LINER LT1082IT 1a high voltage, efficiency switching voltage regulator Datasheet

LT1082
1A High Voltage, Efficiency
Switching Voltage Regulator
U
DESCRIPTIO
FEATURES
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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
UO
APPLICATI
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S
Telecom 5V Supply at 0.7A from –48V
90V Supply at 120mA from 15V
All Applications Using LT1072 (See Below for
Specification Differences)
LT1082 and LT1072 Major Specification Differences
LT1082C
LT1072HV
3V to 75V
3V to 60V
100V
75V
1A
1.25A
4.5mA
6mA
60kHz
40kHz
16.2 + 0.6 (35kΩ/RFB) 16 + 0.35 (7kΩ/RFB)
VIN
VSW
Switch Current Limit
Quiescent Current
Operating Frequency
Flyback Reference Voltage
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.
The LT1082 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 quiescent current. It can deliver load power up to 20W with no
external power devices. By utilizing current-mode switching 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.
Telecom 5V Supply Maximum Output
Current vs Input Voltage
Negative-to-Positive Telecom 5V Supply
3.83k
VIN
33µF
80V
CHEMI-CON
SXE SERIES
+
VSW
Q1
2N5401
VOUT
5V, 0.7A
470µF
10V
CHEMI-CON
SXE SERIES
NOTE: MAXIMUM OUTPUT
CURRENT IS A FUNCTION
OF INPUT VOLTAGE. SEE
THE GRAPH ON THE RIGHT.
LT1082
GND
FB
VC
1.1k
4.7k
0.01µF
0.22µF
–20V
TO –70V
+
1082 TA01
* 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.
1.0
MAXIMUM OUTPUT CURRENT (A)
**250µH
*D1
0.9
0.8
f = 45kHz
ISW LIMIT = 1.07A
L=550µH
L=450µH
L=350µH
L=250µH
0.7
0.6
L=150µH
0.5
L=100µH
0.4
0 –10 –20 –30 –40 –50 –60 –70 –80 –90
INPUT VOLTAGE (V)
1082 TA02
1
LT1082
W W
W
AXI U
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ABSOLUTE
RATI GS
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 ......................................... – 55°C to 150°C
LT1082I ........................................... – 40°C to 125°C
LT1082C ............................................... 0°C to 100°C
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W
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PACKAGE/ORDER I FOR ATIO
TOP VIEW
FRONT VIEW
5
4
3
2
1
E2
GND 1
8
VC 2
7
VSW
FB 3
6
E1
NC 4
5
VIN
J8 PACKAGE
N8 PACKAGE
8-LEAD CERAMIC DIP 8-LEAD PLASTIC DIP
TJMAX = 150°C, θJA = 100°C/W (MJ8)
TJMAX = 100°C, θJA = 90°C/W (CN8)
TJMAX = 125°C, θJA = 90°C/W (IN8)
ORDER PART NUMBER
LT1082MJ8
LT1082CN8
LT1082IN8
FRONT VIEW
VIN
VSW
GND
FB
VC
5
4
3
2
1
Q PACKAGE
5-LEAD DD
VIN
VSW
GND
FB
VC
T PACKAGE
5-LEAD TO-220
TJMAX = 100°C, θJA = 40°C/ W (CQ)
TJMAX = 125°C, θJA = 40°C/ W (IQ)
NOTE: θJA VARIES FROM 25°C/W TO 50°C/W
DEPENDING ON BOARD COMPOSITION.
TJMAX = 100°C, θJA = 75°C/ W, θJC = 8°C/ W (CT)
TJMAX = 125°C, θJA = 75°C/ W, θJC = 8°C/ W (IT)
ORDER PART NUMBER
ORDER PART NUMBER
LT1082CQ
LT1082IQ
LT1082CT
LT1082IT
ELECTRICAL CHARACTERISTICS VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VREF
Reference Voltage
Measured at Feedback Pin
VC = 0.8V
1.224 1.244
1.214 1.244
1.264
1.274
V
V
350
750
1100
nA
nA
IB
Feedback Input Current
●
VFB = VREF
●
gm
AV
Error Amplifier
Transconductance
∆IC = ±25µA
Error Amplifier Source or
Sink Current
VC = 1.5V
Error Amplifier Clamp
Voltage
Hi Clamp, VFB = 1V
Lo Clamp, VFB = 1.5V
Reference Voltage Line Regulation
3V ≤ VIN ≤ VMAX, VC = 0.8V
Error Amplifier Voltage Gain
0.9V ≤ VC ≤ 1.4V
Minimum Input Voltage
2
3000
2400
4400
●
6000
7000
µmho
µmho
150
120
200
●
400
400
µA
µA
2.3
0.36
V
V
0.03
%/V
1.8
0.12
0.22
●
350
●
650
2.6
V/V
3.0
V
LT1082
ELECTRICAL CHARACTERISTICS VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
IQ
Supply Current
3V ≤ VIN ≤ VMAX, VC = 0.6V
Control Pin Threshold
Duty Cycle = 0
TYP
MAX
4.5
7.0
0.7
0.5
0.9
1.1
1.25
V
V
0.58
0.67
0.8
V
50
45
60
●
70
75
kHz
kHz
12
kHz
●
100
115
V
1.5
A/V
●
Normal/Flyback Threshold
on Feedback Pin
f
Switching Frequency
800µA ≥ IFB ≥ 450µA
BV
Output Switch Breakdown Voltage
3V ≤ VIN ≤ VMAX, ISW = 1.5mA
Control Voltage to Switch
Current Transconductance
VFB
Flyback Reference Voltage
IFB = 60µA
●
VSAT
ILIM
Change in Flyback Reference Voltage
60µA ≤ IFB ≤ 200µA
Flyback Reference Voltage Line Regulation
IFB = 60µA, 3V ≤ VIN ≤ VMAX
3.5
Flyback Amplifier Transconductance (gm)
∆IC = ±10µA
Flyback Amplifier Source
and Sink Current
VC = 0.6V Source
IFB = 60µA Sink
●
●
Output Switch “On” Resistance (Note 1)
Switch Current Limit
(LT1082C)
ISW = 0.7A (LT1082C), ISW = 0.5A (LT1082M)
Duty Cycle = 20%
Duty Cycle ≤ 50%
Duty Cycle = 80% (Note 2)
Duty Cycle = 20%
Duty Cycle ≤ 50%
Duty Cycle = 80% (Note 2)
Duty Cycle = 20%
Duty Cycle ≤ 50%
Duty Cycle = 80% (Note 2)
●
Switch Current Limit
(LT1082I)
Switch Current Limit
(LT1082M)
∆IIN
∆ISW
Supply Current Increase
During Switch-On Time
DCMAX
Maximum Switch Duty Cycle
Flyback Sense Delay Time
Shutdown Mode Supply Current
Shutdown Mode
Threshold Voltage
17
16
●
●
●
●
●
●
●
●
●
18.6
The ● denotes the specifications which apply over the operating
temperature range.
Note 1: Measured with VC in hi clamp, VFB = 0.8V.
V
V
4.6
6.5
V
0.03
%/V
150
300
500
µmho
15
30
32
50
70
90
µA
µA
0.8
1.2
2.6
2.6
2.4
2.8
2.8
2.6
3.0
3.0
2.8
45
Ω
A
A
A
A
A
A
A
A
A
mA/A
92
1.5
97
%
µs
120
150
350
250
300
µA
mV
mV
1.07
1.0
0.8
0.85
0.8
0.65
0.75
0.7
0.6
85
●
mA
0.01
35
3V ≤ VIN ≤ VMAX, VC = 0.05V
3V ≤ VIN ≤ VMAX
20.5
21.5
UNITS
70
50
Note 2: For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current decreases linearly.
3
LT1082
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Suggested Core Size and
Inductance for Telecom
5V Supply
Telecom 5V Supply Short-Circuit
Frequency Shift-Down
Telecom 5V Supply Efficiency
80
79
LOAD
CURRENT
TYPE 52
POWDERED
IRON
KOOL Mµ
OR MOLYPERMALLOY
76
100mA
T38 250µH
T38 200µH
73
T38 150µH
T60 250µH
T50 150µH
600mA
T60 250µH
T50 200µH
800mA
T80 350µH
T80 350µH
1082 GA
60
FREQUENCY (kHz)
T50 250µH
400mA
EFFICIENCY (%)
200mA
TA = 25°C
70
VIN = –20V
VIN = –40V
70
67
VIN = –60V
64
50
40
30
20
61
VIN = –70V
58
L = 250µH
R = 0.08Ω
10
0
55
0
2
1
3
4
6
POWER OUTPUT (W)
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.
1082 G01
Short-Circuit Frequency
Shift-Down vs Feedback Current
SWITCH CURRENT (A)
FREQUENCY (kHz)
96
40
30
20
3
95
2
TJ = –55°C
TJ = 25°C
93
91
0
0
100 200 300 400 500 600 700 800
FEEDBACK CURRENT (µA)
0
90
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1082 G04
1082 G03
Flyback Blanking Time
1082 G05
Switch Saturation Voltage
Minimum Input Voltage
2.9
2.0
2.8
MINIMUM INPUT VOLTAGE (V)
2.2
1.8
1.6
1.4
1.2
1.0
–75 –50 –25 0 25 50 75 100 125 150
JUNCTION TEMPERATURE (°C)
1082 G06
2.00
SWITCH SATURATION VOLTAGE (V)
0
TIME (µs)
94
92
TJ = 150°C
1
10
4
0
97
TA = –55°C
TA = 150°C
50
1
Maximum Duty Cycle
Switch Current Limit
DUTY CYCLE (%)
TA = 0°C
60
4
3
2
OUTPUT VOLTAGE (V)
1082 G02
4
70
5
ISW = 1A
2.7
ISW = 0A
2.6
2.5
2.4
2.3
–75 –50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1082 G07
1.75
TJ = 150°C
1.50
TJ = 100°C
TJ = 25°C
1.25
1.00
0.75
TJ = –50°C
0.50
0.25
0
0
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
SWITCH CURRENT (A)
1082 G08
LT1082
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TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage and Switching
Frequency vs Temperature
Isolated Mode Flyback
Reference Voltage
70
25
Line Regulation
1.250
5
RFB = 3k
FREQUENCY (kHz)
21
RFB = 6k
19
RFB = 10k
18
1.245
FREQ
60
1.240
55
1.235
50
17
1.230
16
45
1.225
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
15
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
2
1
–1
–2
0
700
725 FEEDBACK PIN VOLTAGE
(AT THRESHOLD)
700
–22
180
–20
160
675
–18
650
–16
625
–14
600
–12
575 FEEDBACK PIN CURRENT
(AT THRESHOLD)
550
–10
–8
40
525
–6
20
500
–4
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
0
VC PIN VOLTAGE (mV)
200
100
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1082 G12
160
40
0
0
10
50 60
20 30 40
SUPPLY VOLTAGE (V)
70
100
80
60
80
1082 G15
0
10 20 30 40 50 60 70 80 90 100
VC PIN VOLTAGE (mV)
1082 G17
Supply Current vs Input Voltage**
14
90
13
12
SUPPLY CURRENT (mA)
DRIVER CURRENT (mA)
VC = 0V
70
60
50
40
30
11
10
90% DUTY CYCLE
9
8
50% DUTY CYCLE
7
10% DUTY CYCLE
6
5
20
4
10
3
0
80
TJ = 150°C
120
100
80
80
70
–55°C ≤ TJ ≤ 125°C
140
Driver Current* vs Switch Current
200
120
30 40 50 60
INPUT VOLTAGE (V)
1082 G16
Supply Current vs Supply Voltage
(Shutdown Mode)
VC = 50mV
20
Shutdown Mode Supply Current
200
300
10
1082 G10
–24
400
TJ = 25°C
–4
750
500
TJ = 150°C
–3
800
600
TJ = –55°C
0
Normal/Feedback Mode
Threshold on Feedback Pin
VC PIN CURRENT (µA)
FEEDBACK BIAS CURRENT (nA)
3
1082 G11
1082 G09
Feedback Bias Current vs
Temperature
SUPPLY CURRENT (µA)
4
–5
SUPPLY CURRENT (µA)
FLYBACK VOLTAGE (V)
22
20
VREF
65
REFERENCE VOLTAGE (V)
23
REFERENCE VOLTAGE CHANGE (mV)
24
0% DUTY CYCLE
2
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
* AVERAGE SUPPLY CURRENT
= I Q + DC(2.9 + 10 –2 ISW + 10 –5 ISW2)
IQ = QUIESCENT CURRENT, DC = DUTY CYCLE,
1082 G13
ISW = SWITCH CURRENT
0
10
20
30 40 50 60
INPUT VOLTAGE (V)
70
80
**UNDER VERY LOW OUTPUT CURRENT CONDITIONS,
DUTY CYCLE FOR MOST CIRCUITS WILL APPROACH
1082 G14
10% OR LESS.
5
LT1082
U W
TYPICAL PERFOR A CE CHARACTERISTICS
gm =
400
–400
10
350
–350
9
CURRENT
(OUT OF VC PIN)
4000
300
VC VOLTAGE (mV)
3500
3000
2500
2000
1500
–300
250
–250
200
–200
VOLTAGE AT VC PIN
150
–150
100
–100
50
–50
VC CURRENT (µA)
TRANSCONDUCTANCE (µmho)
4500
∆I (VC PIN)
∆V (FB PIN)
1000
500
0
–75 –50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
VC = 0.6V
8
7
6
5
VIN = 75V
4
VIN = 3V
3
2
0
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1082 G19
1082 G18
Feedback Pin Clamp Voltage
1082 G20
Switch “Off” Characteristics
800
500
A. VIN = 3V
B. VIN = 15V
C. VIN = 40V
D. VIN = 55V
E. VIN = 75V
450
700
400
TJ = –55°C
SWITCH CURRENT (µA)
FEEDBACK VOLTAGE (mV)
IDLE SUPPLY CURRENT (mA)
5000
Idle Supply Current vs
Temperature
Shutdown Thresholds
Error Amplifier Transconductance
600
TJ = 25°C
500
TJ = 150°C
400
350
300
250
200
150
A
B
C
D
E
100
300
50
200
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
FEEDBACK CURRENT (mA)
0
10 20 30 40 50 60 70 80 90 100
SWITCH VOLTAGE (V)
1082 G21
1082 G22
Transconductance of Error
Amplifier
VC Pin Characteristics
7000
400
TJ = 25°C
TRANSCONDUCTANCE (µmho)
6000
VFB = 1.5V
(CURRENT INTO VC PIN)
200
100
0
–100
VFB = 0.8V
(CURRENT OUT OF VC PIN)
–200
–300
0
0.5
1.5
2.0
1.0
V C PIN VOLTAGE (V)
2.5
1082 G23
6
5000
30
gm
4000
60
3000
90
2000
120
1000
150
0
180
–1000
–400
0
θ
1k
10k
100k
1M
FREQUENCY (Hz)
210
10M
1082 G24
PHASE (DEG)
VC PIN CURRENT (µA)
300
–30
LT1082
W
BLOCK DIAGRA
VIN
16.2V
SWITCH OUT
FLYBACK
ERROR
AMP
2.3V
REG
OSC
60kHz
14kHz
LOGIC
DRIVER
ANTI-SAT
MODE SELECT
COMP
–
FB
+
ERROR
+ AMP
VC
CURRENT
AMP
GAIN ≈ 5
E1* E2
SHUTDOWN
CIRCUIT
1.24V
REF
0.2Ω
0.2Ω
–
0.15V
GND
* ALWAYS CONNECT E1 TO GROUND PIN ON MiniDIP PACKAGE.
EMITTERS TIED TO GROUND ON TO-220 PACKAGE.
1082 BD
U
OPERATIO
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 closedloop 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
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 IFB of 60µA to 200µA, it programs the LT1082 to
7
LT1082
U
OPERATIO
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 IFB 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 VFB stays at 0.6V when sourcing IFB 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 RFB 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 (VC) 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 (gm)
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 VC pin is pulled to ground through a
diode, placing the LT1082 in an idle mode. Pulling the VC
pin below 0.15V causes total regulator shutdown, with
8
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:
IIN ≈ 4.5mA + ISW (0.004 + DC/28)
ISW = switch current
DC = switch duty cycle
Switch power dissipation is given by:
PSW = (ISW)2 • RSW • DC
RSW = LT1082 switch “on” resistance (1.2Ω maximum)
LT1082
U
OPERATIO
Total power dissipation is the sum of supply current times
input voltage plus switch power:
PTOT = (IIN)(VIN) + PSW
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:
IIN = 4.5mA + 0.5(0.004 + DC/28) = 8.7mA
PSW = (0.5)2 • 1.2Ω • (0.12) = 0.036W
PTOT = (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 ambient 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 I2R switch dissipation under current limit
conditions.
The fourth approach is to clamp the VC pin to a voltage less
than its internal clamp level of 2V. The LT1082 switch
current limit is zero at approximately 1V on the VC pin and
1.6A at 2V on the VC 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 frequency range of 75kHz to 90kHz. This is accomplished as
shown in the accompanying figures. Synchronizing occurs when the VC 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:
 KT 
VC 

  ( tS )( fS) IC + 

 q
R3 
∆VOS =
IC
KT/q = 26mV at 25°C
tS = pulse width
fS = pulse frequency
IC = LT1082 VC source current (≈ 200µA)
VC = LT1082 operating VC voltage (1V to 2V)
R3 = resistor used to set mid-frequency “zero” in LT1082
frequency compensation network.
With tS = 0.6µs, fS = 80kHz, VC = 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 VC pin to within 100mV of ground to
ensure synchronizing.
Synchronizing the LT1082
VIN
LT1082
GND
VC
D1
1N4148
R3
C2
350pF
VN2222*
C1
R2
2.2k
D2
1N4148
*SILICONIX OR EQUIVALENT
FROM 5V
LOGIC
1082 OP01
9
LT1082
UO
TYPICAL APPLICATI
S
Totally Isolated Converter
1.24:1
MUR110
15V
AT 0.3A
+
25V
200µF
MUR110
LPRI
500µH
1
COM
1
+
200µF
VIN
30V
to 70V
VIN
+
–15V
AT 0.3A
VSW
25µF
MUR110
LT1082
FB
GND
VC
3k
0.01µF
MINIMUM LOAD OF 0.15A IS REQUIRED
FOR EACH OUTPUT. (SEE AN19)
7k
1082 TA03
Boost Converter
1mH, 1A
VIN
15V
VIN
+
25µF
MUR110
VOUT
90V AT 120mA
78.8k
VSW
LT1082
+
FB
GND
100µF
VC
4.7k
0.033µF
1.1k
0.22µF
1082 TA04
10
LT1082
U
PACKAGE DESCRIPTIO
Dimensions in inches (milimeters) unless otherwise noted.
J8 Package
8-Lead Ceramic DIP
CORNER LEADS OPTION
(4 PLCS)
0.290 – 0.320
(7.366 – 8.128)
0.008 – 0.018
(0.203 – 0.460)
0.200
(5.080)
MAX
0.023 – 0.045
(0.58 – 1.14)
HALF LEAD
OPTION
0.045 – 0.065
(1.14 – 1.65)
FULL LEAD
OPTION
0° – 15°
0.015 – 0.060
(0.381 – 1.524)
8
6
7
5
0.025
(0.635)
RAD TYP
0.045 – 0.065
(1.14 – 1.65)
0.385 ± 0.025
(9.779 ± 0.635)
0.405
(10.287)
MAX
0.005
(0.127)
MIN
0.220 – 0.310
(5.588 – 7.874)
0.125
3.175
0.100 ± 0.010 MIN
(2.540 ± 0.254)
0.014 – 0.026
(0.360 – 0.660)
1
2
3
4
N8 Package
8-Lead Plastic DIP
0.300 – 0.320
(7.620 – 8.128)
0.045 – 0.065
(1.143 – 1.651)
0.130 ± 0.0
(3.302 ± 0.127)
8
7
6
5
0.065
(1.651)
TYP
0.009 – 0.015
(0.229 – 0.381)
(
0.400
(10.160)
MAX
+0.025
0.325 –0.015
+0.635
8.255
–0.381
0.250 ± 0.010
(6.350 ± 0.254)
0.125
(3.175)
MIN
0.045 ± 0.015
(1.143 ± 0.381)
)
0.020
(0.508)
MIN
1
2
3
4
0.018 ± 0.003
(0.457 ± 0.076)
0.100 ± 0.010
(2.540 ± 0.254)
Q Package
5-Lead Plastic DD
0.401 ± 0.015
(10.185 ± 0.381)
0.060
(1.524)
0.175 ± 0.008
(4.445 ± 0.203)
15° TYP
(
+0.012
0.331 –0.020
+0.305
8.407 –0.508
0.059
(1.499)
TYP
)
0.050 ± 0.008
(1.270 ± 0.203)
(
+0.008
0.004 –0.004
+0.203
0.102 –0.102
)
0.105 ± 0.008
(2.667 ± 0.203)
(
+0.012
0.143 –0.020
+0.305
3.632 –0.508
)
0.067 ± 0.010
(1.702 ± 0.254)
0.032 ± 0.008
(0.813 ± 0.203)
0.022 ± 0.005
(0.559 ± 0.127)
0.050 ± 0.012
(1.270 ± 0.305)
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.
11
LT1082
U
PACKAGE DESCRIPTIO
Dimensions in inches (milimeters) unless otherwise noted.
T Package
5-Lead TO-220
0.380 – 0.420
(9.652 – 10.668)
0.079 – 0.135
(2.007 – 3.429)
0.139 – 0.153
(3.531 – 3.886)
DIA
0.560 – 0.650
(14.224 – 16.510)
0.169 – 0.185
(4.293 – 4.699)
0.460 – 0.500
(11.68 – 12.70)
0.866 – 0.913
(21.996 – 23.190)
0.035 – 0.055
(0.889 – 1.397)
0.620 ± 0.020
(15.75 ± 0.508)
0.700 – 0.728
(17.780 – 18.491)
0.970 – 1.050
(24.64 – 26.67)
0.015 – 0.025
(0.381 – 0.635)
0.057 – 0.077
(1.448 – 1.956)
12
0.028 – 0.035
(0.711 – 0.889)
Linear Technology Corporation
0.210 – 0.240
(5.334 – 6.096)
0.055 – 0.090
(1.397 – 2.286)
0.079 – 0.115
(2.007 – 2.921)
T5 (FORMED) 0392
LT/GP 0193 10K REV 0
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1993