Datasheet LT3680 (Analog Devices) - 10

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APPLICATIONS INFORMATION
maximum operating input voltage. Conversely, a lower ripple current. The LT3680’s switch current limit (ILIM) is switching frequency will be necessary to achieve safe 5.5A at low duty cycles and decreases linearly to 4.5A at operation at high input voltages. DC = 0.8. The maximum output current is a function of If the output is in regulation and no short-circuit, start- the inductor ripple current: up, or overload events are expected, then input voltage IOUT(MAX) = ILIM – ΔIL/2 transients of up to 36V are acceptable regardless of the Be sure to pick an inductor ripple current that provides switching frequency. In this mode, the LT3680 may enter sufficient maximum output current (I pulse skipping operation where some switching pulses OUT(MAX)). are skipped to maintain output regulation. In this mode The largest inductor ripple current occurs at the highest the output voltage ripple and inductor current ripple will VIN. To guarantee that the ripple current stays below the be higher than in normal operation. specified maximum, the inductor value should be chosen according to the following equation: The minimum input voltage is determined by either the LT3680’s minimum operating voltage of ~3.6V or by its V + V V + V maximum duty cycle (see equation in previous section). L OUT D OUT D = 1– The minimum input voltage due to duty cycle is: f I V SW L IN MA ( X) V + V V OUT D where V = – V + V D is the voltage drop of the catch diode (~0.4V), IN MI ( N) 1– f t D SW VIN(MAX) is the maximum input voltage, VOUT is the output SW OFF M ( IN) voltage, fSW is the switching frequency (set by RT), and L where V is in the inductor value. IN(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 switching frequency will increase the minimum input volt- the maximum load current and its saturation current age. If a lower dropout voltage is desired, a lower switch- should be about 30% higher. For robust operation in fault ing frequency should be used. conditions (start-up or short circuit) and high input volt- age (>30V), the saturation current should be above 5A.
Inductor Selection
To keep the efficiency high, the series resistance (DCR) For a given input and output voltage, the inductor value should be less than 0.1 , and the core material should and switching frequency will determine the ripple current. be intended for high frequency applications. Table 1 lists The ripple current ΔIL increases with higher VIN or VOUT several vendors 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 ΔIL = 0.4(IOUT(MAX)) TDK www.componenttdk.com SLF10145 Shielded Toko www.toko.com D75C Shielded where IOUT(MAX) is the maximum output load current. To D75F Open guarantee sufficient output current, peak inductor cur- Sumida www.sumida.com CDRH74 Shielded rent must be lower than the LT3680’s switch current limit CR75 Open (I CDRH8D43 Shielded LIM). The peak inductor current is: NEC www.nec.com MPLC073 Shielded IL(PEAK) = IOUT(MAX) + ΔIL/2 MPBI0755 Shielded where IL(PEAK) is the peak inductor current, IOUT(MAX) is the maximum output load current, and ΔIL is the inductor Rev C 10 For more information www.analog.com Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts