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LT3085

3085fb

The LT3085 can be operated in two modes. Three terminal

mode has the control pin connected to the power input pin

which gives a limitation of 1.35V dropout. Alternatively, the

“control” pin can be tied to a higher voltage and the power

IN pin to a lower voltage giving 275mV dropout on the

IN pin and minimizing the power dissipation. This allows

for a 500mA supply regulating from 2.5VIN to 1.8VOUT or

1.8VIN to 1.2VOUT with low dissipation.

Setting Output Voltage

The LT3085 generates a 10μA reference current that ﬂows

out of the SET pin. Connecting a resistor from SET to

ground generates a voltage that becomes the reference

point for the error ampliﬁer (see Figure 1). The reference

voltage is a straight multiplication of the SET pin current

and the value of the resistor. Any voltage can be generated

and there is no minimum output voltage for the regulator.

Table 1 lists many common output voltages and standard

1% resistor values used to generate that output voltage.

A minimum load current of 1mA is required to maintain

regulation regardless of output voltage. For true zero

voltage output operation, this 1mA load current must be

returned to a negative supply voltage.

APPLICATIONS INFORMATION

With the low level current used to generate the reference

voltage, leakage paths to or from the SET pin can create

errors in the reference and output voltages. High quality

insulation should be used (e.g., Teﬂon, Kel-F); cleaning

of all insulating surfaces to remove ﬂuxes and other resi-

dues will probably be required. Surface coating may be

necessary to provide a moisture barrier in high humidity

environments.

Table 1. 1% Resistors for Common Output Voltages

VOUT

RSET

100k

1.2V

121k

1.5V

150k

1.8V

182k

2.5V

249k

3.3V

332k

499k

Board leakage can be minimized by encircling the SET

pin and circuitry with a guardring operated at a potential

close to itself; the guardring should be tied to the OUT pin.

Guarding both sides of the circuit board is required. Bulk

leakage reduction depends on the guard ring width. Ten

nanoamperes of leakage into or out of the SET pin and

associated circuitry creates a 0.1% error in the reference

voltage. Leakages of this magnitude, coupled with other

sources of leakage, can cause signiﬁcant offset voltage and

reference drift, especially over a wide temperature range.

If guardring techniques are used, this bootstraps any

stray capacitance at the SET pin. Since the SET pin is

a high impedance node, unwanted signals may couple

into the SET pin and cause erratic behavior. This will

be most noticeable when operating with minimum

output capacitors at full load current. The easiest way

to remedy this is to bypass the SET pin with a small

amount of capacitance from SET to ground, 10pF to

20pF is sufﬁcient.

Figure 1. Basic Adjustable Regulator

–

LT3085

10μA

VCONTROL

OUT

3085 F01

SET

COUT

RSET

VOUT

CSET

VIN

VOUT = RSET • 10μA

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