Datasheet LT3470A (Analog Devices) - 9

LT3470A
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 LT3470A 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 ( )(V ) V D + VOUT IN – VSW + VD f = L • ΔI L where VD is the forward voltage drop of the catch diode (~0.6V) and V where f is the switching frequency, ΔI SW is the voltage drop of the internal switch L is the ripple current at maximum load (~0.4V). Given DC in the inductor (~200mA), V MAX = 0.90, this leads D 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) = ⎜ ⎟ + VSW – VD cycles (VIN close to VOUT), it is important to look at the ⎝ DCMAX ⎠ calculated value of the switch off-time: This analysis assumes the part has started up such that the 1– DC tOFF-TIME = capacitor tied between the BOOST and SW pins is charged f to more than 2V. For proper start-up, the minimum input The calculated t voltage is limited by the boost circuit as detailed in the OFF-TIME should be more than LT3470A’s minimum t section BOOST Pin Considerations. OFF-TIME (See Electrical Characteristics), so the application circuit is capable of delivering full rated output The maximum input voltage is limited by the absolute current. If the full output current of 250mA is not required, maximum VIN rating of 40V, provided an inductor of suf- the calculated tOFF-TIME can be made less than minimum ficient value is used. tOFF-TIME possibly allowing the use of a smaller inductor. See Table 1 for an inductor value selection guide.
Inductor Selection Table 1. Recommended Inductors for Loads up to 250mA
The switching action of the LT3470A during continuous
V
operation produces a square wave at the SW pin that results
OUT VIN Up to 16V VIN Up to 40V
2.5V 10μH 33μH in a triangle wave of current in the inductor. The hysteretic 3.3V 10μH 33μH mode control regulates the top and bottom current limits 5V 15μH 33μH (see Electrical Characteristics) such that the average induc- 12V 33μH 47μH tor current equals the load current. For safe operation, it must be noted that the LT3470A cannot turn the switch Choose an inductor that is intended for power applications. on for less than ~150ns. If the inductor is small and the Table 2 lists several manufacturers and inductor series. input voltage is high, the current through the switch may For robust output short-circuit protection at high V exceed safe operating limit before the LT3470A is able to IN (up to 40V) use at least a 33μH inductor with a minimum turn off. To prevent this from happening, the following 450mA saturation current. If short-circuit performance is equation provides a minimum inductor value: not required, inductors with ISAT of 300mA or more may V be used. It is important to note that inductor saturation L IN(MAX) • tON-TIME(MIN) MIN = current is reduced at high temperatures—see inductor IMAX vendors for more information. 3470afb 9