Datasheet LT3685 (Analog Devices) - 10

LT3685
APPLICATIONS INFORMATION
V + V frequency. A reasonable starting point for selecting the V OUT D = – V + V IN MA ( X) ripple current is: f t D SW SW ON M ( IN) ΔIL = 0.4(IOUT(MAX)) where VIN(MAX) is the maximum operating input voltage, where I V OUT(MAX) is the maximum output load current. To OUT is the output voltage, VD is the catch diode drop guarantee suffi cient output current, peak inductor current (~0.5V), VSW is the internal switch drop (~0.5V at max must be lower than the LT3685’s switch current limit (I load), f LIM). SW is the switching frequency (set by RT), and The peak inductor current is: tON(MIN) is the minimum switch on time (~150ns). Note that a higher switching frequency will depress the maximum IL(PEAK) = IOUT(MAX) + ΔIL/2 operating input voltage. Conversely, a lower switching where IL(PEAK) is the peak inductor current, IOUT(MAX) is frequency will be necessary to achieve safe operation at the maximum output load current, and ΔIL is the inductor high input voltages. ripple current. The LT3685’s switch current limit (ILIM) is If the output is in regulation and no short-circuit, start- at least 3.5A at low duty cycles and decreases linearly to up, or overload events are expected, then input voltage 2.5A at DC = 0.8. The maximum output current is a func- transients of up to 60V are acceptable regardless of the tion of the inductor ripple current: switching frequency. In this mode, the LT3685 may enter IOUT(MAX) = ILIM – ΔIL/2 pulse skipping operation where some switching pulses are skipped to maintain output regulation. In this mode Be sure to pick an inductor ripple current that provides the output voltage ripple and inductor current ripple will suffi cient maximum output current (IOUT(MAX)). be higher than in normal operation. Above 38V switching The largest inductor ripple current occurs at the highest will stop. VIN. To guarantee that the ripple current stays below the The minimum input voltage is determined by either the specifi ed maximum, the inductor value should be chosen LT3685’s minimum operating voltage of ~3.6V or by its according to the following equation: maximum duty cycle (see equation in previous section). ⎛ V + V ⎞ ⎛ V + V ⎞ The minimum input voltage due to duty cycle is: L OUT D OUT D = 1– ⎝⎜ ⎜ ⎟ f I Δ ⎠⎟ V SW L ⎝ IN MA ( X) ⎠ V + V V OUT D = – V + V IN MIN ( ) – 1 f t D SW where V SW OFF M ( IN) D is the voltage drop of the catch diode (~0.4V), VIN(MAX) is the maximum input voltage, VOUT is the output where VIN(MIN) is the minimum input voltage, and tOFF(MIN) voltage, fSW is the switching frequency (set by RT), and is the minimum switch off time (150ns). Note that higher L is in the inductor value. switching frequency will increase the minimum input The inductor’s RMS current rating must be greater than the voltage. If a lower dropout voltage is desired, a lower maximum load current and its saturation current should be switching frequency should be used. about 30% higher. For robust operation in fault conditions
Inductor Selection
(start-up or short circuit) and high input voltage (>30V), the saturation current should be above 3.5A. To keep the For a given input and output voltage, the inductor value effi ciency high, the series resistance (DCR) should be less and switching frequency will determine the ripple current. than 0.1 , and the core material should be intended for The ripple current ΔIL increases with higher VIN or VOUT high frequency applications. Table 1 lists several vendors and decreases with higher inductance and faster switching and suitable types. 3685fb 10