LT1121IS8-PBF Просмотр технического описания (PDF) - Linear Technology

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LT1121IS8-PBF

Micropower Low Dropout Regulators with Shutdown

Linear Technology 

LT1121/LT1121-3.3/LT1121-5

APPLICATIONS INFORMATION

NC leads were connected to the ground plane.

Table 1. N8 Package*

COPPER AREA

THERMAL RESISTANCE

TOPSIDE BACKSIDE BOARD AREA JUNCTION TO AMBIENT

2500 sq mm 2500 sq mm 2500 sq mm

80ºC/W

1000 sq mm 2500 sq mm 2500 sq mm

80ºC/W

225 sq mm 2500 sq mm 2500 sq mm

85ºC/W

1000 sq mm 1000 sq mm 1000 sq mm

91ºC/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

1000 sq mm 2500 sq mm 2500 sq mm

225 sq mm 2500 sq mm 2500 sq mm

100 sq mm 1000 sq mm 1000 sq mm

* Device is mounted on topside.

THERMAL RESISTANCE

JUNCTION TO AMBIENT

120ºC/W

120ºC/W

125ºC/W

131ºC/W

Table 3. AS8 Package*

COPPER AREA

TOPSIDE** BACKSIDE BOARD AREA

2500 sq mm 2500 sq mm 2500 sq mm

1000 sq mm 2500 sq mm 2500 sq mm

225 sq mm 2500 sq mm 2500 sq mm

100 sq mm 2500 sq mm 2500 sq mm

* Pins 3, 6 and 7 are ground.

** Device is mounted on topside.

THERMAL RESISTANCE

JUNCTION TO AMBIENT

60ºC/W

60ºC/W

68ºC/W

74ºC/W

Table 4. SOT-223 Package

(Thermal Resistance Junction-to-Tab 20ºC/W)

COPPER AREA

THERMAL RESISTANCE

TOPSIDE* BACKSIDE BOARD AREA JUNCTION TO AMBIENT

2500 sq mm 2500 sq mm 2500 sq mm

50ºC/W

1000 sq mm 2500 sq mm 2500 sq mm

50ºC/W

225 sq mm 2500 sq mm 2500 sq mm

58ºC/W

100 sq mm 2500 sq mm 2500 sq mm

64ºC/W

1000 sq mm 2500 sq mm 1000 sq mm

57ºC/W

1000 sq mm

0

1000 sq mm

60ºC/W

* Tab of device attached to topside copper.

10

Table 5. TO-92 Package THERMAL RESISTANCE

Package alone

Package soldered into PC board with plated through

holes only

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

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

Aavid type 5754

220ºC/W

175ºC/W

145ºC/W

160ºC/W

135ºC/W

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:

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

where, IOUT MAX = 100mA

VIN MAX = 7V

IGND at (IOUT = 100mA, VIN = 7V) = 5mA

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 ambi-

ent 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:

TJMAX = 50°C + 24°C = 74°C

Output Capacitance and Transient Performance

The LT1121 is designed to be stable with a wide range of

output capacitors. The minimum recommended value is 1µF

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

is very limited, capacitors as low as 0.33µF can be used if

combined with a small series resistor. Assuming that the

ESR of the capacitor is low (ceramic) the suggested series

1121fg

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