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LTC1701B Datasheet(PDF) 10 Page - Linear Technology |
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LTC1701B Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LTC1701/LTC1701B SW VFB VIN ITH/RUN GND LTC1701 D1 L1 R2 R1 COUT 1701 F04 VOUT VIN 1 2 3 5 4 CIN CC RC RS BOLD LINES INDICATE HIGH CURRENT PATHS + + Figure 4. LTC1701 Layout Diagram (See Board Layout Checklist) APPLICATIO S I FOR ATIO The following table lists thermal resistance for several different board sizes and copper areas. All measurements were taken in still air on 3/32" FR-4 board with one ounce copper. Table 1. Measured Thermal Resistance COPPER AREA THERMAL RESISTANCE TOPSIDE* BACKSIDE BOARD AREA θJA 2500mm2 2500mm2 2500mm2 125 °C/W 1000mm2 2500mm2 2500mm2 125 °C/W 225mm2 2500mm2 2500mm2 130 °C/W 100mm2 2500mm2 2500mm2 135 °C/W 50mm2 2500mm2 2500mm2 150 °C/W *Device is mounted on topside. Calculating Junction Temperature In a majority of applications, the LTC1701 does not dissi- pate much heat due to its high efficiency. However, in applications where the switching regulator is running at high duty cycles or the part is in dropout with the switch turned on continuously (DC), some thermal analysis is required. The goal of the thermal analysis is to determine whether the power dissipated by the regulator exceeds the maximum junction temperature. The temperature rise is given by: TRISE = PD • θJA where PD is the power dissipated by the regulator and θJA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature is given by: TJ = TRISE + TAMBIENT As an example, consider the case when the LTC1701 is in dropout at an input voltage of 3.3V with a load current of 0.5A. The ON resistance of the P-channel switch is approxi- mately 0.30 Ω. Therefore, power dissipated by the part is: PD = I2 • RDS(ON) = 75mW The SOT package junction-to-ambient thermal resistance, θJA, will be in the range of 125°C/W to 150°C/W. Therefore, the junction temperature of the regulator operating in a 25 °C ambient temperature is approximately: TJ = 0.075 • 150 + 25 = 36°C Remembering that the above junction temperature is ob- tained from a RDS(ON) at 25°C, we might recalculate the junction temperature based on a higher RDS(ON) since it increases with temperature. However, we can safely as- sume that the actual junction temperature will not exceed the absolute maximum junction temperature of 125 °C. Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1701. These items are also illustrated graphically in the layout diagram of Figure 4. Check the following in your layout: 1. Does the capacitor CIN connect to the power VIN (Pin 5) and GND (Pin 2) as close as possible? This capacitor provides the AC current to the internal P-channel MOSFET and its driver. 2. Is the Schottky diode closely connected between the ground (Pin 2) and switch output (Pin 1)? 3. Are the COUT, L1 and D1 closely connected? The Schottky anode should connect directly to the input capacitor ground. 4. The resistor divider, R1 and R2, must be connected between the (+) plate of COUT and a ground line terminated near GND (Pin 2). The feedback signal FB should be routed away from noisy components and traces, such as the SW line (Pin 1). 5. Keep sensitive components away from the SW pin. The input capacitor CIN, the compensation capacitor CC and all the resistors R1, R2, RC and RS should be routed away from the SW trace and the components L1 and D1. |
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