Datasheet LT3470 (Analog Devices) - 9

LT3470
APPLICATIONS INFORMATION Input Voltage Range
where VIN(MAX) is the maximum input voltage for the ap- The minimum input voltage required to generate a par- plication, tON-TIME(MIN) is ~150ns and IMAX is the maximum ticular output voltage in an LT3470 application is limited allowable increase in switch current during a minimum by either its 4V undervoltage lockout or by its maximum switch on-time (150mA). While this equation provides a duty cycle. The duty cycle is the fraction of time that the safe inductor value, the resulting application circuit may internal switch is on and is determined by the input and switch at too high a frequency to yield good efficiency. output voltages: It is advised that switching frequency be below 1.2MHz during normal operation: V DC = OUT + VD (1– DC)(VD + VOUT ) VIN – VSW + VD f = L • where V ∆IL D is the forward voltage drop of the catch diode (~0.6V) and VSW is the voltage drop of the internal switch where f is the switching frequency, ∆IL is the ripple current at maximum load (~0.4V). Given DCMAX = 0.90, this leads in the inductor (~150mA), VD is the forward voltage drop to a minimum input voltage of: of the catch diode, and VOUT is the desired output voltage. ⎛ V ⎞ If the application circuit is intended to operate at high duty V OUT + VD IN(MIN) = cycles (V ⎝⎜ DC IN close to VOUT), it is important to look at the MAX ⎠⎟ + VSW – VD calculated value of the switch off-time: This analysis assumes the part has started up such that the 1– DC capacitor tied between the BOOST and SW pins is charged t OFF-TIME = f to more than 2V. For proper start-up, the minimum input voltage is limited by the boost circuit as detailed in the The calculated tOFF-TIME should be more than LT3470’s section BOOST Pin Considerations. minimum tOFF-TIME (See Electrical Characteristics), so the application circuit is capable of delivering full rated The maximum input voltage is limited by the absolute output current. If the full output current of 200mA is not maximum VIN rating of 40V, provided an inductor of suf- required, the calculated t ficient value is used. OFF-TIME can be made less than minimum tOFF-TIME possibly allowing the use of a smaller
Inductor Selection
inductor. See Table 1 for an inductor value selection guide. The switching action of the LT3470 during continuous
Table 1. Recommended Inductors for Loads up to 200mA
operation produces a square wave at the SW pin that
VOUT VIN UP TO 16V VIN UP TO 40V
results in a triangle wave of current in the inductor. The 2.5V 10µH 33µH hysteretic mode control regulates the top and bottom 3.3V 10µH 33µH current limits (see Electrical Characteristics) such that 5V 15µH 33µH the average inductor current equals the load current. For 12V 33µH 47µH safe operation, it must be noted that the LT3470 cannot Choose an inductor that is intended for power applications. turn the switch on for less than ~150ns. If the inductor is Table 2 lists several manufacturers and inductor series. small and the input voltage is high, the current through the switch may exceed safe operating limit before the LT3470 For robust output short-circuit protection at high VIN (up is able to turn off. To prevent this from happening, the to 40V) use at least a 33µH inductor with a minimum following equation provides a minimum inductor value: 450mA saturation current. If short-circuit performance is not required, inductors with ISAT of 300mA or more may V L IN(MAX) • tON-TIME(MIN) MIN = IMAX Rev. E For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Related Parts