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LT1762 Series

APPLICATIONS INFORMATION

across temperature, while the X5R is less expensive and

is available in higher values.

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress,

similar to the way a piezoelectric accelerometer or micro-

phone 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 6’s trace in response to light tapping from a pencil.

Similar vibration induced behavior can masquerade as

increased output voltage noise.

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 LT1762 series regulators have internal thermal limit-

ing designed to protect the device during overload condi-

tions. 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.

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 table lists 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. Measured Thermal Resistance

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm

110

°C/W

1000mm

2500mm

115

°C/W

225mm

2500mm

120

°C/W

100mm

2500mm

130

°C/W

50mm

2500mm

140

°C/W

*Device is mounted on topside.

Calculating Junction Temperature

Example: Given an output voltage of 3.3V, an input voltage

range of 4V to 6V, an output current range of 0mA to 50mA

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:

IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX))

Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor

LT1762-5

COUT = 10µF

CBYP = 0.01µf

ILOAD = 100mA

VOUT

500

µV/DIV

100ms/DIV

1762 F05

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. GND pin current multiplied by the input voltage:

(IGND)(VIN).

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