW 1
GND 2
NFB 3
4 SHDN
5 V
IN
V
IN
, SHDN Voltage ................................................... 15V
SW Voltage .............................................................. 36V
NFB Voltage ............................................................. – 3V
Current into NFB 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
ORDER PART
NUMBER
LT1617ES5
LT1617ES5-1
S5 PART MARKING
LTKF
LTKA
S5 PACKAGE
5-LEAD PLASTIC SOT-23
T
JMAX
= 125°C,
θ
JA
= 256°C/W
Consult factory for Industrial and Military grade parts.
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
NFB
= –1.23V
CONDITIONS
LT1617-1
LT1617
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
µA
ns
mV
mV
mA
mA
µA
µA
V
V
µA
30
1
–1.255
0.1
2.7
120
350
125
400
3
12
0.25
–1.205
–1.23
8
0.05
q
1.3
2
400
85
250
I
SW
= 60mA (LT1617-1)
I
SW
= 300mA (LT1617)
LT1617-1
LT1617
V
SHDN
= 1.2V
V
SHDN
= 5V
0.9
75
300
100
350
2
8
0.01
5
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1617 and LT1617-1 are guaranteed to meet 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.
Note 3:
Bias current flows out of the NFB pin.
2
U
W
U
U
W W
W
LT1617/LT1617-1
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)
300
250
200
150
LT1617
V
IN
= 1.2V
SHUTDOWN PIN CURRENT (µA)
SWITCH CURRENT LIMIT (mA)
450
400
350
300
250
–50
V
IN
= 1.2V
V
IN
= 12V
–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.
NFB (Pin 3):
Feedback Pin. Set the output voltage by
selecting values for R1 and R2 (see Figure 1):
R1
=
V
OUT
−
1.23
1.23
+
2 • 10
−
6
R2
U W
75
Feedback Pin Voltage and
Bias Current
–1.25
5
25
Quiescent Current
V
FB
= 1.23V
NOT SWITCHING
–1.24
VOLTAGE
–1.23
4
BIAS CURRENT (µA)
23
3
21
V
IN
= 12V
19
V
IN
= 1.2V
17
–1.22
CURRENT
–1.21
2
1
1617/-1 G01
1617/-1 G02
1617/-1 G03
Switch Current Limit
V
IN
= 12V
25
Shutdown Pin Current
20
15
25°C
10
100°C
5
LT1617-1
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
1617/-1 G06
1617/-1 G04
1617/-1 G05
U
U
U
(
)
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.
3
LT1617/LT1617-1
BLOCK DIAGRA
V
IN
V
OUT
R1
(EXTERNAL)
R2
(EXTERNAL)
NFB
OPERATIO
The LT1617 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 NFB pin is slightly below
–1.23V, comparator A1 disables most of the internal
circuitry. Output current is then provided by capacitor C2,
which slowly discharges until the voltage at the NFB pin
goes above the hysteresis point of A1 (typical hysteresis
at the NFB pin is 8mV). A1 then enables the internal
circuitry, turns on power switch Q3, and the current in
4
W
L1
C1
5
C3
L2
V
OUT
V
IN
4
SHDN
1
SW
D1
C2
R5
80k
R6
80k
+
–
Q1
Q2
X10
R3
60k
R4
280k
3
A1
ENABLE
400ns
ONE-SHOT
DRIVER
RESET
Q3
+
0.12Ω
A2
–
42mV*
2
GND
1617/-1 BD
* 12mV FOR LT1617-1
Figure 1. LT1617 Block Diagram
U
inductors L1 and L2 begins ramping up. Once the switch
current reaches 350mA, comparator A2 resets the one-
shot, which turns off Q3 for 400ns. L2 continues to deliver
current to the output while Q3 is off. Q3 turns on again and
the inductor currents ramp back up until the switch
current reaches 350mA, then A2 again resets the one-
shot. This switching action continues until the output
voltage is charged up (until the NFB pin reaches –1.23V),
then A1 turns off the internal circuitry and the cycle
repeats. The LT1617-1 operates in the same manner,
except the switch current is limited to 100mA (the A2
reference voltage is 12mV instead of 42mV).
LT1617/LT1617-1
APPLICATIO S I FOR ATIO
Choosing an Inductor
Several recommended inductors that work well with the
LT1617 and LT1617-1 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
D01608-223
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
VENDOR
Murata
(714) 852-2001
www.murata.com
Sumida
(847) 956-0666
www.sumida.com
Coilcraft
(847) 639-6400
www.coilcraft.com
Inductor Selection—Inverting Regulator
The formula below calculates the appropriate inductor
value to be used for an inverting regulator using the
LT1617 or LT1617-1 (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 (both inductors should be the same 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:
V
OUT
+
V
D
L
=
2
I
LIM
t
OFF
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA or
100mA, and t
OFF
= 400ns.
U
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD bias application), a 47µH inductor is called for with
the above equation, but a 10µH or 22µH inductor could be
used without excessive reduction in maximum output
current.
Inductor Selection—Inverting Charge Pump Regulator
For the inverting regulator, the voltage seen by the internal
power switch is equal to the sum of the absolute value of
the input and output voltages, so that generating high
output voltages from a high input voltage source will often
exceed the 36V maximum switch rating. For instance, a
12V to – 30V converter using the inverting topology would
generate 42V on the SW pin, exceeding its maximum
rating. For such a system, an inverting charge pump is the
best topology.
The formula below calculates the approximate inductor
value to be used for an inverting charge pump regulator
using the LT1617. As for the boost inductor selection, a
larger or smaller value can be used. For designs with
varying V
IN
such as battery powered applications, use the
minimum V
IN
value in the equation below.
L
=
V
OUT
−
V
IN
(
MIN
)
+
V
D
I
LIM
t
OFF
W
U
U
Current Limit Overshoot
For the constant off-time control scheme of the LT1617,
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-
5
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