Datasheet LT3681 (Analog Devices) - 10

LT3681
APPLICATIONS INFORMATION
frequency will be necessary to achieve safe operation at at least 3.5A at low duty cycles and decreases linearly to high input voltages. 2.5A at DC = 0.8. The maximum output current is a func- tion of the inductor ripple current: If the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to IOUT(MAX) = ILIM – ΔIL/2 36V are acceptable regardless of the switching frequency. Be sure to pick an inductor ripple current that provides In this mode, the LT3681 may enter pulse skipping opera- suffi cient maximum output current (I tion where some switching pulses are skipped to maintain OUT(MAX)). output regulation. In this mode the output voltage ripple The largest inductor ripple current occurs at the highest and inductor current ripple will be higher than in normal VIN. To guarantee that the ripple current stays below the operation. specifi ed maximum, the inductor value should be chosen according to the following equation: The minimum input voltage is determined by either the LT3681’s minimum operating voltage of ~3.6V or by its ⎛ ⎞ ⎛ ⎞ maximum duty cycle (see equation in previous section). V + V V + V L OUT D OUT D = ⎜1– ⎟ The minimum input voltage due to duty cycle is: ⎝⎜ f I ∆ ⎠⎟ ⎜ V ⎟ L ⎝ IN MA ( X) ⎠ V + V where VD is the voltage drop of the integrated Schottky V OUT D = – V + V IN MIN diode (~0.55V), V 1– f t D SW ( ) IN(MAX) is the maximum input voltage, SW ( ) OFF MIN VOUT is the output voltage, fSW is the switching frequency (set by RT), and L is in the inductor value. where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time (150ns). Note that higher The inductor’s RMS current rating must be greater than the switching frequency will increase the minimum input maximum load current and its saturation current should be voltage. If a lower dropout voltage is desired, a lower about 30% higher. For robust operation in fault conditions switching frequency should be used. (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 3.5A. To keep the
Inductor Selection
effi ciency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for For a given input and output voltage, the inductor value high frequency applications. Table 1 lists several vendors and switching frequency will determine the ripple current. and suitable types. The ripple current ΔIL increases with higher VIN or VOUT and decreases with higher inductance and faster switch-
Table 1. Inductor Vendors
ing frequency. A reasonable starting point for selecting
VENDOR URL PART SERIES TYPE
the ripple current is: Murata www.murata.com LQH55D Open TDK www.componenttdk.com SLF7045 Shielded ΔIL = 0.4(IOUT(MAX)) SLF10145 Shielded where I Toko www.toko.com D75C Shielded OUT(MAX) is the maximum output load current. To guarantee suffi cient output current, peak inductor current D75F Open must be lower than the LT3681’s switch current limit (I FDV0620 Shielded LIM). The peak inductor current is: Sumida www.sumida.com CDRH74 Shielded CDRH6D38 Shielded IL(PEAK) = IOUT(MAX) + ΔIL/2 CR75 Open where IL(PEAK) is the peak inductor current, IOUT(MAX) is CDRH8D43 Shielded the maximum output load current, and ΔIL is the inductor NEC www.nec.tokin.com PLC-0745 Shielded ripple current. The LT3681’s switch current limit (ILIM) is 3681f 10