1
LT1082
sn1082 1082fas
FEATURES
APPLICATIO S
U
DESCRIPTIO
U
1A High Voltage, Efficiency
Switching Voltage Regulator
LT1082C LT1072HV
V
IN
3V to 75V 3V to 60V
V
SW
100V 75V
Switch Current Limit 1A 1.25A
Quiescent Current 4.5mA 6mA
Operating Frequency 60kHz 40kHz
Flyback Reference Voltage 16.2 + 0.6 (35kΩ/R
FB
) 16 + 0.35 (7kΩ/R
FB
)
LT1082 and LT1072 Major Specification Differences
USER NOTE: This data sheet is only intended to provide specifications, graphs, and a general
functional description of the LT1082. Application circuits are included to show the capability of the
LT1082. A complete design manual (AN19) and Switcher CAD (LTC Switching Power Supply Design
Program) should be obtained to assist in developing new designs. This manual contains a
comprehensive discussion of both the LT1070 and the external components used with it, as well as
complete formulas for calculating the values of these components. The manual can also be used for
the LT1082 by factoring in the lower switch current rating.
■
Telecom 5V Supply at 0.7A from –48V
■
90V Supply at 120mA from 15V
■
All Applications Using LT1072 (See Below for
Specification Differences)
■
Wide Input Voltage Range: 3V to 75V
■
High Switch Voltage: 100V
■
Low Quiescent Current: 4.5mA
■
Internal 1A Switch
■
Shutdown Mode Draws Only 120µA Supply Current
■
Isolated Flyback Regulation Mode for Fully Floating
Outputs
■
Can Be Externally Synchronized
■
Available in MiniDIP and TO-220 Packages
■
Same Pinout as LT1072
The LT
®
1082 is a monolithic high voltage switching
regulator. It can be operated in all standard switching
configurations including buck, boost, flyback, forward,
and inverting. A 1A high efficiency switch is included on
the die along with all oscillator, control, and protection
circuitry.
The LT1082 operates with supply voltages from 3V to 75V,
switch voltage up to 100V and draws only 4.5mA quies-
cent current. It can deliver load power up to 20W with no
external power devices. By utilizing current-mode switch-
ing techniques, it provides excellent AC and DC load and
line regulation.
An externally activated shutdown mode reduces total
supply current to 120µA typical for standby operation.
Totally isolated and regulated outputs can be generated by
using the optional “isolated flyback regulation mode” built
into the LT1082, without the need for optocouplers or
extra transformer windings.
The LT1082 has a unique feature to provide high voltage
short-circuit protection. When the FB pin is pulled down to
0.6V and the current out of the pin reaches approximately
350µA, the switching frequency will shift down from
60kHz to 12kHz.
The LT1082 is nearly identical to the lower voltage LT1072.
For the major differences in specifications, see the table on
the left.
Negative-to-Positive Telecom 5V Supply
INPUT VOLTAGE (V)
0
MAXIMUM OUTPUT CURRENT (A)
–20 –40 –50 –90
1082 TA02
–10 –30 –60 –70 –80
1.0
0.9
0.8
0.7
0.6
0.5
0.4
L=550µH
L=450µH
L=350µH
L=250µH
L=150µH
L=100µH
f = 45kHz
I
SW
LIMIT = 1.07A
Telecom 5V Supply Maximum Output
Current vs Input Voltage
+
V
IN
V
SW
GND FB
V
C
LT1082
4.7k
0.22µF
33µF
80V
CHEMI-CON
SXE SERIES
–20V
TO –70V
**250µH
3.83k
+
0.01µF
1.1k
Q1
2N5401
470µF
10V
CHEMI-CON
SXE SERIES
V
OUT
5V, 0.7A
*D1
MOTOROLA MUR110 (100V, 1A)
69 TURNS OF #28 AWG WIRE ON A
MICROMETALS T60 TYPE 52 CORE.
NOTE: THIS CORE IS LOW COST, BUT
HAS HIGHER CORE LOSS AND IS LARGER
THAN NECESSARY FOR LOWER CURRENT
APPLICATIONS. FOR SMALLER INDUCTORS
OR HIGHER EFFICIENCY, USE A LOW LOSS
CORE SUCH AS MAGNETICS INC. KOOL Mµ
OR MOLYPERMALLOY.
*
**
1082 TA01
NOTE: MAXIMUM OUTPUT
CURRENT IS A FUNCTION OF
INPUT VOLTAGE. SEE THE
GRAPH ON THE RIGHT.
TYPICAL APPLICATIO
U
, LTC and LT are registered trademarks of Linear Technology Corporation
2
LT1082
sn1082 1082fas
A
U
G
W
A
W
U
W
ARBSOLUTEXI T
IS
Supply Voltage ....................................................... 75V
Switch Output Voltage .......................................... 100V
Feedback Pin Voltage (Transient, 1ms) ................ ±15V
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
Operating Junction Temperature Range
LT1082M (OBSOLETE) ............... – 55°C to 150°C
LT1082I ........................................... – 40°C to 125°C
LT1082C............................................... 0°C to 100°C
ELECTRICAL C CHARA TERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
REF
Reference Voltage Measured at Feedback Pin 1.224 1.244 1.264 V
V
C
= 0.8V ●1.214 1.244 1.274 V
I
B
Feedback Input Current V
FB
= V
REF
350 750 nA
●1100 nA
g
m
Error Amplifier ∆I
C
= ±25µA 3000 4400 6000 µmho
Transconductance ●2400 7000 µmho
Error Amplifier Source or V
C
= 1.5V 150 200 400 µA
Sink Current ●120 400 µA
Error Amplifier Clamp Hi Clamp, V
FB
= 1V 1.8 2.3 V
Voltage Lo Clamp, V
FB
= 1.5V 0.12 0.22 0.36 V
Reference Voltage Line Regulation 3V ≤ V
IN
≤ V
MAX
, V
C
= 0.8V ● 0.03 %/V
A
V
Error Amplifier Voltage Gain 0.9V ≤ V
C
≤ 1.4V 350 650 V/V
Minimum Input Voltage ●2.6 3.0 V
1
2
3
4
8
7
6
5
TOP VIEW
GND
V
C
FB
NC
E2
V
SW
E1
V
IN
N8 PACKAGE
8-LEAD PLASTIC DIP
Q PACKAGE
5-LEAD DD
V
IN
V
SW
GND
FB
V
C
FRONT VIEW
5
4
3
2
1
T PACKAGE
5-LEAD TO-220
V
IN
V
SW
GND
FB
V
C
FRONT VIEW
5
4
3
2
1
T
JMAX
= 100°C, θ
JA
= 90°C/W (CN8)
T
JMAX
= 125°C, θ
JA
= 90°C/W (IN8) T
JMAX
= 100°C, θ
JA
= 40°C/W (CQ)
T
JMAX
= 125°C, θ
JA
= 40°C/W (IQ)
NOTE: θ
JA
VARIES FROM 25°C/W TO 50°C/W
DEPENDING ON BOARD COMPOSITION.
T
JMAX
= 100°C, θ
JA
= 75°C/W, θ
JC
= 8°C/W (CT)
T
JMAX
= 125°C, θ
JA
= 75°C/W, θ
JC
= 8°C/W (IT)
WU
U
PACKAGE/ORDER I FOR ATIO
ORDER PART NUMBER
ORDER PART NUMBER
ORDER PART NUMBER
LT1082CN8
LT1082IN8
LT1082CQ
LT1082IQ LT1082CT
LT1082IT
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, output pin open, unless otherwise
specified.
(Note 1)
LT1082MJ8
J8 PACKAGE
8-LEAD CERAMIC DIP
T
JMAX
= 150°C, θ
JA
= 100°C/W (MJ8)
OBSOLETE PACKAGE
Consider the Q Package for Alternate Source
3
LT1082
sn1082 1082fas
ELECTRICAL C CHARA TERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
Q
Supply Current 3V ≤ V
IN
≤ V
MAX
, V
C
= 0.6V 4.5 7.0 mA
Control Pin Threshold Duty Cycle = 0 0.7 0.9 1.1 V
●0.5 1.25 V
Normal/Flyback Threshold 0.58 0.67 0.8 V
on Feedback Pin
f Switching Frequency 50 60 70 kHz
●45 75 kHz
800µA ≥ I
FB
≥ 450µA 12 kHz
BV Output Switch Breakdown Voltage 3V ≤ V
IN
≤ V
MAX
, I
SW
= 1.5mA ●100 115 V
Control Voltage to Switch 1.5 A/V
Current Transconductance
V
FB
Flyback Reference Voltage I
FB
= 60µA 17 18.6 20.5 V
●16 21.5 V
Change in Flyback Reference Voltage 60µA ≤ I
FB
≤ 200µA 3.5 4.6 6.5 V
Flyback Reference Voltage Line Regulation I
FB
= 60µA, 3V ≤ V
IN
≤ V
MAX
0.01 0.03 %/V
Flyback Amplifier Transconductance (g
m
)∆I
C
= ±10µA 150 300 500 µmho
Flyback Amplifier Source V
C
= 0.6V Source ●15 32 70 µA
and Sink Current I
FB
= 60µA Sink ●30 50 90 µA
V
SAT
Output Switch “On” Resistance (Note 2) I
SW
= 0.7A (LT1082C), I
SW
= 0.5A (LT1082M) ●0.8 1.2 Ω
I
LIM
Switch Current Limit Duty Cycle = 20% ● 1.07 2.6 A
(LT1082C) Duty Cycle ≤ 50% ●1.0 2.6 A
Duty Cycle = 80% (Note 3) ●0.8 2.4 A
Switch Current Limit Duty Cycle = 20% ● 0.85 2.8 A
(LT1082I) Duty Cycle ≤ 50% ●0.8 2.8 A
Duty Cycle = 80% (Note 3) ● 0.65 2.6 A
Switch Current Limit Duty Cycle = 20% ● 0.75 3.0 A
(LT1082M) Duty Cycle ≤ 50% ●0.7 3.0 A
Duty Cycle = 80% (Note 3) ●0.6 2.8 A
∆I
IN
Supply Current Increase 35 45 mA/A
∆I
SW
During Switch-On Time
DC
MAX
Maximum Switch Duty Cycle 85 92 97 %
Flyback Sense Delay Time 1.5 µs
Shutdown Mode Supply Current 3V ≤ V
IN
≤ V
MAX
, V
C
= 0.05V 120 350 µA
Shutdown Mode 3V ≤ V
IN
≤ V
MAX
70 150 250 mV
Threshold Voltage ●50 300 mV
Note 1: Absolute Maximum Ratings are those values beyond which the
life of a device may be impaired.
Note 2: Measured with V
C
in hi clamp, V
FB
= 0.8V.
Note 3: For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current decreases linearly.
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, output pin open, unless otherwise
specified.
4
LT1082
sn1082 1082fas
CCHARA TERISTICS
UW
ATYPICALPER
FORCE
Suggested Core Size and
Inductance for Telecom
5V Supply Telecom 5V Supply Short-Circuit
Frequency Shift-Down
Minimum Input Voltage
TEMPERATURE (°C)
–75
MINIMUM INPUT VOLTAGE (V)
2.9
2.8
2.7
2.6
2.5
2.4
2.3 –50 50 100
1082 G07
25 150
–25 075 125
I
SW
= 1A
I
SW
= 0A
Switch Saturation Voltage
SWITCH CURRENT (A)
0
SWITCH SATURATION VOLTAGE (V)
2.00
1082 G08
0.25 0.50 1.50
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
00.75 1.00 1.25 1.75
T
J
= 150°C
T
J
= 100°C
T
J
= 25°C
T
J
= –50°C
JUNCTION TEMPERATURE (°C)
–75
TIME (µs)
2.2
2.0
1.8
1.6
1.4
1.2
1.0 –50 50 100
1082 G06
25 150
–25 075 125
Flyback Blanking Time
Switch Current Limit
TEMPERATURE (°C)
–75
DUTY CYCLE (%)
97
96
95
94
93
92
91
90 –50 50 100
1082 G05
25 150 175
–25 075 125
Maximum Duty Cycle
DUTY CYCLE (%)
0
SWITCH CURRENT (A)
3
4
40
1082 G04
2
1
010 20 30 50 60 70 80 90 100
T
J
= –55°C
T
J
= 150°C
T
J
= 25°C
Short-Circuit Frequency
Shift-Down vs Feedback Current
FEEDBACK CURRENT (µA)
0 300
1082 G03
100 200 800
70
60
50
40
30
20
10
0400 500 600 700
FREQUENCY (kHz)
T
A
= 150°C
T
A
= 0°CT
A
= –55°C
LOAD
CURRENT
100mA
200mA
400mA
600mA
800mA
TYPE 52
POWDERED
IRON
T38 250µH
T50 250µH
T60 250µH
T60 250µH
T80 350µH
KOOL Mµ
OR MOLY-
PERMALLOY
T38 200µH
T38 150µH
T50 150µH
T50 200µH
T80 350µH
1082 GA
POWER OUTPUT (W)
0
EFFICIENCY (%)
4
1082 G01
123
79
76
73
70
67
64
61
58
55
V
IN
= –20V
V
IN
= –40V
V
IN
= –60V
V
IN
= –70V
NOTE: THIS GRAPH IS BASED ON LOW CORE LOSS
PERMALLOY INDUCTOR. IF POWDERED IRON CORE
INDUCTOR IS USED, THE CORE LOSS IS TYPICALLY
100mW HIGHER.
L = 250µH
R = 0.08Ω
Telecom 5V Supply Efficiency
OUTPUT VOLTAGE (V)
6
FREQUENCY (kHz)
54 32
1082 G02
80
70
60
50
40
30
20
10
010
T
A
= 25°C
5
LT1082
sn1082 1082fas
CCHARA TERISTICS
UW
ATYPICALPER
FORCE
Isolated Mode Flyback
Reference Voltage
TEMPERATURE (°C)
FLYBACK VOLTAGE (V)
25
24
23
22
21
20
19
18
17
16
15
1082 G09
–75 –50 50 100
25 150 175
–25 075 125
R
FB
= 3k
R
FB
= 6k
R
FB
= 10k
Supply Current vs Input Voltage**
SUPPLY VOLTAGE (V)
0
SUPPLY CURRENT (µA)
200
160
120
80
40
030 50 80
1082 G15
10 20 40 60 70
V
C
= 50mV
V
C
= 0V
Supply Current vs Supply Voltage
(Shutdown Mode)
INPUT VOLTAGE (V)
0
SUPPLY CURRENT (mA)
20 40 60 80
1082 G14
14
13
12
11
10
9
8
7
6
5
4
3
210 30 50 70
90% DUTY CYCLE
50% DUTY CYCLE
10% DUTY CYCLE
0% DUTY CYCLE
**UNDER VERY LOW OUTPUT CURRENT CONDITIONS,
DUTY CYCLE FOR MOST CIRCUITS WILL APPROACH
10% OR LESS.
Feedback Bias Current vs
Temperature
TEMPERATURE (°C)
FEEDBACK BIAS CURRENT (nA)
1082 G12
800
700
600
500
400
300
200
100
0
–75 –50 50 100
25 150 175
–25 075 125
Normal/Feedback Mode
Threshold on Feedback Pin
TEMPERATURE (°C)
V
C
PIN VOLTAGE (mV)
750
725
700
675
650
625
600
575
550
525
500
1082 G16
–75 –50 50 100
25 150 175
–25 075 125
–24
–22
–20
–18
–16
–14
–12
–10
–8
–6
–4
V
C
PIN CURRENT (µA)
FEEDBACK PIN VOLTAGE
(AT THRESHOLD)
FEEDBACK PIN CURRENT
(AT THRESHOLD)
V
C
PIN VOLTAGE (mV)
SUPPLY CURRENT (µA)
200
180
160
140
120
100
80
60
40
20
0
1082 G17
010 50 70
40 90 100
20 30 60 80
–55°C ≤ T
J
≤ 125°C
T
J
= 150°C
Shutdown Mode Supply Current
Reference Voltage and Switching
Frequency vs Temperature
TEMPERATURE (°C)
FREQUENCY (kHz)
70
65
60
55
50
45
1082 G11
1.250
1.245
1.240
1.235
1.230
1.225
REFERENCE VOLTAGE (V)
–75 –50 50 100
25 150 175
–25 075 125
FREQ
V
REF
INPUT VOLTAGE (V)
0
REFERENCE VOLTAGE CHANGE (mV)
5
4
3
2
1
0
–1
–2
–3
–4
–5 20 40 50
1082 G10
10 30 60 70 80
T
J
= 25°C
T
J
= –55°C
T
J
= 150°C
Line Regulation
SWITCH CURRENT (A)
DRIVER CURRENT (mA)
100
90
80
70
60
50
40
30
20
10
0
1082 G13
00.2 1.0 1.4
0.8 1.8 2.0
0.4 0.6 1.2 1.6
* AVERAGE SUPPLY CURRENT
= I
Q
+ DC(2.9 + 10
–2
I
SW
+ 10
–5
I
SW
2
)
I
Q
= QUIESCENT CURRENT, DC = DUTY CYCLE,
I
SW
= SWITCH CURRENT
Driver Current* vs Switch Current
6
LT1082
sn1082 1082fas
CCHARA TERISTICS
UW
ATYPICALPER
FORCE
VC Pin Characteristics
FREQUENCY (Hz)
TRANSCONDUCTANCE (µmho)
1k 100k 1M 10M
1082 G24
10k
7000
6000
5000
4000
3000
2000
1000
0
–1000
θ
g
m
–30
0
30
60
90
120
150
180
210
PHASE (DEG)
Error Amplifier Transconductance
TEMPERATURE (°C)
–75
TRANSCONDUCTANCE (µmho)
–50 50 100
1082 G18
25 150
–25 075 125
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
g
m
= ∆I
∆V(V
C
PIN)
(FB PIN)
Idle Supply Current vs
Temperature
TEMPERATURE (°C)
IDLE SUPPLY CURRENT (mA)
1082 G20
10
9
8
7
6
5
4
3
2
1
–75 –50 50 100
25 150 175
–25 075 125
V
C
= 0.6V
V
IN
= 75V
V
IN
= 3V
TEMPERATURE (°C)
V
C
VOLTAGE (mV)
1082 G19
400
350
300
250
200
150
100
50
0
–75 –50 50 100
25 150 175
–25 075 125
–400
–350
–300
–250
–200
–150
–100
–50
0
V
C
CURRENT (µA)
VOLTAGE AT V
C
PIN
CURRENT
(OUT OF V
C
PIN)
Shutdown Thresholds
Feedback Pin Clamp Voltage
SWITCH VOLTAGE (V)
SWITCH CURRENT (µA)
500
450
400
350
300
250
200
150
100
50
0
1082 G22
010 50 70
40 90 100
20 30 60 80
A. V
IN
= 3V
B. V
IN
= 15V
C. V
IN
= 40V
D. V
IN
= 55V
E. V
IN
= 75V
AB DE
C
FEEDBACK CURRENT (mA)
0
FEEDBACK VOLTAGE (mV)
800
700
600
500
400
300
200 0.1
1082 G21
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
T
J
= –55°C
T
J
= 25°C
T
J
= 150°C
Switch “Off” Characteristics
Transconductance of Error
Amplifier
V
C
PIN VOLTAGE
(V)
V
C
PIN CURRENT (µA)
400
300
200
100
0
–100
–200
–300
–400
1082 G23
01.0 2.5
0.5 1.5 2.0
V
FB
= 0.8V
(CURRENT OUT OF V
C
PIN)
V
FB
= 1.5V
(CURRENT INTO V
C
PIN)
T
J
= 25°C
7
LT1082
sn1082 1082fas
OPERATIO
U
–
+
–
+
2.3V
REG FLYBACK
ERROR
AMP
COMP
ERROR
AMP
SHUTDOWN
CIRCUIT
CURRENT
AMP
OSC
60kHz
14kHz
MODE SELECT
1.24V
REF
ANTI-SAT
DRIVER
LOGIC
0.15V
GND
* ALWAYS CONNECT E1 TO GROUND PIN ON MiniDIP PACKAGE.
EMITTERS TIED TO GROUND ON TO-220 PACKAGE.
GAIN ≈ 5
0.2Ω0.2Ω
16.2V SWITCH OUT
V
IN
V
C
FB
E1* E2
1082 BD
W
IDAGRA
BLOCK
low dropout internal regulator provides a 2.3V supply for
all internal circuitry on the LT1082. This low dropout
design allows input voltage to vary from 3V to 75V with
virtually no change in device performance. A 60kHz
oscillator is the basic clock for all internal timing. It turns
“on” the output switch via the logic and driver circuitry.
Special adaptive anti-sat circuitry detects onset of
saturation in the power switch and adjusts driver current
instantaneously to limit switch saturation. This minimizes
driver dissipation and provides very rapid turn-off of the
switch.
A 1.2V bandgap reference biases the positive input of the
error amplifier. The negative input is brought out for
output voltage sensing. This feedback pin has a second
function: when pulled low with an external resistor and
with I
FB
of 60µA to 200µA, it programs the LT1082 to
The LT1082 is a current mode switcher. This means that
switch duty cycle is directly controlled by switch current
rather than by output voltage. Referring to the block
diagram, the switch is turned “on” at the start of each
oscillator cycle. It is turned “off” when switch current
reaches a predetermined level. Control of output voltage is
obtained by using the output of a voltage sensing error
amplifier to set current trip level. This technique has
several advantages. First, it has immediate response to
input voltage variations, unlike ordinary switchers which
have notoriously poor line transient response. Second, it
reduces the 90° phase shift at mid-frequencies in the
energy storage inductor. This greatly simplifies closed-
loop frequency compensation under widely varying input
voltage or output load conditions. Finally, it allows simple
pulse-by-pulse current limiting to provide maximum switch
protection under output overload or short conditions. A
8
LT1082
sn1082 1082fas
OPERATIO
U
disconnect the main error amplifier output and connects
the output of the flyback amplifier to the comparator input.
The LT1082 will then regulate the value of the flyback pulse
with respect to the supply voltage. This flyback pulse is
directly proportional to output voltage in the traditional
transformer coupled flyback topology regulator. By
regulating the amplitude of the flyback pulse, the output
voltage can be regulated with no direct connection between
input and output. The output is fully floating up to the
breakdown voltage of the transformer windings. Multiple
floating outputs are easily obtained with additional
windings. A special delay network inside the LT1082
ignores the leakage inductance spike at the leading edge of
the flyback pulse to improve output regulation.
When I
FB
drawn out of the FB pin reaches 350µA, the
LT1082 shifts the switching frequency down to 12kHz.
This unique feature provides high voltage short-circuit
protection in systems like the telecom 5V supplies with
input voltages down to –70V; lower frequency is needed
under short-circuit conditions with current mode switchers
because minimum “on” time cannot be forced below the
internally set blanking time. Referring to the telecom 5V
supply circuit on the front page, with output shorted to
ground, the V
FB
stays at 0.6V when sourcing I
FB
up to
1mA. If the FB pin is forced to source more than 1mA, the
frequency shifting function may be defeated. Therefore,
the minimum suggested value for R
FB
is 1k and the
maximum suggested value is 1.2k. Also, no capacitance
more than 1nF should be used on the FB pin, because it
may cause unstable switching frequency in this low
frequency mode.
The error signal developed at the comparator input is
brought out externally. This pin (V
C
) has four different
functions. It is used for frequency compensation, current
limit adjustment, soft starting, and total regulator shutdown.
During normal regulator operation this pin sits at a voltage
between 0.9V (low output current) and 2V (high output
current). The error amplifiers are current output (g
m
)
types, so this voltage can be externally clamped for
adjusting current limit. Likewise, a capacitor-coupled
external clamp will provide soft start. Switch duty cycle
goes to zero if the V
C
pin is pulled to ground through a
diode, placing the LT1082 in an idle mode. Pulling the V
C
pin below 0.15V causes total regulator shutdown, with
only 120µA supply current for shutdown circuitry biasing.
See AN19 for full application details.
Extra Pins on the MiniDIP Packages
The miniDIP LT1082 has the emitters of the power
transistor brought out separately from the ground pin.
This eliminates errors due to ground pin voltage drops and
allows the user to reduce switch current limit by a factor
of 2:1 by leaving the second emitter (E2) disconnected.
The first emitter (E1) should always be connected to the
ground pin. Note that switch “on” resistance doubles
when E2 is left open, so efficiency will suffer somewhat
when switch currents exceed 100mA. Also, note that chip
dissipation will actually
increase
with E2 open during
normal load operation, even though dissipation in current
limit mode will
decrease
. See “Thermal Considerations.”
Thermal Considerations When Using the
MiniDIP Packages
The low supply current and high switch efficiency of the
LT1082 allow it to be used without a heat sink in most
applications when the TO-220 package is selected.
This package is rated at 50°C/W. The miniDIPs, however,
are rated at 100°C/W in ceramic (J) and 90°/W in plastic
(N).
Care should be taken for miniDIP applications to ensure
that the worst case input voltage and load current conditions
do not cause excessive die temperatures. The following
formulas can be used as a rough guide to calculate LT1082
power dissipation. For more details, the reader is referred
to Application Note 19 (AN19), “Efficiency Calculations”
section.
Average supply current (including driver current) is:
I
IN
≈ 4.5mA + I
SW
(0.004 + DC/28)
I
SW
= switch current
DC = switch duty cycle
Switch power dissipation is given by:
P
SW
= (I
SW
)
2
• R
SW
• DC
R
SW
= LT1082 switch “on” resistance (1.2Ω maximum)
9
LT1082
sn1082 1082fas
OPERATIO
U
KT/q = 26mV at 25°C
t
S
= pulse width
f
S
= pulse frequency
I
C
= LT1082 V
C
source current (≈ 200µA)
V
C
= LT1082 operating V
C
voltage (1V to 2V)
R3 = resistor used to set mid-frequency “zero” in LT1082
frequency compensation network.
With t
S
= 0.6µs, f
S
= 80kHz, V
C
= 1.5V, and R3 = 2k, offset
voltage shift is ≈5mV. This is not particularly bothersome,
but note that high offset could result if R3 were reduced to
a much lower value. Also, the synchronizing transistor
must sink higher currents with low values of R3, so larger
drives may have to be used. The transistor must be
capable of pulling the V
C
pin to within 100mV of ground to
ensure synchronizing.
Total power dissipation is the sum of supply current times
input voltage plus switch power:
P
TOT
= (I
IN
)(V
IN
) + P
SW
In a typical example, using negative-to-positive converter
to generate 5V at 0.5A from a –45V input, duty cycle is
approximately 12%, and switch current is about 0.5A,
yielding:
I
IN
= 4.5mA + 0.5(0.004 + DC/28) = 8.7mA
P
SW
= (0.5)
2
• 1.2Ω • (0.12) = 0.036W
P
TOT
= (45V)(8.7mA) + 0.036 = 0.43W
Temperature rise in a plastic miniDIP would be 90°C/W
times 0.43W, or approximately 39°C. The maximum am-
bient temperature would be limited to 100°C (commercial
temperature limit) minus 39°C, or 61°C.
In most applications, full load current is used to calculate
die temperature. However, if overload conditions must
also be accounted for, four approaches are possible. First,
if loss of regulated output is acceptable under overload
conditions, the internal
thermal limit
of the LT1082 will
protect the die in most applications by shutting off switch
current.
Thermal limit
is not a tested parameter, however,
and should be considered only for noncritical applications
with temporary overloads. A second approach is to use the
larger TO-220 (T) package which, even without a heat sink,
may limit die temperatures to safe levels under overload
conditions. In critical situations, heat sinking of these
packages is required; especially if overload conditions
must be tolerated for extended periods of time.
The third approach for lower current applications is to
leave the second switch emitter (miniDIP only) open. This
increases switch “on” resistance by 2:1, but reduces
switch current limit by 2:1 also, resulting in a net 2:1
reduction in I
2
R switch dissipation under current limit
conditions.
The fourth approach is to clamp the V
C
pin to a voltage less
than its internal clamp level of 2V. The LT1082 switch
current limit is zero at approximately 1V on the V
C
pin and
1.6A at 2V on the V
C
pin. Peak switch current can be
externally clamped between these two levels with a diode.
See AN19 for details.
LT1082 Synchronizing
The LT1082 can be externally synchronized in the fre-
quency range of 75kHz to 90kHz. This is accomplished as
shown in the accompanying figures. Synchronizing oc-
curs when the V
C
pin is pulled to ground with an external
transistor. To avoid disturbing the DC characteristics of
the internal error amplifier, the width of the synchronizing
pulse should be under 1µs. C2 sets the pulse width at ≈
0.6µs. The effect of a synchronizing pulse on the LT1082
amplifier offset can be calculated from:
∆V
KT
qtfI V
R
I
OS
SSC C
C
=
()()
+
3
V
IN
GND V
C
LT1082
VN2222*
C1
R3
C2
350pF
D2
1N4148
R2
2.2k
D1
1N4148
*SILICONIX OR EQUIVALENT
1082 OP01
FROM 5V
LOGIC
Synchronizing the LT1082
10
LT1082
sn1082 1082fas
U
SA
O
PPLICATITYPICAL
Totally Isolated Converter
V
IN
GND V
C
LT1082
V
IN
30V
to 70V
MINIMUM LOAD OF 0.15A IS REQUIRED
FOR EACH OUTPUT. (SEE AN19)
1082 TA03
+
V
SW
FB
+
+
MUR110
3k
0.01µF
COM
25µF
25V
L
PRI
500µH
MUR110
MUR110
200µF
200µF
1
1
15V
AT 0.3A
–15V
AT 0.3A
7k
1.24:1
Boost Converter
V
IN
GND V
C
LT1082
V
IN
15V
1082 TA04
V
SW
FB
VOUT
90V AT 120mA
4.7k 0.033µF
25µF
1mH, 1A
1.1k
+
78.8k
0.22µF
100µF
MUR110
+
J8 1298
0.014 – 0.026
(0.360 – 0.660)
0.015 – 0.060
(0.381 – 1.524)
0.125
3.175
MIN
0.100
(2.54)
BSC
0.300 BSC
(0.762 BSC)
0.008 – 0.018
(0.203 – 0.457) 0° – 15°
0.045 – 0.065
(1.143 – 1.651)
0.045 – 0.068
(1.143 – 1.727)
FULL LEAD
OPTION
0.023 – 0.045
(0.584 – 1.143)
HALF LEAD
OPTION
CORNER LEADS OPTION
(4 PLCS)
0.200
(5.080)
MAX
0.005
(0.127)
MIN
0.405
(10.287)
MAX
0.220 – 0.310
(5.588 – 7.874)
1234
8765
0.025
(0.635)
RAD TYP
NOTE: LEAD DIMENSIONS APPLY TO SOLDER
DIP/PLATE OR TIN PLATE LEADS
OBSOLETE PACKAGE
J8 Package
8-Lead CERDIP (Narrow .300 Inch, Hermetic)
(Reference LTC DWG # 05-08-1110)
U
PACKAGE DESCRIPTIO
11
LT1082
sn1082 1082fas
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
PACKAGE DESCRIPTIO
N8 1098
0.100
(2.54)
BSC
0.065
(1.651)
TYP
0.045 – 0.065
(1.143 – 1.651)
0.130 ± 0.005
(3.302 ± 0.127)
0.020
(0.508)
MIN
0.018 ± 0.003
(0.457 ± 0.076)
0.125
(3.175)
MIN 12 34
8765
0.255 ± 0.015*
(6.477 ± 0.381)
0.400*
(10.160)
MAX
0.009 – 0.015
(0.229 – 0.381)
0.300 – 0.325
(7.620 – 8.255)
0.325 +0.035
–0.015
+0.889
–0.381
8.255
()
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
N8 Package
8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
Q Package
5-Lead Plastic DD Pak
(Reference LTC DWG # 05-08-1461)
Q(DD5) 0502
.028 – .038
(0.711 – 0.965)
TYP
.143 +.012
–.020
()
3.632+0.305
–0.508
.067
(1.702)
BSC
.013 – .023
(0.330 – 0.584)
.095 – .115
(2.413 – 2.921)
.004 +.008
–.004
()
0.102+0.203
–0.102
.050 ± .012
(1.270 ± 0.305)
.059
(1.499)
TYP
.045 – .055
(1.143 – 1.397)
.165 – .180
(4.191 – 4.572)
.330 – .370
(8.382 – 9.398)
.060
(1.524)
TYP
.390 – .415
(9.906 – 10.541)
15° TYP
.420
.350
.565
.090
.042
.067
RECOMMENDED SOLDER PAD LAYOUT
.325
.205
.080
.565
.090
RECOMMENDED SOLDER PAD LAYOUT
FOR THICKER SOLDER PASTE APPLICATIONS
.042
.067
.420
.276
.320
NOTE:
1. DIMENSIONS IN INCH/(MILLIMETER)
2. DRAWING NOT TO SCALE
.300
(7.620)
.075
(1.905)
.183
(4.648)
.060
(1.524)
.060
(1.524)
.256
(6.502)
BOTTOM VIEW OF DD PAK
HATCHED AREA IS SOLDER PLATED
COPPER HEAT SINK
12
LT1082
sn1082 1082fas
LINEAR TECHNOLOGY CORPORATION 1993
U
PACKAGE DESCRIPTIO
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com
LT/CPI 0202 1.5K REV A • PRINTED IN USA
T Package
5-Lead Plastic TO-220 (Standard)
(Reference LTC DWG # 05-08-1421)
T5 (TO-220) 0399
0.028 – 0.038
(0.711 – 0.965)
0.067
(1.70) 0.135 – 0.165
(3.429 – 4.191)
0.700 – 0.728
(17.78 – 18.491)
0.045 – 0.055
(1.143 – 1.397)
0.095 – 0.115
(2.413 – 2.921)
0.013 – 0.023
(0.330 – 0.584)
0.620
(15.75)
TYP
0.155 – 0.195*
(3.937 – 4.953)
0.152 – 0.202
(3.861 – 5.131)
0.260 – 0.320
(6.60 – 8.13)
0.165 – 0.180
(4.191 – 4.572)
0.147 – 0.155
(3.734 – 3.937)
DIA
0.390 – 0.415
(9.906 – 10.541)
0.330 – 0.370
(8.382 – 9.398)
0.460 – 0.500
(11.684 – 12.700)
0.570 – 0.620
(14.478 – 15.748)
0.230 – 0.270
(5.842 – 6.858)
BSC
SEATING PLANE
* MEASURED AT THE SEATING PLANE
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LT1765 1.25MHz, 3A, Step-Down Regulator V
IN
: 3V to 25V, V
REF
= 1.2V, SO8 Package
LT1767 1.25MHz, 1.25A, Step-Down Regulator V
IN
: 3V to 25V, V
REF
= 1.2V, MSOP-8 Package
LTC3401 3MHz, 1A, Step-Up Regulator V
IN
: 0.5V to 5V, MSOP-10 Package
LTC3404 1.4MHz, I
OUT
= 600mA, Step-Down Regulator V
IN
: 2.65V to 6V, V
REF
= 0.8V, MSOP-8 Package
LTC3411 2MHz, 1.25A, Step-Down Regulator V
IN
: 2.5V to 5.5V, V
REF
= 0.8V, MSOP-10, TSSOP-16E Package
