MIC5204

Micrel

February 1999

62

MIC5204

Thermal Considerations

Applications Information

External Capacitors

A 2.2

µF capacitor is recommended between the MIC5204

output and ground to prevent oscillations due to instability.

Larger values serve to improve the regulator's transient re-

sponse. Most types of tantalum or aluminum electrolytics will

be adequate; film types will work. Many aluminum electrolytics

have electrolytes that freeze at about –30

°C,sosolidtantalums

are recommended for operation below –25

°C. The important

parameters of the capacitor are an effective series resistance

of about 5

Ω or less and a resonant frequency above 500kHz.

The value of this capacitor may be increased without limit.

A 1

µF capacitor should be placed from the MIC5204 input to

ground if there is more than 10 inches of wire between the

input and the AC filter capacitor or if a battery is used as the

input.

The MIC5204 will remain stable and in regulation with no load

in addition to the internal voltage divider.

PC Board

θθθθθ

JA

Dielectric

FR4

160

°C/W

Ceramic

120

°C/W

Multi-layer boards having a ground plane, wide traces near

the pads, and large supply bus lines provide better thermal

conductivity. The "worst case" value of 160

°C/W assumes no

ground plane, minimum trace widths, and a FR4 material

board.

Part II. Nominal Power Dissipation and Die Temperature

The MIC5204BM at a 25

°C ambient temperature will operate

reliably at up to 625mW power dissipation when mounted in

the "worst case" manner described above. At an ambient

temperature of 55

°C, the device may safely dissipate 440mW.

These power levels are equivalent to a die temperature of

125

°C, the recommended maximum temperature for non-

military grade silicon integrated circuits. In normal SCSI

terminator applications, the average power dissipation is very

small and this minimum geometry heat sink is suitable. The

total dissipation does not approact the 400mW to 625mW

range described above.

For MIC5204BS (SOT-223 package) heat sink characteris-

tics, please refer to Micrel Application Hint 17, “P.C. Board

Heat Sinking”. As with the SO-8, average power dissipation

in SCSI terminator applications is low and a minimum pad size

is generally adequate.

Part I. Layout

The MIC5204BM (8-pin surface mount package) has the

following thermal characteristics when mounted on a single

layer copper-clad printed circuit board.

245 mil

30 mil

50 mil

50 mil

150 mil

Minimum recommended board pad size, SO-8.