LT3080EST datasheet(12/26 Pages) LINER

12 / 26 page

LT3080

3080fb

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 (1nARMS 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

(40μVRMS 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

VCONTROL

OUT

3080 F07

SET RSET

PARASITIC

RESISTANCE

LOAD

APPLICATIONS INFORMATION

LT3080EST Datasheet(PDF) 12 Page - Linear Technology

LT3080EST Datasheet(HTML) 12 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL