Datasheet LT1912 (Analog Devices) - 10

LT1912
APPLICATIONS INFORMATION
pulse-skipping operation where some switching pulses The largest inductor ripple current occurs at the highest are skipped to maintain output regulation. In this mode VIN. To guarantee that the ripple current stays below the the output voltage ripple and inductor current ripple will specified maximum, the inductor value should be chosen be higher than in normal operation. according to the following equation: The minimum input voltage is determined by either the ⎛ V ⎞ ⎛ V ⎞ LT1912’s minimum operating voltage of ~3.6V or by its L OUT + VD OUT + VD = ⎜ ⎟ maximum duty cycle (see equation in previous section). ⎝⎜ f ⎠⎟ 1– V SWΔIL ⎝ IN(MAX) ⎠ The minimum input voltage due to duty cycle is: where VD is the voltage drop of the catch diode (~0.4V), V V OUT + VD VIN(MAX) is the maximum input voltage, VOUT is the output IN(MAX) = – V f D + VSW voltage, f SW tON(MIN) SW 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) The inductor’s RMS current rating must be greater than the is the minimum switch off time (150ns). Note that higher maximum load current and its saturation current should be switching frequency will increase the minimum input about 30% higher. For robust operation in fault conditions voltage. If a lower dropout voltage is desired, a lower (start-up or short circuit) and high input voltage (>30V), switching frequency should be used. the saturation current should be above 3.5A. To keep the efficiency high, the series resistance (DCR) should be less
Inductor Selection
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
Table 1. Inductor Vendors
and decreases with higher inductance and faster switch-
VENDOR URL PART SERIES TYPE
ing frequency. A reasonable starting point for selecting Murata www.murata.com LQH55D Open the ripple current is: TDK www.componenttdk.com SLF7045 Shielded ΔI SLF10145 Shielded L = 0.4(IOUT(MAX)) Toko www.toko.com D62CB Shielded where IOUT(MAX) is the maximum output load current. To D63CB Shielded guarantee sufficient output current, peak inductor current D75C Shielded must be lower than the LT1912’s switch current limit (ILIM). D75F Open The peak inductor current is: Sumida www.sumida.com CR54 Open I CDRH74 Shielded L(PEAK) = IOUT(MAX) + ΔIL/2 CDRH6D38 Shielded where IL(PEAK) is the peak inductor current, IOUT(MAX) is CR75 Open the maximum output load current, and ΔIL is the inductor ripple current. The LT1912’s switch current limit (ILIM) is at least 3.5A at low duty cycles and decreases linearly to Of course, such a simple design guide will not always re- 2.5A at DC = 0.8. The maximum output current is a func- sult in the optimum inductor for your application. A larger tion of the inductor ripple current: value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your IOUT(MAX) = ILIM – ΔIL/2 load is lower than 2A, then you can decrease the value of Be sure to pick an inductor ripple current that provides the inductor and operate with higher ripple current. This sufficient maximum output current (IOUT(MAX)). allows you to use a physically smaller inductor, or one 1912fa 10 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