Datasheet LT3980 (Analog Devices) - 10

LT3980
applicaTions inFormaTion
drop (~0.5V), VSW is the internal switch drop (~0.5V at where IL(PEAK) is the peak inductor current, IOUT(MAX) is max load), fSW is the switching frequency (set by RT), the maximum output load current, and
Δ
IL is the inductor and tON(MIN) is the minimum switch on time (~200ns). ripple current. The LT3980’s switch current limit (ILIM) is Note that a higher switching frequency will depress the 4A at low duty cycles and decreases linearly to 3A at DC maximum operating input voltage. Conversely, a lower = 0.8. The maximum output current is a function of the switching frequency will be necessary to achieve safe inductor ripple current: operation at high input voltages. IOUT(MAX) = ILIM –
Δ
IL/2 Input voltages up to 58V are acceptable regardless of the Be sure to pick an inductor ripple current that provides switching frequency. In this mode, the LT3980 may enter sufficient maximum output current (I pulse-skipping operation where some switching pulses OUT(MAX)). are skipped to maintain safe inductor current. The largest inductor ripple current occurs at the highest V The minimum input voltage is determined by either the IN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen LT3980’s minimum operating voltage of ~3.6V or by its according to the following equation: maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is:  V + V   V + V  L OUT D OUT D =  1–  V + V  f ∆I   V  V OUT D = – V + V SW L  IN MA ( X)  IN MI ( N) 1– f t D SW SW OFF M ( IN) where VD is the voltage drop of the catch diode (~0.4V), V where V IN(MAX) is the maximum input voltage, VOUT is the output IN(MIN) is the minimum input voltage, and tOFF(MIN) voltage, f is the minimum switch off time (200ns). Note that higher SW is the switching frequency (set by RT), and L is in the inductor value. switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower The inductor’s RMS and saturation current rating must switching frequency should be used. be greater than the maximum load current. For robust operation in fault conditions (start-up or short circuit) and
Inductor Selection
high input voltage (>40V), the saturation current should For a given input and output voltage, the inductor value be above 3.5A. To keep the efficiency high, the series and switching frequency will determine the ripple current. resistance (DCR) should be less than 0.1Ω, and the core The ripple current ΔI material should be intended for high frequency applications. L increases with higher VIN or VOUT and decreases with higher inductance and faster switching Table 1 lists several vendors and suitable types. frequency. A reasonable starting point for selecting the
Table 1. Inductor Vendors
ripple current is:
VENDOR URL PART SERIES TYPE Δ
I Murata www.murata.com LQH55D Open L = 0.4(IOUT(MAX)) TDK www.component.tdk.com SLF10145 Shielded where IOUT(MAX) is the maximum output load current. To Toko www.toko.com D75C Shielded guarantee sufficient output current, peak inductor current D75F Open must be lower than the LT3980’s switch current limit (ILIM). Sumida www.sumida.com CDRH74 Shielded The peak inductor current is: CR75 Open CDRH8D43 Shielded IL(PEAK) = IOUT(MAX) +
Δ
IL/2 NEC www.nec-tokin.com MPLC073 Shielded MPBI0755 Shielded Vishay www.vishay.com IHLP2525CE01 Shielded 3980fa 10 For more information www.linear.com/LT3980 Document Outline Features Description Applications 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