9

LT1121/LT1121-3.3/LT1121-5

S

APPLICATI

I FOR ATIO

taken in still air, on 3/32" FR-4 board with 1oz copper. All

NC leads were connected to the ground plane.

Calculating Junction Temperature

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

range of 4.5V to 7V, an output current range of 0mA to

100mA, and a maximum ambient temperature of 50

°C,

what will the maximum junction temperature be?

Power dissipated by the device will be equal to:

so,

P = 100mA

× (7V – 3.3V) + (5mA × 7V)

= 0.405W

If we use an SOT-223 package, then the thermal resistance

will be in the range of 50

°C/W to 65°C/W depending on

copper area. So the junction temperature rise above

ambient will be less than or equal to:

0.405W

× 60°C/W = 24°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

Table 5. TO-92 Package

THERMAL

RESISTANCE

Package alone

220

°C/W

Package soldered into PC board with plated

175

°C/W

through holes only

Package soldered into PC board with 1/4 sq. inch of

145

°C/W

copper trace per lead

Package soldered into PC board with plated through holes

in board, no extra copper trace, and a clip-on type

heat sink:

Thermalloy type 2224B

160

°C/W

Aavid type 5754

135

°C/W

Output Capacitance and Transient Performance

The LTC1121 is designed to be stable with a wide range of

output capacitors. The minimum recommended value is

1

µF with an ESR of 3Ω or less. For applications where

space is very limited, capacitors as low as 0.33

µF can be

used if combined with a small series resistor. Assuming

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

Table 1. N8 Package*

COPPER AREA

TOPSIDE

BACKSIDE

BOARD AREA

2500 sq mm

2500 sq. mm

2500 sq. mm

80

°C/W

1000 sq mm

2500 sq. mm

2500 sq. mm

80

°C/W

225 sq mm

2500 sq. mm

2500 sq. mm

85

°C/W

1000 sq mm

1000 sq. mm

1000 sq. mm

91

°C/W

* Device is mounted on topside. Leads are through hole and are soldered

to both sides of board.

Table 2. S8 Package

COPPER AREA

TOPSIDE*

BACKSIDE

BOARD AREA

2500 sq. mm 2500 sq. mm

2500 sq. mm

120

°C/W

1000 sq. mm 2500 sq. mm

2500 sq. mm

120

°C/W

225 sq. mm

2500 sq. mm

2500 sq. mm

25

°C/W

100 sq. mm

1000 sq. mm

1000 sq. mm

131

°C/W

* Device is mounted on topside.

Table 4. SOT-223 Package

(Thermal Resistance Junction-to-Tab 20

°C/W)

COPPER AREA

TOPSIDE*

BACKSIDE

BOARD AREA

2500 sq. mm 2500 sq. mm

2500 sq. mm

50

°C/W

1000 sq. mm 2500 sq. mm

2500 sq. mm

50

°C/W

225 sq. mm

2500 sq. mm

2500 sq. mm

58

°C/W

100 sq. mm

2500 sq. mm

2500 sq. mm

64

°C/W

1000 sq. mm 1000 sq. mm

1000 sq. mm

57

°C/W

1000 sq. mm

0

1000 sq. mm

60

°C/W

* Tab of device attached to topside copper

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

Table 3. AS8 Package*

COPPER AREA

TOPSIDE**

BACKSIDE

BOARD AREA

2500 sq. mm 2500 sq. mm

2500 sq. mm

60

°C/W

1000 sq. mm 2500 sq. mm

2500 sq. mm

60

°C/W

225 sq. mm

2500 sq. mm

2500 sq. mm

68

°C/W

100 sq. mm

2500 sq. mm

2500 sq. mm

74

°C/W

*

Pins 3, 6, and 7 are ground.

** Device is mounted on topside.