LT1963 datasheet(10/26 Pages) LINER

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LT3088

3088fb

For more information www.linear.com/LT3088

APPLICATIONS INFORMATION

Withthe50µAcurrentsourceusedtogeneratethereference

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 residues

is required. Surface coating may be necessary to provide

a moisture barrier in high humidity environments.

Minimize board leakage by encircling the SET pin and

circuitry with a guard ring operated at a potential close

to itself. Tie the guard ring to the OUT pin. Guarding both

sidesofthecircuitboardisrequired.Bulkleakagereduction

depends on the guard ring width. 50nA of leakage into or

out of the SET pin and its associated circuitry creates a

0.1% reference voltage error. Leakages of this magnitude,

coupled with other sources of leakage, can cause signifi-

cant offset voltage and reference drift, especially over the

possible operating temperature range. Figure 2 depicts an

example guard ring layout.

If guard ring 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.

Using the LT3088 as a Replacement for the LT1117

The LT3088 can be used as an upgrade or replacement

for the LT1117 regulator. The LT3088 offers superior

performance over the LT1117, including extended input

voltage range, lower output voltage capability, extended

safeoperatingarea,andprotectionfeaturessuchasreverse

voltage/currentprotection.Figure 3showshowtheLT1117

is used as a basic adjustable regulator. Two methods are

shown in Figures 4 and 5 to change from the LT1117 to

the LT3088. The first method (shown in Figure 4) requires

no changes to existing board layouts: replace the LT1117

with the LT3088, change resistor R2 to set the desired

output voltage, and do not stuff resistor R1 (the minimum

loadrequirementof 2mAforthe LT3088muststillbemet).

The second method is shown in Figure 5: a 25k resistor is

Figure 2. Guard Ring Layout Example of DD Package

3088 F02

OUT

GND

SET

Figure 3. LT1117 Basic Adjustable Regulator

Figure 4. Upgrade to LT1117 Requires No Layout Changes

Figure 5. Resistor in Series with SET Pin Matches

LT1117 Operation

3088 F03

VOUT

VIN

VREF

1.25V

IADJ

50µA

LT1117

ADJ

OUT

VOUT = VREF 1 +

+ IADJ R2

—

( )

3088 F04

R1*

VOUT

VIN

ISET

50µA

LT3088

SET

OUT

VOUT = ISET • R2

*DO NOT STUFF R1

3088 F05

VOUT

VIN

1.25V

25k

ISET

50µA

LT3088

SET

OUT

VOUT = 1.25V 1 +

+ ISET R2

—

( )

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