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LT1766 Datasheet(PDF) 10 Page - Linear Technology |
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LT1766 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3474/LT3474-1 10 3474fd APPLICATIONS INFORMATION Setting the Switching Frequency The LT3474 uses a constant frequency architecture that can be programmed over a 200kHz to 2MHz range with a single external timing resistor from the RT pin to ground. The current that flows into the timing resistor is used to charge an internal oscillator capacitor. A graph for selecting the value of RT for a given operating frequency is shown in the Typical Performance Characteristics section. Table 1 shows suggested RT selections for a variety of switching frequencies. Table 1. Switching Frequencies SWITCHING FREQUENCY (MHz) RT (kΩ) 210 1.5 18.7 1 33.2 0.7 52.3 0.5 80.6 0.3 147 0.2 232 Operating Frequency Selection The choice of operating frequency is determined by sev- eral factors. There is a tradeoff between efficiency and component size. Higher switching frequency allows the use of smaller inductors at the cost of increased switching losses and decreased efficiency. Another consideration is the maximum duty cycle. In certain applications, the converter needs to operate at a high duty cycle in order to work at the lowest input voltage possible. The LT3474 has a fixed oscillator off-time and a variable on-time. As a result, the maximum duty cycle increases as the switching frequency is decreased. Input Voltage Range The minimum operating voltage is determined either by the LT3474’s undervoltage lockout of 4V, or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC VV VV V OUT F IN SW F = + () + () – where VF is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.4V at maximum load). This leads to a minimum input voltage of: V VV DC VV IN MIN OUT F MAX FSW () = + + – with DCMAX = 1–tOFF(MIN) • f where t0FF(MIN) is equal to 200ns and f is the switching frequency. Example: f = 500kHz, VOUT = 4V DC ns kHz V VV MAX IN MIN = − = = + () 1 200 500 0 90 40 4 0 •. . .9 9 04 04 49 –. . . VV V += The maximum operating voltage is determined by the absolute maximum ratings of the VIN and BOOST pins, and by the minimum duty cycle. V VV DC VV IN MAX OUT F MIN FSW () = + + – with DCMIN = tON(MIN) • f where tON(MIN) is equal to 160ns and f is the switching frequency. Example: f = 500kHz, VOUT = 2.5V DC ns kHz V VV MIN IN MAX == = + () 160 500 0 08 25 04 0 •. .. .0 08 04 04 36 –. . VV V += The minimum duty cycle depends on the switching fre- quency. Running at a lower switching frequency might allow a higher maximum operating voltage. Note that this is a restriction on the operating input voltage; the circuit will tolerate transient inputs up to the Absolute Maximum Rating. |
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