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LM3402 Folha de dados(PDF) 4 Page - National Semiconductor (TI) |
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LM3402 Folha de dados(HTML) 4 Page - National Semiconductor (TI) |
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4 / 18 page  Application Circuit Calculations To better explain the improvements made to the COT LM3402 / 04 demonstration board, a comparison is shown between the unmodified average output LED current circuit to the improved circuit. Design examples 1 and 2 use two orig- inal LM3402 / 04 circuits. The switching frequencies will be maximized to provide a small solution size. Design example 3 is an improved average current application. Example 3 will be compared against example 2 to illustrate the improvements. Example 4 will use the same conditions and circuit as exam- ple 3, but the switching frequency will be reduced to improve efficiency. The reduced switching frequency can further re- duce any variations in average LED current with a wide operating range of series LEDs and input voltages. Design Example 1 • V IN = 48V (±20%) • Driving three HB LEDs with V F = 3.4V • V OUT = (3 x 3.4V +200 mV) = 10.4V • I F = 500 mA (typical application) • Estimated efficiency = 82% • f SW = fast as possible • Design for typical application within t ON and tOFF limitations LED (inductor) ripple current of 10% to 60% is acceptable when driving LEDs. With this much allowed ripple current, you can see that there is no need for an output capacitor. Elimi- nating the output capacitor is actually desirable. An LED connected to an inductor without a capacitor creates a near perfect current source, and this is what we are trying to create. In this design we will choose 50% ripple current. Δi L = 500 mA x 0.50 = 250 mA I PEAK = 500 mA + 125 mA = 625 mA Calculate t ON, tOFF & RON From the datasheet there are minimum control MOSFET ON and OFF times that need to be met. t OFF minimum = 300 ns t ON minimum = 300 ns The minimum ON time will occur when V IN is at its maximum value. Therefore calculate R ON at VIN = 60V, and set tON = 300 ns. A quick guideline for maximum switching frequency allowed versus input and output voltages are shown below in the two graphs (figures 6 & 7). 30061615 FIGURE 6. V OUT-MAX vs fSW 30061616 FIGURE 7. V OUT-MIN vs fSW R ON = 135 kΩ (use standard value of 137 kΩ) t ON = 306 ns Check to see if t OFF minimum is satisfied. This occurs when V IN is at its minimum value. At V IN = 36V, and RON = 137 kΩ calculate tON from previous equation. t ON = 510 ns We know that: www.national.com 4 |
Nº de peça semelhante - LM3402_1 |
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Descrição semelhante - LM3402_1 |
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