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LT3990 Datasheet(PDF) 9 Page - Linear Technology |
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LT3990 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LT3990 9 3990p 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 VOUT 12 121 1 = ⎛ ⎝⎜ ⎞ ⎠⎟ . – Reference designators refer to the Block Diagram. Note that choosing larger resistors will decrease the quiescent current of the application circuit. Setting the Switching Frequency The LT3990 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.2MHz 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 Table 1. Table 1. Switching Frequency vs RT Value SWITCHING FREQUENCY (MHz) RT VALUE (kΩ) 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 768 499 357 280 226 158 124 100 80.6 68.1 57.6 49.9 42.2 Operating Frequency Trade-Offs Selection of the operating frequency is a trade-off 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 OUT D ON MIN IN SW D () () – = + + () where VIN is the typical input voltage, VOUT is the output voltage, VD is the integrated catch diode drop (~0.7V), and VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to accommodate high VIN/VOUT ratio. Lower frequency also allows a lower dropout voltage. The input voltage range depends on the switching frequency because the LT3990 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 ~160ns (note that the minimum on-time is a strong function of temperature). This means that the minimum and maximum duty cycles are: DCMIN = fSW • tON(MIN) DCMAX = 1 – fSW • tON(MIN) 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 (~160ns). 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 minimum input voltage is determined by either the LT3990’s minimum operating voltage of 4.2V or by its maximum duty cycle (as explained in previous section). The minimum input voltage due to duty cycle is: V V ft VV IN MIN D SW OFF MIN DSW () () –• – = + + VOUT 1 where VIN(MIN) is the minimum input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.7V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch off-time (160ns). Note that higher switch- ing frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used. |
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