Datasheet LT1121, LT1121-3.3, LT1121-5 (Analog Devices) - 10

LT1121/LT1121-3.3/LT1121-5
APPLICATIONS INFORMATION
NC leads were connected to the ground plane.
Table 5. TO-92 Package THERMAL RESISTANCE
Package alone 220ºC/W
Table 1. N8 Package*
Package soldered into PC board with plated through 175ºC/W holes only
COPPER AREA THERMAL RESISTANCE
Package soldered into PC board with 1/4 sq. inch of 145ºC/W
TOPSIDE BACKSIDE BOARD AREA JUNCTION TO AMBIENT
copper trace per lead 2500 sq mm 2500 sq mm 2500 sq mm 80ºC/W Package soldered into PC board with plated through 160ºC/W holes in board, no extra copper trace, and a clip-on type 1000 sq mm 2500 sq mm 2500 sq mm 80ºC/W heat sink: Thermalloy type 2224B 225 sq mm 2500 sq mm 2500 sq mm 85ºC/W Aavid type 5754 135ºC/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.
Calculating Junction Temperature
Example: given an output voltage of 3.3V, an input voltage
Table 2. S8 Package
range of 4.5V to 7V, an output current range of 0mA to
COPPER AREA THERMAL RESISTANCE
100mA, and a maximum ambient temperature of 50°C,
TOPSIDE* BACKSIDE BOARD AREA JUNCTION TO AMBIENT
what will the maximum junction temperature be? 2500 sq mm 2500 sq mm 2500 sq mm 120ºC/W Power dissipated by the device will be equal to: 1000 sq mm 2500 sq mm 2500 sq mm 120ºC/W 225 sq mm 2500 sq mm 2500 sq mm 125ºC/W IOUT MAX • (VIN MAX – VOUT) + (IGND • VIN) 100 sq mm 1000 sq mm 1000 sq mm 131ºC/W where, IOUT MAX = 100mA * Device is mounted on topside. VIN MAX = 7V IGND at (IOUT = 100mA, VIN = 7V) = 5mA
Table 3. AS8 Package*
so, P = 100mA • (7V – 3.3V) + (5mA • 7V)
COPPER AREA THERMAL RESISTANCE
= 0.405W
TOPSIDE** BACKSIDE BOARD AREA JUNCTION TO AMBIENT
If we use an SOT-223 package, then the thermal resistance 2500 sq mm 2500 sq mm 2500 sq mm 60ºC/W will be in the range of 50°C/W to 65°C/W depending on 1000 sq mm 2500 sq mm 2500 sq mm 60ºC/W copper area. So the junction temperature rise above ambi- 225 sq mm 2500 sq mm 2500 sq mm 68ºC/W ent will be less than or equal to: 100 sq mm 2500 sq mm 2500 sq mm 74ºC/W * Pins 3, 6 and 7 are ground. 0.405W • 60°C/W = 24°C ** Device is mounted on topside. The maximum junction temperature will then be equal to the maximum junction temperature rise above ambient
Table 4. SOT-223 Package
plus the maximum ambient temperature or:
(Thermal Resistance Junction-to-Tab 20ºC/W) COPPER AREA
T
THERMAL RESISTANCE
JMAX = 50°C + 24°C = 74°C
TOPSIDE* BACKSIDE BOARD AREA JUNCTION TO AMBIENT
2500 sq mm 2500 sq mm 2500 sq mm 50ºC/W
Output Capacitance and Transient Performance
1000 sq mm 2500 sq mm 2500 sq mm 50ºC/W The LT1121 is designed to be stable with a wide range of 225 sq mm 2500 sq mm 2500 sq mm 58ºC/W output capacitors. The minimum recommended value is 1µF 100 sq mm 2500 sq mm 2500 sq mm 64ºC/W with an ESR of 3Ω or less. For applications where space 1000 sq mm 2500 sq mm 1000 sq mm 57ºC/W is very limited, capacitors as low as 0.33µF can be used if 1000 sq mm 0 1000 sq mm 60ºC/W combined with a small series resistor. Assuming that the * Tab of device attached to topside copper. ESR of the capacitor is low (ceramic) the suggested series 1121fg 10 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Applications Information Package Description Revision History Related Parts