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3508FE Datasheet(PDF) 10 Page - Linear Technology |
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3508FE Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LT3508 10 3508fb Table 1. Programming the Switching Frequency SWITCHING FREQUENCY (MHz) RT (kΩ) 2.5 7.50 2.2 9.76 2 11.5 1.8 14 1.6 16.9 1.4 20.5 1.2 26.1 1 33.2 0.9 38.3 0.8 44.2 0.7 52.3 0.6 61.9 0.5 76.8 0.45 88.7 0.4 100 0.35 115 0.3 140 0.25 169 Inductor Selection and Maximum Output Current A good first choice for the inductor value is: LV V MHz f OUT F =+ ()• . 12 where VFisthevoltagedropofthecatchdiode(~0.4V)andL is in μH. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. For highest efficiency, the series resistance (DCR) should be less than 0.1 Ω. Table 2 lists several vendors and types that are suitable. Table 2. Inductor Vendors VENDOR URL PART SERIES TYPE Coilcraft www.coilcraft MSS7341 Shielded Murata www.murata.com LQH55D Open TDK www.component.tdk.com SLF7045 SLF10145 Shielded Shielded Toko www.toko.com DC62CB D63CB D75C D75F Shielded Shielded Shielded Open Sumida www.sumida.com CR54 CDRH74 CDRH6D38 CR75 Open Shielded Shielded Open Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and fSW = 1.5MHz Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V and fSW = 1.5MHz. Output Ripple and Peak Inductor Current Increase APPLICATIONS INFORMATION VOUT 100mV/DIV (AC) IL 500mA/DIV 2μs/DIV 3508 F02 VOUT 100mV/DIV (AC) IL 500mA/DIV 2μs/DIV 3508 F03 Setting the Switching Frequency The switching frequency is programmed either by driving the RT/SYNC pin with a logic level SYNC signal or by tying a resistor from the RT/SYNC pin to ground. A graph for selecting the value of RT for a given operating frequency is shown in the Typical Application section. Suggested programming resistors for various switching frequencies are shown in Table 1. Choosing a high switching frequency will allow the smallest overall solution size. However, at high input voltages the efficiency can drop significantly with increasing switching frequency. The choice of switching frequency will also impact the input voltage range, inductor and capacitor selection, and compensation. See the related sections for details. |
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