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LT3470AEDDB-PBF Datasheet(PDF) 10 Page - Linear Technology |
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LT3470AEDDB-PBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3470A 10 3470afa Input Voltage Range The minimum input voltage required to generate a par- ticular output voltage in an LT3470A application is limited by either its 4V undervoltage lockout 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 = VOUT + VD VIN –VSW + VD where VD is the forward voltage drop of the catch diode (~0.6V) and VSW is the voltage drop of the internal switch at maximum load (~0.4V). Given DCMAX = 0.90, this leads to a minimum input voltage of: VIN(MIN) = VOUT + VD DCMAX + VSW –VD This analysis assumes the part has started up such that the capacitor tied between the BOOST and SW pins is charged to more than 2V. For proper start-up, the minimum input voltage is limited by the boost circuit as detailed in the section BOOST Pin Considerations. The maximum input voltage is limited by the absolute maximum VIN rating of 40V, provided an inductor of suf- ficient value is used. Inductor Selection The switching action of the LT3470A during continuous operation produces a square wave at the SW pin that results in a triangle wave of current in the inductor. The hysteretic mode control regulates the top and bottom current limits (see Electrical Characteristics) such that the average induc- tor current equals the load current. For safe operation, it must be noted that the LT3470A cannot turn the switch on for less than ~150ns. If the inductor is small and the input voltage is high, the current through the switch may exceed safe operating limit before the LT3470A is able to turn off. To prevent this from happening, the following equation provides a minimum inductor value: LMIN = VIN(MAX) •tON-TIME(MIN) IMAX APPLICATIONS INFORMATION where VIN(MAX) is the maximum input voltage for the ap- plication, tON-TIME(MIN)is ~150ns and IMAX is the maximum allowable increase in switch current during a minimum switch on-time (150mA). While this equation provides a safe inductor value, the resulting application circuit may switch at too high a frequency to yield good efficiency. It is advised that switching frequency be below 1.2MHz during normal operation: f = 1– DC () V D + VOUT () L• ΔIL where f is the switching frequency, ΔIL is the ripple cur- rent in the inductor (~200mA), VD is the forward voltage drop of the catch diode, and VOUT is the desired output voltage. If the application circuit is intended to operate at high duty cycles (VIN close to VOUT), it is important to look at the calculated value of the switch off-time: tOFF-TIME = 1– DC f The calculated tOFF-TIME should be more than LT3470A’s minimum tOFF-TIME (See Electrical Characteristics), so the application circuit is capable of delivering full rated output current. If the full output current of 250mA is not required, the calculated tOFF-TIME can be made less than minimum tOFF-TIME possibly allowing the use of a smaller inductor. See Table 1 for an inductor value selection guide. Table 1. Recommended Inductors for Loads up to 250mA VOUT VIN Up to 16V VIN Up to 40V 2.5V 10μH 33μH 3.3V 10μH 33μH 5V 15μH 33μH 12V 33μH 47μH Choose an inductor that is intended for power applications. Table 2 lists several manufacturers and inductor series. For robust output short-circuit protection at high VIN (up to 40V) use at least a 33μH inductor with a minimum 450mA saturation current. If short-circuit performance is not required, inductors with ISAT of 300mA or more may |
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