Datasheet LT3693 (Analog Devices) - 10

LT3693
APPLICATIONS INFORMATION
operating input voltage. Conversely, a lower switching ripple current. The LT3693’s switch current limit (ILIM) is frequency will be necessary to achieve safe operation at 5.5A at low duty cycles and decreases linearly to 4.5A at high input voltages. DC = 0.8. The maximum output current is a function of the inductor ripple current: If the output is in regulation and no short-circuit, start- 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 LT3693 may enter suffi cient 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. specifi ed maximum, the inductor value should be chosen according to the following equation: The minimum input voltage is determined by either the LT3693’s minimum operating voltage of ~3.6V or by its ⎛ V + V ⎞ ⎛ V + V ⎞ OUT D OUT D maximum duty cycle (see equation in previous section). L = ⎜1– ⎟ ⎝⎜ f I Δ ⎠⎟ V The minimum input voltage due to duty cycle is: ⎝ ( ) ⎠ SW L IN MAX V + V where V V OUT D = – V + V D is the voltage drop of the catch diode (~0.4V), IN MIN 1– f t D SW ( ) V SW ( ) IN(MAX) is the maximum input voltage, VOUT is the output OFF MIN voltage, fSW is the switching frequency (set by RT), and where V L 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 the maximum load current and its saturation current voltage. If a lower dropout voltage is desired, a lower should be about 30% higher. For robust operation in fault switching 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 effi ciency high, the series resistance (DCR) For a given input and output voltage, the inductor value should be less than 0.05 , 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 ΔI several vendors and suitable types. L 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 ΔI TDK www.componenttdk.com SLF10145 Shielded L = 0.4(IOUT(MAX)) Toko www.toko.com D75C Shielded where IOUT(MAX) is the maximum output load current. To D75F Open guarantee suffi cient output current, peak inductor current Sumida www.sumida.com CDRH74 Shielded must be lower than the LT3693’s switch current limit (I CR75 Open LIM). The peak inductor current is: CDRH8D43 Shielded 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 3693f 10