LT1109
Micropower Low Cost
DC/DC Converter
Adjustable and Fixed 5V, 12V
FEATURES
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DESCRIPTIO
Uses Off-the-Shelf Inductors
Only 33µH Inductor Required
Low Cost
3-Lead TO-92, SO8, or 8-Pin DIP
Adjustable or Fixed 5V or 12V Output
120kHz Oscillator
Only Three External Components Required
320µA I
Q
1.6V Minimum Start-Up Voltage
Logic Controlled Shutdown
The LT1109 is a simple step-up DC/DC converter. Avail-
able in 8-pin SO, 3-lead TO-92 (fixed output only) or
miniDIP packages, the devices require only three external
components to construct a complete DC/DC converter.
Current drain is just 320µA at no load, making the device
ideal for cost-sensitive applications where standby cur-
rent must be kept to a minimum.
The LT1109-5 can deliver 5V at 100mA from a 3V input
and the LT1109-12 can deliver 12V at 60mA from a 5V
input. The 8-pin versions also feature a logic controlled
SHUTDOWN pin that turns off the oscillator when taken
low. The gated-oscillator design requires no frequency
compensation components. The high frequency 120kHz
oscillator permits the use of small surface mount induc-
tors and capacitors. For a 5V to 12V at 120mA converter,
see the LT1109A. Foa a 5V to 12V at 200mA converter
with 20µA shutdown current, see the LT1301.
APPLICATI
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Flash Memory VPP Generators
3V to 5V Converters
5V to 12V Converters
Disk Drives
PC Plug-In Cards
Peripherals
Battery-Powered Equipment
TYPICAL APPLICATI
L1
¦
33µH
3
V
IN
5V
1
SW
V
IN
SENSE
8
MBRS120T3
All Surface Mount
Flash Memory VPP Generator
15
Output Voltage
V
IN
= 5V
12
OUTPUT VOLTAGE (V)
V
OUT
12V
80mA
9
SHUTDOWN
5V/DIV
6
1ms/DIV
LT1109 • TA02
LT1109CS8-12
7
SHUTDOWN*
GND
4
SHUTDOWN
PROGRAM
LT1109 • TA01
+
C1**
22µF
16V
3
* 8-PIN PACKAGE ONLY
¦
L1 = SUMIDA CD54-330LC (I
OUT
= 80mA)
COILTRONICS CTX33-1 (80mA)
MURATA-ERIE LQH4N330K (I
OUT
= 50mA)
ISI LCS2414-330K (I
OUT
= 50mA)
**C1 = MATSUO 267M1602226 OR EQUIVALENT
0
0 10 20 30 40 50 60 70 80 90 100
OUTPUT CURRENT (mA)
LT1109 • TPC01
U
Flash Memory Program Output
V
OUT
5V/DIV
OV
UO
UO
1
LT1109
ABSOLUTE
AXI U
RATI GS
Operating Temperature Range ..................... 0°C to 70°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................. 300°C
Switch Current ........................................................ 1.2A
(Voltages Referred to GND Pin)
Supply Voltage (V
OUT
) .............................. – 0.4V to 20V
SW Pin Voltage .......................................... – 0.4V to 50V
SHUTDOWN Pin Voltage......................................... 6.0V
Maximum Power Dissipation ............................. 300mW
PACKAGE/ORDER I FOR ATIO
TOP VIEW
VIN 1
NC 2
SW 3
GND 4
8
7
6
5
SENSE
SHUTDOWN
NC
NC
V
IN
1
NC 2
SW 3
GND 4
N8 PACKAGE
8-LEAD PLASTIC DIP
LT1109 • PO102
S8 PACKAGE
8-LEAD PLASTIC SOIC
T
JMAX
= 100°C,
θ
JA
= 130°C/W
T
JMAX
= 100°C,
θ
JA
= 150°C/W
ORDER PART
NUMBER
LT1109CN8
LT1109CN8-5
LT1109CN8-12
ORDER PART
NUMBER
LT1109CS8
LT1109CS8-5
LT1109CS8-12
ELECTRICAL CHARACTERISTICS
T
A
= 25°C, V
IN
= 3V (LT1109CN8, LT1109CS8), unless otherwise specified.
SYMBOL
I
Q
PARAMETER
Quiescent Current
Minimum Start-Up Voltage
at V
OUT
Pin (Z Package)
V
IN
V
OUT
Input Voltage (N8, S8 Package)
Comparative Trip Point Voltage
Output Voltage
Comparator Hysteresis
Output Voltage Ripple
f
OSC
t
ON
DC
V
CESAT
Oscillator Frequency
q
q
CONDITIONS
Switch Off
q
LT1109
LT1109-5; 3V
≤
V
IN
≤
5V
LT1109-12; 3V
≤
V
IN
≤
12V
LT1109
LT1109-5
LT1109-12
Switch ON Time
q
Duty Cycle
Switch Saturation Voltage
Full Load
I
SW
= 500mA
LT1109-5: V
IN
= 3V; LT1109-12: V
IN
= 5V
2
U
U
W
W W
U
W
TOP VIEW
8
7
6
5
SENSE
SHUTDOWN
NC
NC
BOTTOM VIEW
3
V
OUT
2
SW
1
GND
LT1109 • PO103
Z PACKAGE
3-LEAD TO-92 PLASTIC
LT1109 • POI01
T
JMAX
= 100°C,
θ
JA
= 160°C/W
S8 PART
MARKING
1109
10905
10912
ORDER PART
NUMBER
LT1109CZ-5
LT1109CZ-12
MIN
1.6
3
1.20
4.75
11.52
TYP
320
MAX
550
UNITS
µA
V
V
q
q
q
q
q
q
1.25
5.00
12.00
8
25
60
1.30
5.25
12.55
12.5
50
120
140
150
5.3
5.5
60
0.7
0.8
V
V
V
mV
mV
mV
kHz
kHz
µs
µs
%
V
V
100
90
3.3
3.0
45
120
4.2
50
0.4
0.5
q
q
LT1109
ELECTRICAL CHARACTERISTICS
T
A
= 25°C, V
IN
= 3V (LT1109CN8, LT1109CS8), unless otherwise specified.
SYMBOL
V
IH
V
IL
I
IH
I
IL
PARAMETER
Switch Leakage Current
SHUTDOWN Pin High
SHUTDOWN Pin Low
SHUTDOWN Pin Input Current
SHUTDOWN Pin Input Current
CONDITIONS
V
SW
= 12V
N8, S8 Package
N8, S8 Package
N8, S8 Package, V
SHUTDOWN
= 4V
N8, S8 Package, V
SHUTDOWN
= 0V
q
q
q
q
MIN
2.0
TYP
1
MAX
10
0.8
10
20
UNITS
µA
V
V
µA
µA
The
q
denotes the specifications which apply over the full operating
temperature range.
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
160
140
OSCILLATOR FREQUENCY (kHz)
FREQUENCY (kHz)
124
122
120
118
116
114
112
120
SWITCH ON TIME (µs)
0
2
4
6
8
10 12 14 16 18 20
LT1109 • TPC03
100
80
60
–50
–25
0
25
50
TEMPERATURE (°C)
LT1109 • TPC02
Duty Cycle
70
65
60
700
600
500
DUTY CYCLE (%)
V
CESAT
(mV)
400
300
200
100
0
–50
50
45
40
35
30
–50
–25
0
25
50
75
100
V
CESAT
(V)
55
TEMPERATURE (°C)
LT1109 • TPC05
U W
75
Oscillator Frequency
130
128
126
6
7
Switch ON Time
5
4
100
110
INPUT VOLTAGE (V)
3
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
LT1109 • TPC04
Switch Saturation Voltage
1.2
I
SW
= 500mA
1.0
0.8
0.6
0.4
0.2
0
–25
0
25
50
75
100
Switch Saturation Voltage
V
IN
= 5V
T
A
= 25°C
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
SWITCH CURRENT (A)
LT1109 • TPC07
TEMPERATURE (°C)
LT1109 • TPC06
3
LT1109
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum/Maximum
Oscillator Frequency vs t
ON
160
150
OSCILLATOR FREQUENCY (kHz)
0°C
≤
T
A
≤
70°C
QUIESCENT CURRENT (µA)
140
130
120
110
100
90
80
2.0 2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
T
ON
(µs)
LT1109 • TPC08
QUIESCENT CURRENT (µA)
0
2
4
6
8
10 12 14 16 18 20
LT1109 • TPC09
T
A
= 25°C
BLOCK DIAGRA S
LT1109-5, -12 Z Package
V
OUT
V
IN
R2
250k
1.25V
REFERENCE
+
A1
–
COMPARATOR
R1
120kHz
OSCILLATOR
DRIVER
GND
LT1109-5: R1 = 83k
LT1109-12: R1 = 29k
LT1109 • TA03
LT1109Z OPERATIO
The LT1109Z-5 and LT1109Z-12 are fixed output voltage
step-up DC/DC converters in a 3-pin TO-92 package.
Power for internal regulator circuitry is taken from the
V
OUT
pin, a technique known as “bootstrapping.” Circuit
operation can be best understood by referring to the block
diagram. V
OUT
, attenuated by R1 and R2, is applied to the
negative input of comparator A1. When this voltage falls
below the 1.25V reference voltage, the oscillator is turned
on and the power switch Q1 cycles at the oscillator
4
U W
U
Quiescent Current
450
T
A
= 25°C
400
400
380
360
340
320
300
280
260
240
220
200
INPUT VOLTAGE (V)
Quiescent Current
350
300
250
200
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
LT1109 • TPC10
W
LT1109-5, -12 N8, S8 Package
FB SENSE
R2
250k
SW
SW
1.25V
REFERENCE
COMPARATOR
+
A1
120kHz
OSCILLATOR
DRIVER
R1
Q1
Q1
–
GND
SHUTDOWN
LT1109 • TA04
ON FIXED VERSION PIN 8 IS SENSE
ON ADJUSTABLE VERSION PIN 8 IS FB AND R1 AND R2 ARE DISCONNECTED
frequency of 120kHz. Switch cycling alternately builds
current in the inductor, then dumps it into the output
capacitor, increasing the output voltage. When A1’s nega-
tive input rises above 1.25V, it turns off the oscillator. A
small amount of hysteresis in A1 obviates the need for
frequency compensation circuitry. When Q1 is off, current
into the V
OUT
pin drops to just 320µA. Quiescent current
from the battery will be higher because the device oper-
ates off the
stepped-up
voltage.
LT1109
LT1109 S8 A D 8 OPERATIO
The 8-pin versions of the LT1109 have separate pins for
V
IN
and SENSE or FB and also have a SHUTDOWN pin.
Separating the device V
IN
pin from the SENSE pin allows
the device to be powered from the (lower) input voltage
rather than the (higher) output voltage. Although quies-
cent
current
remains constant, quiescent
power
will be
APPLICATI
S I FOR ATIO
Inductor Selection
A DC/DC converter operates by storing energy as mag-
netic flux in an inductor core, and then switching this
energy into the load. To operate as an efficient energy
transfer element, the inductor must fulfill three require-
ments. First, the inductance must be low enough for the
inductor to store adequate energy under the worst case
condition of minimum input voltage and switch-ON time.
The inductance must also be high enough so that maxi-
mum current ratings of the LT1109 and inductor are not
exceeded at the other worst case condition of maximum
input voltage and ON time. Additionally, the inductor core
must be able to store the required flux; i.e., it must not
saturate.
At power levels generally encountered with
LT1109 designs, small ferrite surface-mount inductors
will function well. Lastly, the inductor must have suffi-
ciently low DC resistance so that excessive power is not
lost as heat in the windings. Look for DCR values in the
inductors’ specification tables; values under 0.5Ω will give
best efficiency. An additional consideration is Electro-
Magnetic Interference (EMI). Toroid and pot core type
inductors are recommended in applications where EMI
must be kept to a minimum; for example, where there are
sensitive analog circuitry or transducers nearby. Rod core
types are a less expensive choice where EMI is not a
problem.
Specifying a proper inductor for an application requires
first establishing minimum and maximum input voltage,
output voltage, and output current. In a step-up converter,
the inductive events add to the input voltage to produce the
output voltage. Power required from the inductor is deter-
mined by
P
L
= (V
OUT
+ V
D
– V
IN
) (I
OUT
)
(01)
U
reduced by using the 8-pin version since the quiescent
current flows from a lower voltage source. The SHUT-
DOWN pin disables the oscillator when taken to a logic “0.”
If left floating or tied high, the converter operates nor-
mally. With SHUTDOWN low, quiescent current remains
at 320µA.
where V
D
is the diode drop (0.5V for a 1N5818 Schottky).
Energy required by the inductor per cycle must be equal or
greater than
P
L
F
OSC
U
W
U
U U
UO
(
02
)
in order for the converter to regulate the output.
When the switch is closed, current in the inductor builds
according to
–R't
V
IN
I
L
t
=
1– e
L
R'
()
(
03
)
where R' is the sum of the switch equivalent resistance
(0.8 typical at 25°C) and the inductor DC resistance. When
the drop across the switch is small compared to V
IN
, the
simple lossless equation
V
I
L
t
=
IN
t
L
()
(
04
)
can be used. These equations assume that at t = 0,
inductor current is zero. This situation is called “discon-
tinuous mode operation” in switching regulator parlance.
Setting “t” to the switch-ON time from the LT1109 speci-
fication table (typically 4.2µs) will yield I
PEAK
for a specific
“L” and V
IN
. Once I
PEAK
is known, energy in the inductor
at the end of the switch-ON time can be calculated as
E
L
=
1
2
LI
PEAK
2
(
05
)
E
L
must be greater than P
L
/F
OSC
for the converter to deliver
the required power. For best efficiency I
PEAK
should be
5
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