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LT1110

Datasheet LT1110 Datasheet (Linear Technology)

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L T1110
LOAD CURRENT (mA)
0
EFFICIENCY (%)
50
60
70
80
85
90
10 20 30 40
LT1110 • TA02
75
65
55
5152535
V
IN
= 1.50V
VIN = 1.25V
VIN = 1.00V
Micropower
DC-DC Converter
Adjustable and Fixed 5V, 12V
EATU
F
■
Operates at Supply Voltages From 1.0V to 30V
■
Works in Step-Up or Step-Down Mode
■
Only Three External Off-the-Shelf Components
RE
S
Required
■
Low-Battery Detector Comparator On-Chip
■
User-Adjustable Current Limit
■
Internal 1A Power Switch
■
Fixed or Adjustable Output Voltage Versions
■
Space-Saving 8-Pin MiniDIP or S8 Package
U
O
PPLICATI
A
■
Pagers
■
Cameras
■
Single-Cell to 5V Converters
■
Battery Backup Supplies
■
Laptop and Palmtop Computers
■
Cellular Telephones
■
Portable Instruments
■
Laser Diode Drivers
■
Hand-Held Inventory Computers
S
DUESCRIPTIO
The LT1110 is a versatile micropower DC-DC converter. The device requires only three external components to deliver a fixed output of 5V or 12V. The very low minimum supply voltage of 1.0V allows the use of the LT1110 in applications where the primary power source is a single cell. An on-chip auxiliary gain block can function as a low battery detector or linear post regulator.
The device can easily be configured as a step-up or step-down converter, although for most step-down applications or input sources greater than 3V, the LT1111 is recommended. Switch current limiting is user-adjustable by adding a single external resistor. Unique reverse battery protection circuitry limits reverse current to safe, non­destructive levels at reverse supply voltages up to 1.6V.
A
PPLICATITYPICAL
All Surface Mount
Single Cell to 5V Converter
SUMIDA
CD54-470K
47µH
1
I
LIM
1.5V AA CELL*
OPERATES WITH CELL VOLTAGE 1.0V
*ADD 10 F DECOUPLING CAPACITOR IF BATTERY
µ
IS MORE THAN 2" AWAY FROM LT1110.
LT1110-5
GND SW2
54
V
SENSE
IN
2
SW1
≥
3
8
U
O
MBRS120T3
+
5V
15µF TANTALUM
LT1110 • TA01
Efficiency
1
LT1110
WU
U
PACKAGE
/
O
RDER I FOR ATIO
A
W
O
LUTEXI T
S
A
WUW
ARB
U G
I
S
Supply Voltage, Step-Up Mode................................ 15V
Supply Voltage, Step-Down Mode ........................... 36V
SW1 Pin Voltage...................................................... 50V
SW2 Pin Voltage.........................................– 0.5V to V
Feedback Pin Voltage (LT1110) .............................. 5.5V
Switch Current........................................................ 1.5A
Maximum Power Dissipation ............................. 500mW
Operating Temperature Range .....................0°C to 70°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec.).................300°C
TOP VIEW
I
1
LIM
V
2
IN
Consult factory for Industrial and Military grade parts.
IN
SW1
3
SW2
4
N8 PACKAGE
8-LEAD PLASTIC DIP
*FIXED VERSIONS
T
= 90°C, θJA = 130°C/W
JMAX
I
1
LIM
V
2
IN
SW1
3
SW2
4
S8 PACKAGE
8-LEAD PLASTIC SOIC
*FIXED VERSIONS
T
= 90°C, θJA = 150°C/W
JMAX
TOP VIEW
FB (SENSE)*
8
SET
7
A0
6
GND
5
FB (SENSE)*
8
SET
7
A0
6
GND
5
ORDER PART
NUMBER
LT1110CN8 LT1110CN8-5 LT1110CN8-12
LT1110CS8 LT1110CS8-5 LT1110CS8-12
S8 PART MARKING
1110 11105 11012
LECTRICAL C CHARA TERIST
E
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
Q
V
IN
V
OUT
f
OSC
DC Duty Cycle Full Load (VFB < V t
ON
I
FB
I
SET
V
AO
Quiescent Current Switch Off ● 300 µA Input Voltage Step-Up Mode ● 1.15 12.6 V
Step-Down Mode ● 30 V Comparator Trip Point Voltage LT1110 (Note 1) ● 210 220 230 mV Output Sense Voltage LT1110-5 (Note 2) ● 4.75 5.00 5.25 V
LT1110-12 (Note 2) ● 11.4 12.00 12.6 V Comparator Hysteresis LT1110 ● 48 mV Output Hysteresis LT1110-5 ● 90 180 mV
LT1110-12 ● 200 400 mV Oscillator Frequency ● 52 70 90 kHz
Switch ON Time ● 7.5 10 12.5 µs Feedback Pin Bias Current LT1110, VFB = 0V ● 70 150 nA Set Pin Bias Current V AO Output Low I Reference Line Regulation 1.0V ≤ VIN ≤ 1.5V ● 0.35 1.0 %/V
AO
1.5V ≤ VIN ≤ 12V ● 0.05 0.1 %/V
ICS
= V
SET
REF
= –300µA, V
TA = 25°C, VIN = 1.5V, unless otherwise noted.
1.0 12.6 V
) ● 62 69 78 %
REF
● 100 300 nA
= 150mV ● 0.15 0.4 V
SET
2
L T1110
LECTRICAL C CHARA TERIST
E
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
CESAT
A
V
I
REV
I
LIM
I
LEAK
V
SW2
The ● denotes the specifications which apply over the full operating temperature range.
Note 1: This specification guarantees that both the high and low trip point of the comparator fall within the 210mV to 230mV range.
Note 2: This specification guarantees that the output voltage of the fixed versions will always fall within the specified range. The waveform at the sense pin will exhibit a sawtooth shape due to the comparator hysteresis.
Switch Saturation Voltage VIN = 1.5V, ISW = 400mA 300 400 mV Step-Up Mode ● 600 mV
VIN = 1.5V, ISW = 500mA 400 550 mV
VIN = 5V, ISW = 1A 700 1000 mV A2 Error Amp Gain RL = 100kΩ (Note 3) ● 1000 5000 V/V Reverse Battery Current (Note 4) 750 mA Current Limit 220Ω Between I Current Limit Temperature – 0.3 %/°C
Coefficient Switch OFF Leakage Current Measured at SW1 Pin 1 10 µA Maximum Excursion Below GND I
SW1
TA = 25°C, V
ICS
LIM
≤ 10µA, Switch Off – 400 – 350 mV
= 1.5V, unless otherwise noted.
IN
● 750 mV
and V
IN
Note 3: 100kΩ resistor connected between a 5V source and the AO pin. Note 4: The LT1110 is guaranteed to withstand continuous application of
+1.6V applied to the GND and SW2 pins while VIN, I grounded.
400 mA
, and SW1 pins are
LIM
UW
Y
PICA
100
90
80
70
60
OSCILLATOR FREQUENCY (KHz)
50
40
LPER
F
O
R
AT
Oscillator Frequency Oscillator Frequency Switch On Time
–50
–25 0
TEMPERATURE (°C)
25
50
75
LT1110 • TPC01
CCHARA TERIST
E
C
80 78 76
74 72 70
68
FREQUENCY (KHz)
66 64 62
100
60
36
0
ICS
15 18 21
912
INPUT VOLTAGE (V)
242730
LT1110 • TPC02
14
13
12
11
10
ON TIME (µs)
9
8
7
–50 –25 0 25
50 75 100
TEMPERATURE (°C)
LT1110 • TPC03
3
LT1110
I (A)
0
0
V (V)
0.2
0.4
0.6
1.2
1.4
0.2 0.4 0.8 1.2
LT1110 • TPC06
1.0
1.4 1.6
SWITCH
CESAT
V
IN
= 1.0V
V = 1.2V
IN
V
IN
= 1.5V
V
IN
= 5.0V
V = 2.0V
IN
0.6 1.0
0.8
V
IN
= 3.0V
INPUT VOLTAGE (V)
QUIESCENT CURRENT (µA)
0
LT1110 • TPC09
3
400
380 360 340 320
280 260
6
240 220
200
300
912151821242730
R
LIM
(Ω)
SWITCH CURRENT (A)
10
LT1110 • TPC12
100
1.5
1.3
1.1
0.9
1000
0.7
0.5
0.3
0.1
STEP-DOWN MODE V
IN
= 12V
UW
Y
PICA
78 76 74 72 70 68 66
DUTY CYCLE (%)
64
62 60
58
–50 –25 0 25
1.4
1.2
1.0
0.8
0.6
ON VOLTAGE (V)
0.4
0.2
0
LPER
F
O
R
AT
CCHARA TERIST
E
C
ICS
Saturation Voltage
Duty Cycle Switch Saturation Voltage Step-Up Mode
50 75 100
TEMPERATURE (°C)
LT1110 • TPC04
500
VIN = 1.5V
450 400 350 300
(mV)
250
CESAT
200
V
150 100
50
0
= 500mA
I
SW
–50 – 25 0 25
50 75 100
TEMPERATURE (°C)
LT1110 • TPC05
Switch On Voltage Minimum/Maximum Frequency vs Step-Down Mode On Time Quiescent Current
VIN = 12V
0 0.2 0.4 0.6
I
(A)
SWITCH
0.8 1.0
LT1110 • TPC07
100
95 90
85 80
75 70
 65 60 55
OSCILLATOR FREQUENCY (KHz)
50 45
40
7
9
8
10
SWITCH ON TIME (µs)
0°C ≤ TA ≤ 70°C
11
12
LT1110 • TPC08
13
500
450
400
350
300
250
200
QUIESCENT CURRENT (µA)
150
100
–50
4
Quiescent Current R
–25
0 25 50 75 100
TEMPERATURE (°C)
LT1110 • TPC10
1.5
1.3
1.1
0.9
0.7
0.5
SWITCH CURRENT (A)
0.3
0.1
Maximum Switch Current vs Maximum Switch Current vs
Step-Up R
LIM
STEP-UP MODE
≤ 5V
V
IN
10
100
R
(Ω)
LIM
1000
LT1110 • TPC11
Step-Down
LIM
UW
Y
PICA
160
140
120
100
80
60
BIAS CURRENT (nA)
40
20
0
–50
LPER
Set Pin Bias Current FB Pin Bias Current Reference Voltage
–25
0 25 50 75 100
TEMPERATURE (°C)
F
O
R
AT
LT1110 • TPC13
CCHARA TERIST
E
C
120 110
100
90 80 70
60 50 40
BIAS CURRENT (nA)
30 20 10
0
–25
–50
ICS
0 25 50 75 100
TEMPERATURE (°C)
LT1110 • TPC14
226
224
222
220
(mV)
REF
218
V
216
214
212
–50 –25 0 25
50 75 100
TEMPERATURE (°C)
L T1110
LT1110 • TPC15
U
PI
I
FUUC
(Pin 1): Connect this pin to VIN for normal use. Where
LIM
TI
O
U
S
lower current limit is desired, connect a resistor between I
and VIN. A 220Ω resistor will limit the switch current
LIM
to approximately 400mA.
V
(Pin 2): Input supply voltage.
IN
SW1 (Pin 3): Collector of power transistor. For step-up
mode connect to inductor/diode. For step-down mode connect to VIN.
SW2 (Pin 4): Emitter of power transistor. For step-up
mode connect to ground. For step-down mode connect to inductor/diode. This pin must never be allowed to go more than a Schottky diode drop below ground.
W
LT
1110
BLOCK
IDAGRA
V
IN
SET
+
A2
–
GAIN BLOCK/ERROR AMP
GND (Pin 5): Ground. AO (Pin 6): Auxiliary Gain Block (GB) output. Open collector,
can sink 300µA. SET (Pin 7): GB input. GB is an op amp with positive input
connected to SET pin and negative input connected to 220mV reference.
FB/SENSE (Pin 8): On the LT1110 (adjustable) this pin goes to the comparator input. On the LT1110-5 and LT1110-12, this pin goes to the internal application resistor that sets output voltage.
AO
220mV
REFERENCE
GND
I
LIM
A1
COMPARATOR
FB
OSCILLATOR
DRIVER
SW1
Q1
SW2
LT1110 • BD01
5
LT1110
-
VmV
R
R
OUT
=
()
+
 
 
220
2
1
101.()
1110
LT
OPER
AT
U
O
I
The oscillator is set internally for 10µs ON time and 5µs OFF time, optimizing the device for step-up circuits where V
≈ 3VIN, e.g., 1.5V to 5V. Other step-up ratios as well
OUT
as step-down (buck) converters are possible at slight losses in maximum achievable power output.
A2 is a versatile gain block that can serve as a low battery detector, a linear post regulator, or drive an under voltage lockout circuit. The negative input of A2 is internally connected to the 220mV reference. An external resistor divider from VIN to GND provides the trip point for A2. The AO output can sink 300µA (use a 47k resistor pull up to +5V). This line can signal a microcontroller that the battery voltage has dropped below the preset level. To prevent the gain block from operating in its linear region, a 2MΩ resistor can be connected from AO to SET. This provides positive feedback.
A resistor connected between the I
pin and VIN adjusts
LIM
maximum switch current. When the switch current ex­ceeds the set value, the switch is turned off. This feature is especially useful when small inductance values are used with high input voltages. If the internal current limit of 1.5A is desired, I
should be tied directly to VIN. Propagation
LIM
delay through the current limit circuitry is about 700ns. In step-up mode, SW2 is connected to ground and SW1
drives the inductor. In step-down mode, SW1 is con­nected to VIN and SW2 drives the inductor. Output voltage is set by the following equation in either step-up or step­down modes where R1 is connected from FB to GND and R2 is connected from V
OUT
to FB.
V
IN
-5, -12
SET
R1
300kΩ
+
–
R2
1110
LT
220mV
REF
GND
6
BLOCK
A2
GAIN BLOCK/ERROR AMP
A1
COMPARATOR
SENSE
IDAGRA
AO
OSCILLATOR
LT1110-5:
LT1110-12:
I
LIM
DRIVER
R1 = 13.8kΩ R2 = 5.6kΩ
LT1110 • BD02
W
SW1
SW2
U
LT
1110
-5, -12
OPEROAT
The LT1110-5 and LT1110-12 fixed output voltage ver­sions have the gain setting resistors on-chip. Only three external components are required to construct a 5V or 12V output converter. 16µA flows through R1 and R2 in the LT1110-5, and 39µA flows in the LT1110-12. This current represents a load and the converter must cycle from time
Q1
to time to maintain the proper output voltage. Output ripple, inherently present in gated oscillator designs, will typically run around 90mV for the LT1110-5 and 200mV for the LT1110-12 with the proper inductor/capacitor selection. This output ripple can be reduced considerably by using the gain block amp as a pre-amplifier in front of the FB pin. See the Applications section for details.
I
L T1110
P
f
L
OSC
()02
It
V
R
e
L
IN
Rt
L
()
'
–()
–'
=
 
 
103
It
V
L
t
L
IN
()
= ()04
ELI
L
PEAK
=
1 2
052()
P V V V mA mW
L
=+
()()
=12 0 5 4 5 120 960 06.–. .()
P
f
mW
kHz
J
L
OSC
==
960
70
13 7 07.. ()µ
U
O
PPLICATI
A
Inductor Selection — General
saturate
LT1110 based designs, small surface mount ferrite core units with saturation current ratings in the 300mA to 1A range and DCR less than 0.4Ω (depending on application) are adequate. Lastly, the inductor must have sufficiently low DC resistance so excessive power is not lost as heat in the windings. An additional consideration is Electro­Magnetic 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. Minimum and maximum input voltage, output voltage and output current must be established before an inductor can be selected.
Inductor Selection — Step-Up Converter
In a step-up, or boost converter (Figure 4), power gener­ated by the inductor makes up the difference between input and output. Power required from the inductor is determined by
. At power levels generally encountered with
S
I FOR ATIO
WU
U
Energy required by the inductor per cycle must be equal or greater than
in order for the converter to regulate the output. When the switch is closed, current in the inductor builds
according to
where R' is the sum of the switch equivalent resistance (0.8Ω typical at 25°C) and the inductor DC resistance. When the drop across the switch is small compared to VIN, the simple lossless equation
can be used. These equations assume that at t = 0, inductor current is zero. This situation is called “discon­tinuous mode operation” in switching regulator parlance. Setting “t” to the switch ON time from the LT1110 speci­fication table (typically 10µs) will yield I “L” and VIN. Once I at the end of the switch ON time can be calculated as
EL must be greater than PL/f the required power. For best efficiency I kept to 1A or less. Higher switch currents will cause excessive drop across the switch resulting in reduced efficiency. In general, switch current should be held to as low a value as possible in order to keep switch, diode and inductor losses at a minimum.
As an example, suppose 12V at 120mA is to be generated from a 4.5V to 8V input. Recalling equation (01),
is known, energy in the inductor
PEAK
for the converter to deliver
OSC
for a specific
PEAK
should be
PEAK
PV VV I
=+
()()
L OUT D IN
where VD is the diode drop (0.5V for a 1N5818 Schottky).
–()01
MIN
OUT
Energy required from the inductor is
7
LT1110
L
VVV
I
t
IN MIN SW OUT
PEAK
ON
=•
––
()11
I
mA
mA
PEAK
=
()
+
+
 
 
=
2 250
069
505
91505
498 12
.
.
–. .
.( )
L
mA
sH=•=
9155
498
10 50 13
–.–
.()µµ
PV VI
L OUT D OUT
=+
()()
|| . ()14
U
O
PPLICATI
A
Picking an inductor value of 47µH with 0.2Ω DCR results in a peak switch current of
V
45
I
PEAK
Substituting I
EHAJ
L
Since 17.5µJ > 13.7µJ, the 47µH inductor will work. This trial-and-error approach can be used to select the opti­mum inductor. Keep in mind the switch current maximum rating of 1.5A. If the calculated peak current exceeds this, an external power transistor can be used.
A resistor can be added in series with the I switch current limit. The resistor should be picked such that the calculated I Maximum Switch Current (from Typical Performance Characteristic curves). Then, as VIN increases, switch current is held constant, resulting in increasing efficiency.
.
=−
W
10
.
PEAK
1
=
47 0 862 17 5 09
µµ...()
()( )
2
S
I FOR ATIO
−•
10 10
1 862 08
 
.
emA
47
into Equation 05 results in
at minimum VIN is equal to the
PEAK
s
Wm
H
m
2
=
WU
=
 
.()
pin to invoke
LIM
U
V
= output voltage
OUT
VIN = minimum input voltage
VSW is actually a function of switch current which is in turn a function of VIN, L, time and V be used for VSW as a very conservative value.
Once I
where tON = switch ON time (10µs). Next, the current limit resistor R
I
PEAK
of this resistor keeps maximum switch current constant as the input voltage is increased.
As an example, suppose 5V at 250mA is to be generated from a 9V to 18V input. Recalling Equation (10),
is known, inductor value can be derived from
PEAK
from the R
Step-Down Mode curve. The addition
LIM
. To simplify, 1.5V can
OUT
is selected to give
LIM
Inductor Selection — Step-Down Converter
The step-down case (Figure 5) differs from the step-up in that the inductor current flows through the load during both the charge and discharge periods of the inductor. Current through the switch should be limited to ~800mA in this mode. Higher current can be obtained by using an external switch (see Figure 6). The I successful operation over varying inputs.
After establishing output voltage, output current and input voltage range, peak switch current can be calculated by the formula
I
2
I
PEAK
where DC = duty cycle (0.69)
VSW = switch drop in step-down mode VD = diode drop (0.5V for a 1N5818) I
OUT OUT D
=
DC
= output current
OUT
VV
+
VV V
IN SW D
pin is the key to
LIM
 
+
10–()
Next, inductor value is calculated using Equation (11)
Use the next lowest standard value (47µH). Then pick R
R
= 82Ω.
LIM
Inductor Selection — Positive-to-Negative Converter
In this mode the switch is arranged in common collector or step-down mode. The switch drop can be modeled as a 0.75V source in series with a 0.65Ω resistor. When the
from the curve. For I
LIM
= 500mA,
PEAK
8
L T1110
PPLICATI
A
U
O
S
I FOR ATIO
WU
U
switch closes, current in the inductor builds according to
V
L
I
+
()
L
=
–()
115
'
R
where R' = 0.65Ω + DCR
e
L
 
L
Rt
–'
VL = VIN – 0.75V
As an example, suppose –5V at 75mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14),
PVVmAmW
=− +
()()
L
=||..()5 0 5 75 413 16
Energy required from the inductor is
P
f
OSC
413
L
mW
==
70
kHz
59 17.. ()µ
J
Picking an inductor value of 56µH with 0.2Ω DCR results in a peak switch current of
capacitors provide still better performance at more ex­pense. We recommend OS-CON capacitors from Sanyo Corporation (San Diego, CA). These units are physically quite small and have extremely low ESR. To illustrate, Figures 1, 2 and 3 show the output voltage of an LT1110 based converter with three 100µF capacitors. The peak switch current is 500mA in all cases. Figure 1 shows a Sprague 501D, 25V aluminum capacitor. V
jumps by
OUT
over 120mV when the switch turns off, followed by a drop in voltage as the inductor dumps into the capacitor. This works out to be an ESR of over 240mΩ. Figure 2 shows the same circuit, but with a Sprague 150D, 20V tantalum capacitor replacing the aluminum unit. Output jump is now about 35mV, corresponding to an ESR of 70mΩ. Figure 3 shows the circuit with a 16V OS-CON unit. ESR is now only 20mΩ.
s
•
085 10
–.
45 075
.–.
VV
I
PEAK
Substituting I
EHAJ
()
=
065 02
..
+
ΩΩ
()
PEAK
1
=
56 0 621 10 8 19
µµ...()
()( )
L
2
1 621 18
–.()
 
into Equation (04) results in
Ωµ
emA
56
2
=
H
µ
=
 
Since 10.8µJ > 5.9µJ, the 56µH inductor will work. With this relatively small input range, R
necessary and the I
pin can be tied directly to VIN. As in
LIM
is not usually
LIM
the step-down case, peak switch current should be limited to ~800mA.
Capacitor Selection
50mV/DIV
50mV/DIV
50mV/DIV
5 s/DIV
µ
Figure 1. Aluminum
5 s/DIV
µ
Figure 2. Tantalum
5 s/DIV
µ
Figure 3. OS-CON
LT1110 • TA19
LT1110 • TA20
LT1110 • TA21
9
LT1110
V
R R
mV
OUT
=+
 
 
()
1
2 1
220 21.()
LT1110 • TA15
GND
SW2
SW1
LIM
I
IN
V
R3 220
FB
V
OUT
+
C2
+
C1
D1 1N5818
V
IN
R2
R1
L1
Ω
LT1110
I
V
VV
L
t
PEAK
IN
SW OUT
ON
=
−−
.()22
PPLICATI
A
U
O
S
I FOR ATIO
WU
U
Diode Selection
Speed, forward drop, and leakage current are the three main considerations in selecting a catch diode for LT1110 converters. General purpose rectifiers such as the 1N4001 are
unsuitable
for use in
any
switching regulator applica-
tion. Although they are rated at 1A, the switching time of a 1N4001 is in the 10µs-50µs range. At best, efficiency will be severely compromised when these diodes are used; at worst, the circuit may not work at all. Most LT1110 circuits will be well served by a 1N5818 Schottky diode, or its surface mount equivalent, the MBRS130T3. The combina­tion of 500mV forward drop at 1A current, fast turn ON and turn OFF time, and 4µA to 10µA leakage current fit nicely with LT1110 requirements. At peak switch currents of 100mA or less, a 1N4148 signal diode may be used. This diode has leakage current in the 1nA-5nA range at 25°C and lower cost than a 1N5818. (You can also use them to get your circuit up and running, but beware of destroying the diode at 1A switch currents.)
Immediately after switch turn off, the SW1 voltage pin starts to rise because current cannot instantaneously stop flowing in L1. When the voltage reaches V
+ VD, the
OUT
inductor current flows through D1 into C1, increasing V
. This action is repeated as needed by the LT1110 to
OUT
keep VFB at the internal reference voltage of 220mV. R1 and R2 set the output voltage according to the formula
Step-Down (Buck Mode) Operation
A step-down DC-DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1110 based step-down converter is shown in Figure 5.
Step-Up (Boost Mode) Operation
A step-up DC-DC converter delivers an output voltage higher than the input voltage. Step-up converters are short circuit protected since there is a DC path from input to output.
The usual step-up configuration for the LT1110 is shown in Figure 4. The LT1110 first pulls SW1 low causing VIN – V
to appear across L1. A current then builds up in L1.
CESAT
At the end of the switch ON time the current in L1 is1:
V
IN
t
= ()20
ON
L
L1
R3*
V
I
LIM
GND SW2
Figure 4. Step-Up Mode Hookup.
LT1110
IN
SW1
FB
D1
R2
+
C1
R1
LT1110 • TA14
10
I
PEA K
V
IN
* = OPTIONAL
V
not
OUT
Figure 5. Step-Down Mode Hookup
When the switch turns on, SW2 pulls up to V puts a voltage across L1 equal to VIN – VSW – V
– VSW. This
IN
OUT
, causing a current to build up in L1. At the end of the switch ON time, the current in L1 is equal to
DIODE
. The voltage at SW2 must never be allowed to go
MUST BE A SCHOTTKY
below – 0.5V. A silicon diode such as the 1N4933 will allow SW2 to go to –0.8V, causing potentially destructive power
Note 1: This simple expression neglects the effects of switch and coil resistance. This is taken into account in the “Inductor Selection” section.
L T1110
VVV V
SW R SAT
=+ ≈
1
09 24.. ()
PPLICATI
A
U
O
S
I FOR ATIO
WU
U
dissipation inside the LT1110. Output voltage is deter­mined by
V
OUT
=+
1
R
2
R
1
mV
220 23.()
()
R3 programs switch current limit. This is especially im­portant in applications where the input varies over a wide range. Without R3, the switch stays on for a fixed time each cycle. Under certain conditions the current in L1 can build up to excessive levels, exceeding the switch rating and/or saturating the inductor. The 220Ω resistor pro­grams the switch to turn off when the current reaches approximately 800mA. When using the LT1110 in step­down mode, output voltage should be limited to 6.2V or less. Higher output voltages can be accommodated by inserting a 1N5818 diode in series with the SW2 pin (anode connected to SW2).
Higher Current Step-Down Operation
pin. This also lowers the maximum drop across R1
LIM
to VBE – VD, increasing efficiency. As shown, switch current is limited to 2A. Inductor value can be calculated based on formulas in the “Inductor Selection Step-Down
R1
V
GND
0.3Ω
IN
LT1110
I
SW2
L
SW1
FB
V
25V
MAX
IN
+
C2
Figure 6. Q1 Permits Higher-Current Switching. LT1110 Functions as Controller.
MJE210 OR
ZETEX ZTX789A
R2 220
R3
330
Q1
L1
D1 1N5821
R4
R5
V
= 220mV (1 + )
OUT
V
OUT
+
C1
R4 R5
LT1110 • TA16
Converter” section with the following conservative ex­pression for VSW:
R2 provides a current path to turn off Q1. R3 provides base drive to Q1. R4 and R5 set output voltage.
Inverting Configurations
The LT1110 can be configured as a positive-to-negative converter (Figure 7), or a negative-to-positive converter (Figure 8). In Figure 7, the arrangement is very similar to a step-down, except that the high side of the feedback is referred to ground. This level shifts the output negative. As in the step-down mode, D1 must be a Schottky diode, and V
should be less than 6.2V. More negative out-
OUT
put voltages can be accommodated as in the prior section.
+V
IN
R3
V
I
LIM
+
C2
IN
LT1110
GND
SW1
SW2
FB
L1
D1 1N5818
+
R1
C1
R2
–V
OUT
LT1110 • TA03
Figure 7. Positive-to-Negative Converter
In Figure 8, the input is negative while the output is positive. In this configuration, the magnitude of the input voltage can be higher or lower than the output voltage. A level shift, provided by the PNP transistor, supplies proper polarity feedback information to the regulator.
L1
V
I
+
C2
AO
GND SW2
–V
IN
LIM
IN
SW1
LT1110
FB
Figure 8. Negative-to-Positive Converter
D1
+V
+
C1
R2
R1
V
= 220mV + 0.6V
( )
OUT
R2
OUT
R1
2N3906
LT1110 • TA04
11
LT1110
PPLICATI
A
Using the I
LIM
Pin
U
O
S
I FOR ATIO
WU
U
The LT1110 switch can be programmed to turn off at a set switch current, a feature not found on competing devices. This enables the input to vary over a wide range without exceeding the maximum switch rating or saturating the inductor. Consider the case where analysis shows the LT1110 must operate at an 800mA peak switch current with a 2.0V input. If VIN rises to 4V, peak current will rise to 1.6A, exceeding the maximum switch current rating. With the proper resistor selected (see the “Maximum Switch Current vs R
” characteristic), the switch current
LIM
will be limited to 800mA, even if the input voltage increases.
Another situation where the I
feature is useful occurs
LIM
when the device goes into continuous mode operation. This occurs in step-up mode when
V
+
OUT DIODE
V
<
VV DC
−
IN SW
−11
25.()
switch ON times less than 3µs. Resistor values program­ming switch ON time for 800ns or less will cause spurious response in the switch circuitry although the device will still maintain output regulation.
I
L
ON
SWITCH
OFF
Figure 9. No Current Limit Causes Large Inductor Current Build-Up
PROGRAMMED CURRENT LIMIT
I
L
LT1110 • TA05
When the input and output voltages satisfy this relation­ship, inductor current does not go to zero during the switch OFF time. When the switch turns on again, the current ramp starts from the non-zero current level in the inductor just prior to switch turn on. As shown in Figure 9, the inductor current increases to a high level before the comparator turns off the oscillator. This high current can cause excessive output ripple and requires oversizing the output capacitor and inductor. With the I
feature,
LIM
however, the switch current turns off at a programmed level as shown in Figure 10, keeping output ripple to a minimum.
Figure 11 details current limit circuitry. Sense transistor Q1, whose base and emitter are paralleled with power switch Q2, is ratioed such that approximately 0.5% of Q2’s collector current flows in Q1’s collector. This current is passed through internal 80Ω resistor R1 and out through the I between I switch current flows to develop a VBE across R1 + R
pin. The value of the external resistor connected
LIM
and VIN set the current limit. When sufficient
LIM
LIM
, Q3 turns on and injects current into the oscillator, turning off the switch. Delay through this circuitry is approximately 800ns. The current trip point becomes less accurate for
ON
SWITCH
OFF
LT1110 • TA06
Figure 10. Current Limit Keeps Inductor Current Under Control
R
V
IN
Q3
OSCILLATOR
Figure 11. LT1110 Current Limit Circuitry
(EXTERNAL)
DRIVER
LIM
I
LIM
R1 80Ω (INTERNAL)
Q1
SW1
Q2
SW2
LT1110 • TA17
Using the Gain Block
The gain block (GB) on the LT1110 can be used as an error amplifier, low battery detector or linear post regulator. The gain block itself is a very simple PNP input op amp with an open collector NPN output. The negative input of the gain block is tied internally to the 220mV reference. The posi­tive input comes out on the SET pin.
12
L T1110
L1
LT1110 • TA08
GND SW2
SET
SW1
LIM
I
IN
V
D1
R3 270k
FB
+
V
OUT
R2
R1
C1
V = + 1 220mV
OUT
R2 R1
( ) ( )
LT1110
AO
V
BAT
PPLICATI
A
U
O
S
I FOR ATIO
WU
U
Arrangement of the gain block as a low battery detector is straightforward. Figure 12 shows hookup. R1 and R2 need only be low enough in value so that the bias current of the SET input does not cause large errors. 33kΩ for R2 is adequate. R3 can be added to introduce a small amount of hysteresis. This will cause the gain block to “snap” when the trip point is reached. Values in the 1M-10M range are optimal. The addition of R3 will change the trip point, however.
+5V
V
IN
R1
220mV
V
BAT
R2
REF
SET
–
+
LT1110
AO
GND
R3
47k
TO  PROCESSOR
Output ripple of the LT1110, normally 90mV at 5V
OUT
can be reduced significantly by placing the gain block in front of the FB input as shown in Figure 13. This effectively reduces the comparator hysteresis by the gain of the gain block. Output ripple can be reduced to just a few millivolts using this technique. Ripple reduction works with step­down or inverting modes as well. For this technique to be effective, output capacitor C1 must be large, so that each switching cycle increases V
by only a few millivolts.
OUT
1000µF is a good starting value.
– 220mV
V
LB
R1 =
( )
4.33µA
= BATTERY TRIP POINT
V
LB
R2 = 33kΩ
R3 = 2MΩ
Figure 12. Setting Low Battery Detector Trip Point
Table 1. Inductor Manufacturers
MANUFACTURER PART NUMBERS
Coiltronics International CTX100-4 Series 984 S.W. 13th Court Surface Mount Pompano Beach, FL 33069 305-781-8900
Sumida Electric Co. USA CD54 708-956-0666 CDR74
CDR105 Surface Mount
LT1110 • TA07
Figure 13. Output Ripple Reduction Using Gain Block
Table 2. Capacitor Manufacturers
MANUFACTURER PART NUMBERS
Sanyo Video Components OS-CON Series 2001 Sanyo Avenue San Diego, CA 92173 619-661-6835
Nichicon America Corporation PL Series 927 East State Parkway Schaumberg, IL 60173 708-843-7500
Sprague Electric Company 150D Solid Tantalums Lower Main Street 550D Tantalex Sanford, ME 04073 207-324-4140
Matsuo 267 Series 714-969-2491 Surface Mount
Table 3. Transistor Manufacturers
MANUFACTURER PART NUMBERS
Zetex ZTX Series Commack, NY FZT Series 516-543-7100 Surface Mount
13
LT1110
U
O
PPLICATITYPICAL
SA
All Surface Mount
Flash Memory VPP Generator
+5V
±10%
= PROGRAM
1
= SHUTDOWN
0
L1*
47µH
MMBT4403
10k
V
I
LIM
+
22µF
1k
MMBF170
LT1110CS8-12
GND SW2
*L1= SUMIDA CD105-470M
IN
SW1
SENSE
MBRS12OT3
+
47µF 20V
LT1110 • TA18
V
PP
12V 120MA
1.5V Powered Laser Diode Driver
TOSHIBA
TOLD-9211
22nF
4.7k 2N3906
1N4148
1.5V
ADJUST R1 FOR CHANGE IN LASER OUTPUT POWER
*
✝
TOKO 262LYF-0076M
 LASER DIODE CASE COMMON TO +BATTERY TERMINAL
•
170mA CURRENT DRAIN FROM 1.5V CELL (50mA DIODE)
•
NO OVERSHOOT
• 
12
I
LIM
6
AO
LT1110
8
FB
GND SW2
V
IN
3
SW1
7
SET
45
1.5V Powered Laser Diode Driver
220Ω
10
Ω
1k* R1
MJE210
1N5818
✝
L1
2.2 H
0.22 F
µ
C1
+
100 F OS-CON
µ
CERAMIC
µ
2Ω
LT1110 • TA13
14
L T1110
LT1110 • TA12
GND SW2
SENSE
SW1
LIM
I
IN
V
LT1110
1.5V AA OR AAA CELL
= MBRL120 = COILCRAFT 1812LS-823
4.7µF
L1*
82µH
+5V 4mA
4.7µF
–5V 4mA
4.7µF
+
+
*L1
+
U
O
PPLICATITYPICAL
SA
All Surface Mount
3V to 5V Step-Up Converter
L1*
47µH
220
V
I
LIM
3V 2x AA CELL
GND SW2
*L1 = COILCRAFT 1812LS-473
LT1110-5
IN
SW1
SENSE
+
MBRL120
5V 40mA
10µF
LT1110 • TA09
All Surface Mount
1.5V to +10V, +5V Dual Output Step-Up Converter
All Surface Mount
9V to 5V Step-Down Converter
220
V
I
LIM
9V
GND SW2
*L1 = COILCRAFT 1812LS-473
LT1110-5
IN
SW1
SENSE
47µH
MBRL120
L1*
+
10µF
LT1110 • TA10
All Surface Mount
1.5V to ±5V Dual Output Step-Up Converter
5V 40mA
*L1
I
LIM
GND SW2
LT1110
1.5V AA OR AAA CELL
= MBRL120 = COILCRAFT 1812LS-823
L1*
82µH
V
IN
SW1
FB
4.7µF
+
490k
+
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
4.7µF
11k
+
LT1110 • TA11
+10V 3mA
+5V 3mA
4.7µF
15
LT1110
PACKAGEDESCRIPTI
0.300 – 0.320
(7.620 – 8.128)
U
Dimensions in inches (millimeters) unless otherwise noted.
O
N8 Package
8-Lead Plastic DIP
0.045 – 0.065
(1.143 – 1.651)
0.130 ± 0.005
(3.302 ± 0.127)
0.400
(10.160)
MAX
876
5
0.065
(1.651)
0.009 – 0.015
(0.229 – 0.381)
+0.025
0.325
–0.015 +0.635
8.255
()
–0.381
0.010 – 0.020
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
× 45°
0.016 – 0.050
0.406 – 1.270
TYP
0.045 ± 0.015
(1.143 ± 0.381)
0.100 ± 0.010
(2.540 ± 0.254)
0°– 8° TYP
0.053 – 0.069
(1.346 – 1.752)
0.014 – 0.019
(0.355 – 0.483)
0.125
(3.175)
MIN
0.018 ± 0.003
(0.457 ± 0.076)
S8 Package
8-Lead Plastic SOIC
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
0.020
(0.508)
MIN
12
0.228 – 0.244
(5.791 – 6.197)
3
0.189 – 0.197* (4.801 – 5.004)
7
8
1
2
4
6
3
0.250 ± 0.010
(6.350 ± 0.254)
5
0.150 – 0.157* (3.810 – 3.988)
4
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
●
FAX
: (408) 434-0507
●
TELEX
: 499-3977
LT/GP 0594 2K REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1994
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