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LT1912EDD-PBF Datasheet(PDF) 9 Page - Linear Technology |
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LT1912EDD-PBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT1912 9 1912f 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: R1 =R2 VOUT 0.79V –1 Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT1912 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 500kHz 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 (kHz) RT VALUE (kΩ) 200 300 400 500 187 121 88.7 68.1 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 SW MAX () = V D + VOUT t ON MIN () VD + VIN –VSW () 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 LT1912 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 MIN = fSW tON MIN () DC MAX = 1– fSW tOFF 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 (~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 LT1912 applications depends on switching frequency, the Absolute Maximum Ratings of the VIN and BOOST pins, and the operating mode. While the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be chosen according to the following equation. V IN MIN () = V OUT + VD 1– f SW tOFF MIN () –V D + VSW where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tON(MIN)istheminimumswitchontime(~150ns).Notethat a higher switching frequency will depress the maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit, start- up, or overload events are expected, then input voltage transients of up to 36V are acceptable regardless of the switching frequency. In this mode, the LT1912 may enter pulse skipping operation where some switching pulses APPLICATIONS INFORMATION |
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