ADP3367

REV. 0

–7–

This may be expressed in terms of power dissipation as follows:

θ

where:

°C)

°C)

θ

°C/W)

If the device is being operated at the maximum permitted ambi-

ent temperature of 85

°C, the maximum power dissipation per-

mitted is:

= 40/

θ

JA

where:

θ

Therefore, for a maximum ambient temperature of 85

°C

At lower ambient temperatures the maximum permitted power

dissipation increases accordingly up to the maximum limits

specified in the absolute maximum specifications.

The thermal impedance (

θ

air conditions and are reduced considerably where fan assisted

cooling is employed. Other techniques for reducing the thermal

impedance include large contact pads on the printed circuit

board and wide traces. The copper will act as a heat exchanger

thereby reducing the effective thermal impedance.

POWER DISSIPATION

Low Thermal Resistance Package

The ADP3367 utilizes a patented and proprietary Thermal

Coastline Leadframe which offers significantly lower resistance

to heat flow from die to the PC board.

Heat generated on the die is removed and transferred to the PC

board faster resulting in lower die temperature than standard

packages. Table II is a performance comparison between and

standard and Thermal Coastline package.

Table I. Thermal Resistance Performance Comparison*

Standard Package (SO-8)

Thermal Coastline Package

θ

JC

44

°C/W

40

°C/W

θ

JA

170

°C/W

98

°C/W

PD

235 mW

408 mW

*Data presented in Table II is obtained using SEMI Standard Method G38-47

and SEMI Standard Specification G42-88.

A device operating at room temperature, +25

°C, and +125°C

junction temperature can dissipate 1.15 W.

To maintain this high level of heat removal efficiency, once heat

is removed from the die to the PC board, it should be dissipated

to the air or other mediums to maintain the largest possible tem-

perature differential between the die and PC board; remember,

the rate at which heat is transferred is directly proportional to

the temperature differential.

Various PC board layout techniques could be used to remove

the heat from the immediate vicinity of the package. Consider

the following issues when designing a board layout:

1. PC board traces with larger copper cross section areas will

remove more heat; use PCs with thicker copper and/or wider

traces.

2. Increase the surface area exposed to open air so heat can be

removed by convection or forced air flow.

3. Use larger masses such as heat sinks or thermally conductive

enclosures to distribute and dissipate the heat.

4. Do not solder mask or silk screen the heat dissipating traces;

black anodizing will significantly improve heat dissipation by

means of increased radiation.

High Power Dissipation Recommendations

Where excessive power dissipation due to high input-output

differential voltages and/or high current conditions exists, the

simplest method of reducing the power requirements on the

regulator is to use a series dropper resistor. In this way the

excess power can be dissipated in the external resistor. As an

example, consider an input voltage of +12 V and an output

voltage requirement of +5 V @ 100 mA with an ambient tem-

perature of +85

°C. The package power dissipation under these

conditions is 700 mW which exceeds the maximum ratings. By

using a dropper resistor to drop 4 V, the power dissipation

requirement for the regulator is reduced to 300 mW which is

within the maximum specifications for the SO-8 package at

85

°C. The resistor value is calculated as R = 4/0.1 = 40 Ω.A

resistor power rating of 1/2 W or greater may be used.

IN

OUT

GND

SET

SHDN

ADP3367

+5V

OUTPUT

C2

10µF

+

40

Ω

0.5W

C1

1µF

12V

+

Figure 14. Reducing Regulator Power Dissipation

Transient Response

The ADP3367 exhibits excellent transient performance as illus-

trated in the “Typical Performance Characteristics.” Figure 6

shows that an input step from 10 V to 6 V results in a very small

output disturbance (50 mV). Adding an input capacitor would

improve this even more.

Figure 7 shows how quickly the regulator recovers from an out-

put load change from 10 mA to 100 mA. The offset due to the

load current change is less than 1 mV.

Monitored

µP Power Supply

Figure 15 shows the ADP3367 being used in a monitored

µP

supply application. The ADP3367 supplies +5 V for the micro-