LT3085 datasheet(13/28 Pages) LINER

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LT3085

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APPLICATIONS INFORMATION

The LT3085 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 LT3085,

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 100nV/√Hz

(33μVRMS 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 LT3085 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 LT3085 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 LT3085.

between the devices is within 2°C, showing excellent

tracking between the devices. The board temperature has

reached approximately 40°C 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 ambients, the

temperature must be controlled to prevent device tempera-

ture from exceeding 125°C. A 3-meter-per-second airﬂow

across the devices will decrease the device temperature

about 20°C providing a margin for higher operating ambi-

ent 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.

Quieting the Noise

The LT3085 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 LT3085 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 2.3pA/√Hz (0.7nARMS 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.

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