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LT3080ET#PBF

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LT3080
13
3080fc
from the error amplifier must be considered, and the gain
created by using a resistor divider cannot be forgotten.
Traditional low noise regulators bring the voltage refer-
ence out to an external pin (usually through a large value
resistor) to allow for bypassing and noise reduction of
reference noise. The LT3080 does not use a traditional
voltage reference like other linear regulators, but instead
uses a reference current. That current operates with typi-
cal noise current levels of 3.2pA/√Hz (1nA
RMS
over the
10Hz to 100kHz bandwidth). The voltage noise of this
is equal to the noise current multiplied by the resistor
value. The resistor generates spot noise equal to √4kTR
(k = Boltzmann’s constant, 1.38 • 10
–23
J/°K, and T is
absolute temperature) which is RMS summed with the
reference current noise. To lower reference noise, the
voltage setting resistor may be bypassed with a capacitor,
though this causes start-up time to increase as a factor
of the RC time constant.
The LT3080 uses a unity-gain follower from the SET pin
to drive the output, and there is no requirement to use
a resistor to set the output voltage. Use a high accuracy
voltage reference placed at the SET pin to remove the er-
rors in output voltage due to reference current tolerance
and resistor tolerance. Active driving of the SET pin is
acceptable; the limitations are the creativity and ingenuity
of the circuit designer.
One problem that a normal linear regulator sees with refer-
ence voltage noise is that noise is gained up along with the
output when using a resistor divider to operate at levels
higher than the normal reference voltage. With the LT3080,
the unity-gain follower presents no gain whatsoever from
the SET pin to the output, so noise figures do not increase
accordingly. Error amplifier noise is typically 125nV/√Hz
(40µV
RMS
over the 10Hz to 100kHz bandwidth); this is
another factor that is RMS summed in to give a final noise
figure for the regulator.
Curves in the Typical Performance Characteristics show
noise spectral density and peak-to-peak noise character-
istics for both the reference current and error amplifier
over the 10Hz to 100kHz bandwidth.
Overload Recovery
Like many IC power regulators, the LT3080 has safe operat-
ing area (SOA) protection. The SOA protection decreases
current limit as the input-to-output voltage increases and
keeps the power dissipation at safe levels for all values
of input-to-output voltage. The LT3080 provides some
output current at all values of input-to-output voltage up
to the device breakdown. See the Current Limit curve in
the Typical Performance Characteristics.
When power is first turned on, the input voltage rises and
the output follows the input, allowing the regulator to start
into very heavy loads. During start-up, as the input voltage
is rising, the input-to-output voltage differential is small,
allowing the regulator to supply large output currents.
With a high input voltage, a problem can occur wherein
removal of an output short will not allow the output volt-
age to recover. Other regulators, such as the LT1085 and
LT1764A, also exhibit this phenomenon so it is not unique
to the LT3080.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low. Com-
mon situations are immediately after the removal of a
short circuit. The load line for such a load may intersect
the output current curve at two points. If this happens,
there are two stable operating points for the regulator.
With this double intersection, the input power supply may
need to be cycled down to zero and brought up again to
make the output recover.
Load Regulation
Because the LT3080 is a floating device (there is no ground
pin on the part, all quiescent and drive current is delivered
to the load), it is not possible to provide true remote load
sensing. Load regulation will be limited by the resistance
Figure 7. Connections for Best Load Regulation
+
–
LT3080
IN
V
CONTROL
OUT
3080 F07
SET
R
SET
R
P
PARASITIC
RESISTANCE
R
P
R
P
LOAD
applicaTions inForMaTion
LT3080
14
3080fc
Table 2. MSE Package, 8-Lead MSOP
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm
2
2500mm
2
2500mm
2
55°C/W
1000mm
2
2500mm
2
2500mm
2
57°C/W
225mm
2
2500mm
2
2500mm
2
60°C/W
100mm
2
2500mm
2
2500mm
2
65°C/W
*Device is mounted on topside
Table 3. DD Package, 8-Lead DFN
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm
2
2500mm
2
2500mm
2
60°C/W
1000mm
2
2500mm
2
2500mm
2
62°C/W
225mm
2
2500mm
2
2500mm
2
65°C/W
100mm
2
2500mm
2
2500mm
2
68°C/W
*Device is mounted on topside
Table 4. ST Package, 3-Lead SOT-223
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm
2
2500mm
2
2500mm
2
48°C/W
1000mm
2
2500mm
2
2500mm
2
48°C/W
225mm
2
2500mm
2
2500mm
2
56°C/W
100mm
2
2500mm
2
2500mm
2
62°C/W
*Device is mounted on topside
Table 5. Q Package, 5-Lead DD-Pak
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm
2
2500mm
2
2500mm
2
25°C/W
1000mm
2
2500mm
2
2500mm
2
30°C/W
125mm
2
2500mm
2
2500mm
2
35°C/W
*Device is mounted on topside
T Package, 5-Lead TO-220
Thermal Resistance (Junction-to-Case) = 3°C/W
Calculating Junction Temperature
Example: Given an output voltage of 0.9V, a V
CONTROL
voltage of 3.3V ±10%, an IN voltage of 1.5V ±5%, output
current range from 1mA to 1A and a maximum ambient
temperature of 50°C, what will the maximum junction
temperature be for the DFN package on a 2500mm
2
board
with topside copper area of 500mm
2
?
of the connections between the regulator and the load.
The data sheet specification for load regulation is Kelvin
sensed at the pins of the package. Negative side sensing
is a true Kelvin connection, with the bottom of the voltage
setting resistor returned to the negative side of the load
(see Figure 7). Connected as shown, system load regula-
tion will be the sum of the LT3080 load regulation and the
parasitic line resistance multiplied by the output current.
It is important to keep the positive connection between
the regulator and load as short as possible and use large
wire or PC board traces.
Thermal Considerations
The LT3080 has internal power and thermal limiting cir-
cuitry designed to protect it under overload conditions.
For continuous normal load conditions, maximum junc-
tion temperature must not be exceeded. It is important
to give consideration to all sources of thermal resistance
from junction to ambient. This includes junction-to-case,
case-to-heat sink interface, heat sink resistance or circuit
board-to-ambient as the application dictates. Additional
heat sources nearby must also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Surface mount heat sinks and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
Junction-to-case thermal resistance is specified from the
IC junction to the bottom of the case directly below the
die. This is the lowest resistance path for heat flow. Proper
mounting is required to ensure the best possible thermal
flow from this area of the package to the heat sinking
material. For the TO-220 package, thermal compound is
strongly recommended for mechanical connections to a
heat sink. A thermally conductive spacer can be used for
electrical isolation as long as the added contribution to
thermal resistance is considered. Note that the Tab or
Exposed Pad (depending on package) is electrically
connected to the output.
The following tables list thermal resistance for several
different copper areas given a fixed board size. All mea-
surements were taken in still air on two-sided 1/16” FR-4
board with one ounce copper.
applicaTions inForMaTion
LT3080
15
3080fc
The power in the drive circuit equals:
P
DRIVE
= (V
CONTROL
– V
OUT
)(I
CONTROL
)
where I
CONTROL
is equal to I
OUT
/60. I
CONTROL
is a function
of output current. A curve of I
CONTROL
vs I
OUT
can be found
in the Typical Performance Characteristics curves.
The power in the output transistor equals:
P
OUTPUT
= (V
IN
– V
OUT
)(I
OUT
)
The total power equals:
P
TOTAL
= P
DRIVE
+ P
OUTPUT
The current delivered to the SET pin is negligible and can
be ignored.
V
CONTROL(MAX CONTINUOUS)
= 3.630V (3.3V + 10%)
V
IN(MAX CONTINUOUS)
= 1.575V (1.5V + 5%)
V
OUT
= 0.9V, I
OUT
= 1A, T
A
= 50°C
Power dissipation under these conditions is equal to:
PDRIVE = (V
CONTROL
– V
OUT
)(I
CONTROL
)
I
CONTROL
=
I
OUT
60
=
1A
60
= 17mA
P
DRIVE
= (3.630V – 0.9V)(17mA) = 46mW
P
OUTPUT
= (V
IN
– V
OUT
)(I
OUT
)
P
OUTPUT
= (1.575V – 0.9V)(1A) = 675mW
Total Power Dissipation = 721mW
Junction Temperature will be equal to:
T
J
= T
A
+ P
TOTAL
• θ
JA
(approximated using tables)
T
J
= 50°C + 721mW • 64°C/W = 96°C
In this case, the junction temperature is below the maxi-
mum rating, ensuring reliable operation.
Reducing Power Dissipation
In some applications it may be necessary to reduce
the power dissipation in the LT3080 package without
sacrificing output current capability. Two techniques are
available. The first technique, illustrated in Figure 8, em-
ploys a resistor in series with the regulator’s input. The
voltage drop across R
S
decreases the LT3080’s IN-to-OUT
differential voltage and correspondingly decreases the
LT3080’s power dissipation.
As an example, assume: V
IN
= V
CONTROL
= 5V, V
OUT
= 3.3V
and I
OUT(MAX)
= 1A. Use the formulas from the Calculating
Junction Temperature section previously discussed.
Without series resistor R
S
, power dissipation in the LT3080
equals:
P
TOTAL
= 5V – 3.3V
( )
•
1A
60
+ 5V – 3.3V
( )
• 1A
= 1.73W
If the voltage differential (V
DIFF
) across the NPN pass
transistor is chosen as 0.5V, then R
S
equals:
R
S
=
5V – 3.3V
−
0.5V
1A
= 1.2Ω
Power dissipation in the LT3080 now equals:
P
TOTAL
= 5V – 3.3V
( )
•
1A
60
+ 0.5V
( )
• 1A = 0.53W
The LT3080’s power dissipation is now only 30% compared
to no series resistor. R
S
dissipates 1.2W of power. Choose
appropriate wattage resistors to handle and dissipate the
power properly.
Figure 8. Reducing Power Dissipation Using a Series Resistor
+
–
LT3080
IN
V
CONTROL
OUT
V
OUT
V
IN
ʹ
V
IN
C2
3080 F08
SET
R
SET
R
S
C1
applicaTions inForMaTion
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LT3080ET#PBF

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Buy LT3080ET#PBF
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Analog Devices / Linear Technology
Description:
LDO Voltage Regulators Adj 1.1A 1x Res L Drop Reg
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