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LT1121 Datasheet(PDF) 9 Page - Linear Technology |
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LT1121 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 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: 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 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: TJMAX = 50°C + 24°C = 74°C 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. |
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