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LT1964

1964fb

APPLICATIONS INFORMATION

amounts of noise, especially when a ceramic capacitor is

used for noise bypassing. A ceramic capacitor produced

Figure 4’s trace in response to light tapping from a pencil.

Similar vibration induced behavior can masquerade as

increased output voltage noise.

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 gener-

ated 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 board with one ounce

copper.

Table 1. SOT-23 Thermal Resistance

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

2500mm2

125°C/W

1000mm2

2500mm2

125°C/W

225mm2

2500mm2

130°C/W

100mm2

2500mm2

135°C/W

50mm2

2500mm2

150°C/W

*Device is mounted on topside.

Table 2. DFN Thermal Resistance

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

2500mm2

40°C/W

1000mm2

2500mm2

45°C/W

225mm2

2500mm2

50°C/W

100mm2

2500mm2

62°C/W

*Device is mounted on topside.

The thermal resistance junction-to-case

(θJC), measured

at Pin 2, is 60°C/W. for the SOT-23 package and is 16°C/W

measured at the backside of the exposed pad on the DFN

package

Calculating Junction Temperature

Example: Given an output voltage of –5V, an input voltage

range of –6V to –8V, an output current range of 0mA to

–100mA, and a maximum ambient temperature of 50°C,

what will the maximum junction temperature be?

The power dissipated by the device will be equal to:

Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor

LT1964-5

COUT = 10μF

CBYP = 0.01μF

ILOAD = –200mA

VOUT

1mV/DIV

100ms/DIV

1964 F04

Thermal Considerations

The power handling capability of the device will be limited

by the maximum rated junction temperature (125°C). The

power dissipated by the device will be made up of two

components:

1. Output current multiplied by the input/output voltage

differential: IOUT • (VIN – VOUT), and

2. Ground pin current multiplied by the input voltage:

IGND • VIN

The GND pin current can be found by examining the GND

Pin Current curves in the Typical Performance Character-

istics. Power dissipation will be equal to the sum of the

two components listed above.

The LT1964 series regulators have internal thermal limiting

designed to protect the device during overload conditions.

For continuous normal conditions the maximum junction

temperature rating of 125°C must not be exceeded. It is

important to give careful consideration to all sources of

thermal resistance from junction to ambient. Additional

heat sources mounted nearby must also be considered.

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