LT3080EDD-1#PBF Datasheet LT3080IDD-1#PBF, LT3080IMS8E-1#PBF, LT3080IDD-1#TRPBF | P10-P12

LT3080-1

30801fc

For more information www.linear.com/LT3080-1

temperature and applied voltage. The most common

dielectrics used are specified with EIA temperature char-

acteristic codes of Z5U, Y5V,

5R and X7R. The Z5U and

Y5V dielectrics are good for providing high capacitances

in a small package, but they tend to have strong voltage

and temperature coefficients as shown in Figures 2 and 3.

When used with a 5V regulator, a 16V 10µF Y5V capacitor

can exhibit an effective value as low as 1µF to 2µF for the

DC bias voltage applied and over the operating tempera

ture range. The X5R and X7R dielectrics result in more

stable characteristics and are more suitable for use as the

output capacitor.

The X

7R type has better stability across

temperature, while the X5R is less expensive and is avail

able in higher values. Care still must be exercised when

using X5

R and X7R capacitors; the X5R and X7R codes

only specify operating temperature range and maximum

capacitance change over temperature. Capacitance change

due to DC bias with X5R and X7R capacitors is better than

Y5V and Z5U capacitors, but can still be significant enough

to drop capacitor values below appropriate levels. Capaci

tor DC bias characteristics tend to improve as component

case size increases,

but expected capacitance at operating

voltage should be verified.

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress,

similar to the way a piezoelectric microphone works. For a

ceramic capacitor the stress can be induced by vibrations

in the system or thermal transients.

Paralleling Devices

LT3080-1’s may be directly paralleled to obtain higher

output current. The SET pins are tied together and the

IN pins are tied together. This is the same whether it’s in

three terminal mode or has separate input supplies. The

outputs are connected in common; the internal ballast

resistor equalizes the currents.

The worst-case offset between the SET pin and the output

of only ±2 millivolts allows very small ballast resistors

to be used. As shown in Figure 4, the two devices have

internal ballast resistors, which at full output current gives

better than 90 percent equalized sharing of the current.

The internal resistance of 25 milliohms (per device) only

adds about 25 millivolts of output regulation drop at an

DC BIAS VOLTAGE (V)

CHANGE IN VALUE (%)

30801 F02

–20

–40

–60

–80

–100

2 6

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

Figure 2. Ceramic Capacitor DC Bias Characteristics

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

25 75

3080 F03

–25 0

50 100 125

Y5V

CHANGE IN VALUE (%)

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

Figure 3. Ceramic Capacitor Temperature Characteristics

Figure 4. Parallel Devices

applicaTions inFormaTion

–

LT3080-1

CONTROL

OUT

SET

25mΩ

–

LT3080-1

4.8V TO 28V

OUT

3.3V

2.2A

CONTROL

OUT

10µF

1µF

SET

165k

30801 F04

25mΩ

LT3080-1

30801fc

For more information www.linear.com/LT3080-1

output of 2A. At low output voltage, 1V, this adds 2.5%

regulation. The output can be set 19mV high for lower ab-

solute error ±1.3%. Of course, more than two LT3080-1’s

can be paralleled for even higher output current. They are

spread out on the PC board

spreading the heat. Input

resistors can further spread the heat if the input-to-output

difference is high.

Thermal Performance

In this example, two LT3080-1 3mm

3mm DFN devices

are mounted on a 1oz copper 4-layer PC board. They are

placed approximately 1.5 inches apart and the board is

mounted vertically for convection cooling. Two tests were

set up to measure the cooling performance and current

sharing of these devices.

The first test was done with approximately 0.7V input-

to-output and 1A per device. This gave a 700 milliwatt

dissipation in each device and a 2A output current. The

temperature rise above ambient is approximately 28°C

and both devices were within plus or minus 1°C. Both

the thermal and electrical sharing of these devices is

excellent. The thermograph in Figure 5 shows the tem

perature distribution between these devices and the PC

board reaches ambient temperature within about a half

an inch from the devices.

The power is then increased with

1.7V across each de-

vice. This gives 1.7 watts dissipation in each device and

a device temperature of about

90°C,

about 65°C above

ambient as shown in Figure 6. Again, the temperature

matching between the devices is within 2°C, showing

excellent tracking between the devices. The board tem

perature has reached approximately 40°

within about

0.75 inches of each device.

While 90°C is an acceptable operating temperature for

these devices, this is in 25°C ambient. For higher am-

bients, the temperature must be controlled to prevent

device temperature from exceeding 125°C.

A three meter

per second airflow across the devices will decrease the

device temperature about 20°C providing a margin for

higher operating ambient temperatures.

Both at low power and relatively high power levels de

vices can be paralleled for higher output current. Current

sharing and thermal sharing is excellent,

showing that

acceptable operation can be had while keeping the peak

temperatures below excessive operating temperatures on

a board. This technique allows higher operating current

linear regulation to be used in systems where it could

never be used before.

Figure 6. Temperature Rise at 1.7W Dissipation

Figure 5. Temperature Rise at 700mW Dissipation

applicaTions inFormaTion

LT3080-1

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For more information www.linear.com/LT3080-1

Quieting the Noise

The LT3080-1 offers numerous advantages when it comes

to dealing with noise. There are several sources of noise

in a linear regulator. The most critical noise source for any

LDO is the reference; from there, the noise contribution

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 LT3080-1 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 set-

ting resistor may be bypassed with a capacitor, though

this causes start-up time to increase as a factor of the RC

time constant.

The LT3080-1 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

reference 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 LT3080-1, the unity-gain follower presents no gain

whatsoever from the SET pin to the output, so noise fig-

ures 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 LT3080-1 has safe oper-

ating 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 LT3080-1 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 section.

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

1764A,

also exhibit this phenomenon so it is not unique

to the LT3080-1.

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 cir

cuit

. 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.

applicaTions inFormaTion

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LT3080EDD-1#PBF

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Manufacturer:

Analog Devices / Linear Technology

Description:

LDO Voltage Regulators Parallelable 1.1A Adj 1x Res L Drop Reg

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