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LT3480EMSE-TR Datasheet(PDF) 9 Page - Linear Technology |
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LT3480EMSE-TR Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT3480 9 3480fb APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis- tors according to: RR V V OUT 12 079 1 =− ⎛ ⎝⎜ ⎞ ⎠⎟ . Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT3480 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Figure 1. SWITCHING FREQUENCY (MHz) RT VALUE (kΩ) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 187 121 88.7 68.1 56.2 46.4 40.2 34 29.4 23.7 19.1 16.2 13.3 11.5 9.76 8.66 Figure 1. Switching Frequency vs. RT Value Operating Frequency Tradeoffs Selection of the operating frequency is a tradeoff between efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: f VV tV V V SW MAX D OUT ON MIN D IN SW () () – = + + () where VIN is the typical input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V) and VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown in the next section, lower frequency allows a lower dropout voltage. The reason input voltage range depends on the switching frequency is because the LT3480 switch has finite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. Typical minimum on time at 25°C is 80ns. This means that the minimum and maximum duty cycles are: DC f t DC f t MIN SW ON MIN MAX SW OFF MIN = = () () – 1 where fSW is the switching frequency, the tON(MIN) is the minimum switch on time (~150ns), and the tOFF(MIN) is the minimum switch off time (~150ns). These equations show that duty cycle range increases when switching frequency is decreased. A good choice of switching frequency should allow ad- equate input voltage range (see next section) and keep the inductor and capacitor values small. Input Voltage Range ThemaximuminputvoltageforLT3480applicationsdepends on switching frequency, the Absolute Maximum Ratings of the VIN and BOOST pins, and the operating mode. The LT3480 can operate from input voltages up to 38V, and safely withstand input voltages up 60V. Note that while VIN>38V (typical), the LT3480 will stop switching, allowing the output to fall out of regulation. While the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be chosen according to the following discussion. For safe operation at inputs up to 60V the switching fre- quency must be set low enough to satisfy VIN(MAX) ≥ 40V according to the following equation. If lower VIN(MAX) is desired, this equation can be used directly. |
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