Datasheet LT3684 (Analog Devices) - 10

LT3684
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- If the output is in regulation and no short-circuit or start-up tion of the inductor ripple current: events are expected, then input voltage transients of up to IOUT(MAX) = ILIM – ∆IL/2 36V are acceptable regardless of the switching frequency. Be sure to pick an inductor ripple current that provides In this mode, the LT3684 may enter pulse skipping opera- sufficient 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. specified maximum, the inductor value should be chosen according to the following equation: The minimum input voltage is determined by either the LT3684’s minimum operating voltage of ~3.6V or by its    maximum duty cycle (see equation in previous section). L = VOUT + VD  1− VOUT + VD   f∆I  V  The minimum input voltage due to duty cycle is: L  IN(MAX)  V where VD is the voltage drop of the catch diode (~0.4V), IN(MIN) = VOUT + VD −V 1− f D + VSW V SW tOFF(MIN) IN(MAX) is the maximum input voltage, VOUT is the output voltage, fSW is the switching frequency (set by RT), and L where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is in the inductor value. 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 4A. To keep the
Inductor Selection
efficiency high, the series resistance (DCR) should be less For a given input and output voltage, the inductor value than 0.1Ω, and the core material should be intended for and switching frequency will determine the ripple current. high frequency applications. Table 1 lists several vendors The ripple current ∆IL increases with higher VIN or VOUT and suitable types. 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 ∆IL = 0.4(IOUT(MAX)) TDK www.componenttdk.com SLF7045 Shielded SLF10145 Shielded where IOUT(MAX) is the maximum output load current. To Toko www.toko.com D62CB Shielded guarantee sufficient output current, peak inductor current D63CB Shielded must be lower than the LT3684’s switch current limit (ILIM). D75C Shielded The peak inductor current is: D75F Open IL(PEAK) = IOUT(MAX) + ∆IL/2 Sumida www.sumida.com CR54 Open CDRH74 Shielded where IL(PEAK) is the peak inductor current, IOUT(MAX) is CDRH6D38 Shielded the maximum output load current, and ∆IL is the inductor CR75 Open ripple current. The LT3684’s switch current limit (ILIM) is 3684fa 10 For more information www.linear.com/LT3684