Datasheet LT3088 (Analog Devices) - 10

LT3088
APPLICATIONS INFORMATION
With the 50µA current source used to generate the reference
Using the LT3088 as a Replacement for the LT1117
voltage, leakage paths to or from the SET pin can create The LT3088 can be used as an upgrade or replacement errors in the reference and output voltages. High quality for the LT1117 regulator. The LT3088 offers superior insulation should be used (e.g., Teflon, Kel-F); cleaning of performance over the LT1117, including extended input all insulating surfaces to remove fluxes and other residues voltage range, lower output voltage capability, extended is required. Surface coating may be necessary to provide safe operating area, and protection features such as reverse a moisture barrier in high humidity environments. voltage/current protection. Figure 3 shows how the LT1117 Minimize board leakage by encircling the SET pin and is used as a basic adjustable regulator. Two methods are circuitry with a guard ring operated at a potential close shown in Figures 4 and 5 to change from the LT1117 to to itself. Tie the guard ring to the OUT pin. Guarding both the LT3088. The first method (shown in Figure 4) requires sides of the circuit board is required. Bulk leakage reduction no changes to existing board layouts: replace the LT1117 depends on the guard ring width. 50nA of leakage into or with the LT3088, change resistor R2 to set the desired out of the SET pin and its associated circuitry creates a output voltage, and do not stuff resistor R1 (the minimum 0.1% reference voltage error. Leakages of this magnitude, load requirement of 2mA for the LT3088 must still be met). coupled with other sources of leakage, can cause signifi- The second method is shown in Figure 5: a 25k resistor is cant offset voltage and reference drift, especially over the possible operating temperature range. Figure 2 depicts an LT1117 example guard ring layout. V IN OUT V IN OUT + ADJ V
If guard ring techniques are used, this bootstraps any
REF R1 1.25V
stray capacitance at the SET pin. Since the SET pin is
IADJ 50µA
a high impedance node, unwanted signals may couple
R2 R2
into the SET pin and cause erratic behavior. This will
VOUT = VREF 1 + + I — ADJ R2 ( )R1 3088 F03
be most noticeable when operating with minimum Figure 3. LT1117 Basic Adjustable Regulator 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
LT3088
20pF is sufficient.
V IN OUT V IN OUT + SET R1* ISET 50µA OUT R2 VOUT = ISET • R2 *DO NOT STUFF R1 3088 F04 SET
Figure 4. Upgrade to LT1117 Requires No Layout Changes
LT3088 GND 3088 F02 V IN OUT V IN OUT + SET 1.25V R1
Figure 2. Guard Ring Layout Example of DD Package
25k ISET 50µA R2 V R2 OUT = 1.25V 1 + + I — SET R2 ( )R1 3088 F05
Figure 5. Resistor in Series with SET Pin Matches LT1117 Operation
3088fb 10 For more information www.linear.com/LT3088 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Order Information Typical Performance Characteristics Block Diagram Applications Information Revision History Typical Application Related Parts