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LTC3561EDD Datasheet(PDF) 11 Page - Linear Technology |
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LTC3561EDD Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC3561 11 3561f looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteris- tics curves. Thus, to obtain I2R losses: I2R losses = IOUT2(RSW + RL) 4) Other “hidden” losses such as copper trace and internal battery resistances can account for additional efficiency degradations in portable systems. It is very important to include these “system” level losses in the design of a system. The internal battery and fuse resistance losses can be minimized by making sure that CIN has adequate charge storage and very low ESR at the switching frequency. Other losses including diode conduction losses during dead-time and inductor core losses gen- erally account for less than 2% total additional loss. Thermal Considerations In a majority of applications, the LTC3561 does not dissi- pate much heat due to its high efficiency. However, in applications where the LTC3561 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150 °C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC3561 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maxi- mum junction temperature of the part. 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, TJ, is given by: TJ = TRISE + TAMBIENT As an example, consider the case when the LTC3561 is in dropout at an input voltage of 3.3V with a load current of 1A. From the Typical Performance Characteristics graph of Switch Resistance, the RDS(ON) resistance of the P-channel switch is 0.11 Ω. Therefore, power dissipated by the part is: PD = I2 • RDS(ON) = 110mW The DD8 package junction-to-ambient thermal resistance, θJA, will be in the range of about 43°C/W. Therefore, the junction temperature of the regulator operating in a 70 °C ambient temperature is approximately: TJ = 0.11 • 43 + 70 = 74.7°C Remembering that the above junction temperature is obtained from an 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 assume that the actual junction temperature will not exceed the absolute maximum junction temperature of 125 °C. Design Example As a design example, consider using the LTC3561 in a portable application with a Li-Ion battery (refer to Figure 4 for reference designation). The battery provides a VIN = 2.5V to 4.2V. The load requires a maximum of 1A in active mode and 10mA in standby mode. The output voltage is VOUT = 2.5V. First, calculate the timing resistor: R MHz k T = () = − 9 78 10 1 323 8 11 108 .• . . Use a standard value of 324k. Next, calculate the inductor value for about 40% ripple current at maximum VIN: L V MHz mA V V H = − ⎛ ⎝⎜ ⎞ ⎠⎟ =µ 25 1 400 1 25 42 25 . • • . . . Choosing the closest inductor from a vendor of 2.2 µH, results in a maximum ripple current of: ∆ = µ − ⎛ ⎝⎜ ⎞ ⎠⎟ = I V MHz V V mA L 25 12 2 1 25 42 460 . •. • . . APPLICATIO S I FOR ATIO |
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