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LT3008 Series

3008fc

APPLICATIONS INFORMATION

Figure 5. Noise Resulting from Tapping

on a Ceramic Capacitor

GND pin current is found by examining the GND Pin

Current curves in the Typical Performance Characteristics

section. Power dissipation is equal to the sum of the two

components listed prior.

The LT3008 regulator has internal thermal limiting designed

to protect the device during overload conditions. For

continuous normal conditions, do not exceed the maximum

junction temperature rating of 125°C. Carefully consider

all sources of thermal resistance from junction to ambient

including other heat sources mounted in proximity to

the LT3008. For surface mount devices, heat sinking is

accomplished by using the heat spreading capabilities of

the PC board and its copper traces. Copper board stiffeners

and plated through-holes can also be used to spread the

heat generated by power devices.

The following tables list thermal resistance for several

different board sizes and copper areas. All measurements

were taken in still air on 3/32" FR-4 two-layer boards with

one ounce copper.

PCB layers, copper weight, board layout and thermal vias

affect the resultant thermal resistance. Although Tables

2 and 3 provide thermal resistance numbers for 2-layer

boards with 1 ounce copper, modern multilayer PCBs

provide better performance than found in these tables.

100ms/DIV

VOUT

500μV/DIV

3008 F05

VOUT = 0.6V

COUT = 22μF

ILOAD = 10μA

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress,

similar to the way a piezoelectric accelerometer or

microphone works. For a ceramic capacitor, the stress

can be induced by vibrations in the system or thermal

transients. The resulting voltages produced can cause

appreciable amounts of noise, especially when a ceramic

capacitor is used for noise bypassing. A ceramic capacitor

produced Figure 5’s trace in response to light tapping from a

pencil. Similar vibration induced behavior can masquerade

as increased output voltage noise.

Table 2: Measured Thermal Resistance for DC Package

COPPER AREA

BOARD

AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

2500mm2

65°C/W

1000mm2

2500mm2

70°C/W

225mm2

2500mm2

75°C/W

100mm2

2500mm2

80°C/W

50mm2

2500mm2

85°C/W

*Device is mounted on the topside.

Thermal Considerations

The LT3008’s maximum rated junction temperature of

125°C limits its power-handling capability. Two components

comprise the power dissipated by the device:

1. Output current multiplied by the input/output voltage

differential: IOUT • (VIN – VOUT)

2. GND pin current multiplied by the input voltage:

IGND • VIN

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