Datasheet LT3972 (Analog Devices) - 10

LT3972
APPLICATIONS INFORMATION
V + V where IOUT(MAX) is the maximum output load current. To V OUT D = – V + V IN MAX guarantee suffi cient output current, peak inductor current f t D SW ( ) SW ( ) ON MIN must be lower than the LT3972’s switch current limit (ILIM). The peak inductor current is: where VIN(MAX) is the maximum operating input voltage, V I OUT is the output voltage, VD is the catch diode drop L(PEAK) = IOUT(MAX) + ΔIL/2 (~0.5V), VSW is the internal switch drop (~0.5V at max where IL(PEAK) is the peak inductor current, IOUT(MAX) is load), fSW is the switching frequency (set by RT), and the maximum output load current, and ΔIL is the inductor tON(MIN) is the minimum switch on-time (~100ns). Note that ripple current. The LT3972’s switch current limit (ILIM) is a higher switching frequency will depress the maximum 5.5A at low duty cycles and decreases linearly to 4.5A at operating input voltage. Conversely, a lower switching DC = 0.8. The maximum output current is a function of frequency will be necessary to achieve safe operation at the inductor ripple current: high input voltages. IOUT(MAX) = ILIM – ΔIL/2 If the output is in regulation and no short-circuit, start- up, or overload events are expected, then input voltage Be sure to pick an inductor ripple current that provides transients of up to 33V are acceptable regardless of the suffi cient maximum output current (IOUT(MAX)). switching frequency. In this mode, the LT3972 may enter The largest inductor ripple current occurs at the highest pulse-skipping operation where some switching pulses VIN. To guarantee that the ripple current stays below the are skipped to maintain output regulation. In this mode specifi ed maximum, the inductor value should be chosen the output voltage ripple and inductor current ripple will according to the following equation: be higher than in normal operation. ⎛ V + V ⎞ ⎛ V + V ⎞ The minimum input voltage is determined by either the L OUT D OUT D =⎜ ⎟ ⎜⎜1– ⎟⎟ LT3972’s minimum operating voltage of ~3.6V or by its f ΔI V ⎝ ⎠ ⎝ SW L IN MA ( X) ⎠ maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is: where VD is the voltage drop of the catch diode (~0.4V), V V + V IN(MAX) is the maximum input voltage, VOUT is the output V OUT D = – V + V voltage, fSW is the switching frequency (set by RT), and IN MIN 1– f t D SW ( ) SW ( ) L is in the inductor value. OFF MIN The inductor’s RMS current rating must be greater than the where VIN(MIN) is the minimum input voltage, and tOFF(MIN) maximum load current and its saturation current should be is the minimum switch off-time (150ns). Note that higher about 30% higher. For robust operation in fault conditions switching frequency will increase the minimum input (start-up or short circuit) and high input voltage (>30V), voltage. If a lower dropout voltage is desired, a lower the saturation current should be above 5A. To keep the switching frequency should be used. effi ciency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for
Inductor Selection
high frequency applications. Table 1 lists several vendors For a given input and output voltage, the inductor value and suitable types. and switching frequency will determine the ripple current. The ripple current ΔIL increases with higher VIN or VOUT and decreases with higher inductance and faster switch- ing frequency. A reasonable starting point for selecting the ripple current is: ΔIL = 0.4(IOUT(MAX)) 3972fa 10