Datasheet LTC1701, LTC1701B (Analog Devices) - 10

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