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LTC3245 数据表(PDF) 13 Page - Linear Technology |
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LTC3245 数据表(HTML) 13 Page - Linear Technology |
13 / 18 page LTC3245 13 3245fa For more information www.linear.com/LTC3245 applicaTions inForMaTion ratherthandiscussingthespecifiedcapacitancevalue.For example, over rated voltage and temperature conditions, a 4.7μF, 10V, Y5V ceramic capacitor in an 0805 case may not provide any more capacitance than a 1μF, 10V, X5R or X7R available in the same 0805 case. In fact, over bias and temperature range, the 1μF, 10V, X5R or X7R will provide more capacitance than the 4.7μF, 10V, Y5V. The capacitor manufacturer’s data sheet should be consulted to determine what value of capacitor is needed to ensure minimum capacitance values are met over operating temperature and bias voltage. Below is a list of ceramic capacitor manufacturers and how to contact them: MANUFACTURER WEBSITE AVX www.avxcorp.com Kemet www.kemet.com Murata www.murata.com Taiyo Yuden www.t-yuden.com TDK www.tdk.com Layout Considerations Due to the high switching frequency and transient cur- rents produced by the LTC3245, careful board layout is necessary for optimal performance. A true ground plane and short connections to all capacitors will optimize performance, reduce noise and ensure proper regulation over all conditions. When using the LTC3245 with an external resistor divider it is important to minimize any stray capacitance to the ADJ (OUTS/ADJ pin) node. Stray capacitance from ADJ to C+ or C– can degrade performance significantly and should be minimized and/or shielded if necessary. Thermal Management TheonchippowerdissipationintheLTC3245willcausethe junction to ambient temperature to rise at rate of 40°C/W or more. To reduce the maximum junction temperature, a goodthermalconnectiontothePCboardisrecommended. Connecting the die paddle (Pin 13) with multiple vias to a largegroundplaneunderthedevicecanreducethethermal resistanceofthepackageandPCboardconsiderably.Poor board layout and failure to connect the die paddle (Pin 13) to a large ground plane can result in thermal junction to ambient impedance well in excess of 40°C/W. Because of the wide input operating range it is possible to exceed the specified operating junction temperature and even reach thermal shutdown. Figure 3 shows the avail- able output current vs temperature to ensure the 150°C operating junction temperature is not exceed for input voltages less than 20V. Figure 3 assumes worst-case operating conditions. Under someoperatingconditionsthepartcansupplymorecurrent than shown without exceeding the 150°C operating junc- tion temperature. When operating outside the constraints of Figure 3 it is the responsibility of the user to calculate worst-caseoperatingconditions(temperatureandpower) to make sure the LTC3245’s specified operating junction temperature is not exceeded for extended periods of time. The 2:1 Step-Down, 1:1 Step-Down, and 1:2 Step-Up Charge Pump Operation sections provide equations for calculating power dissipation (PD) in each mode. For example, if it is determined that the maximum power dissipation (PD) is 1.2W under normal operation, then the junction to ambient temperature rise will be: Junction to ambient = 1.2W • 40°C/W = 48°C Thus,theambienttemperatureunderthisconditioncannot exceed102°Cifthejunctiontemperatureistoremainbelow 150°Candiftheambienttemperatureexceedsabout127°C the device will cycle in and out of the thermal shutdown. Figure 3. Available Output Current vs Temperature TEMPERATURE (°C) 70 0 50 150 100 250 200 300 80 3245 F03 90 100 110 130 120 150 140 VIN < 20V |
类似零件编号 - LTC3245 |
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类似说明 - LTC3245 |
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