LT3085EDCB-PBF datasheet(14/28 Pages) LINER

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LT3085

3085fa

APPLICATIONS INFORMATION

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 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.

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 LT3085 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

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 regulation

will be the sum of the LT3085 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.

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