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LT3972 Datasheet(PDF) 9 Page - Linear Technology |
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LT3972 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT3972 9 3972f 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 LT3972 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 2.4 215 140 100 78.7 63.4 53.6 45.3 39.2 34 26.7 22.1 18.2 15 12.7 10.7 9.09 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 DIN 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 LT3972 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 The maximum input voltage for LT3972 applications de- pendsonswitchingfrequency,AbsoluteMaximumRatings of the VIN and BOOST pins, and the operating mode. The LT3972 can operate from input voltages of up to 33V, and withstand voltages up to 62V. Note that while VIN is above 35V typical (33V minimum and 37V maximum) the part will keep the switch off and the output will not be in regulation. To safely allow inputs of up to 62V, be sure to choose a Schottky diode, inductor size, and switching frequency to allow safe operation at 37V according to the following discussions. While the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be chosen according to the following equation: APPLICATIONS INFORMATION |
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