Datasheet LT3480 (Analog Devices) - 10

LT3480
APPLICATIONS INFORMATION
A good choice of switching frequency should allow ad- maximum duty cycle (see equation in previous section). equate input voltage range (see next section) and keep The minimum input voltage due to duty cycle is: the inductor and capacitor values small.
V V OUT
+
VD – V Input Voltage Range IN(MIN)
=
1– f D
+
VSW SW tOFF(MIN)
The maximum input voltage for LT3480 applications depends on switching frequency, the Absolute Maximum Ratings of where VIN(MIN) is the minimum input voltage, and tOFF(MIN) the V is the minimum switch off time (150ns). Note that higher IN and BOOST pins, and the operating mode. switching frequency will increase the minimum input The LT3480 can operate from input voltages up to 38V, voltage. If a lower dropout voltage is desired, a lower and safely withstand input voltages up 60V. Note that while switching frequency should be used. VIN>38V (typical), the LT3480 will stop switching, allowing the output to fall out of regulation.
Inductor Selection
While the output is in start-up, short-circuit, or other For a given input and output voltage, the inductor value overload conditions, the switching frequency should be and switching frequency will determine the ripple current. chosen according to the following discussion. The ripple current ΔIL increases with higher VIN or VOUT For safe operation at inputs up to 60V the switching fre- and decreases with higher inductance and faster switching quency must be set low enough to satisfy V frequency. A reasonable starting point for selecting the IN(MAX) ≥ 40V according to the following equation. If lower V ripple current is: IN(MAX) is desired, this equation can be used directly. ΔIL = 0.4(IOUT(MAX))
V
where I
V OUT
+
VD
OUT(MAX) is the maximum output load current. To
IN(MAX)
=
– VD
+
VSW
guarantee sufficient output current, peak inductor current
f SW tON(MIN)
must be lower than the LT3480’s switch current limit (ILIM). where VIN(MAX) is the maximum operating input voltage, The peak inductor current is: VOUT is the output voltage, VD is the catch diode drop I (~0.5V), V L(PEAK) = IOUT(MAX) + ΔIL/2 SW is the internal switch drop (~0.5V at max load), f where I SW is the switching frequency (set by RT), and L(PEAK) is the peak inductor current, IOUT(MAX) is t the maximum output load current, and ΔI ON(MIN) is the minimum switch on time (~150ns). Note that L is the inductor a higher switching frequency will depress the maximum ripple current. The LT3480’s switch current limit (ILIM) is operating input voltage. Conversely, a lower switching at least 3.5A at low duty cycles and decreases linearly to frequency will be necessary to achieve safe operation at 2.5A at DC = 0.8. The maximum output current is a func- high input voltages. tion of the inductor ripple current: If the output is in regulation and no short-circuit, start- IOUT(MAX) = ILIM – ΔIL/2 up, or overload events are expected, then input voltage Be sure to pick an inductor ripple current that provides transients of up to 60V are acceptable regardless of the sufficient maximum output current (IOUT(MAX)). switching frequency. In this mode, the LT3480 may enter 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. Above 38V switching according to the following equation: will stop.  V   V  L OUT + VD OUT + VD The minimum input voltage is determined by either the =  f   SW∆IL  1– V  IN(MAX)  LT3480’s minimum operating voltage of ~3.6V or by its 3480fe 10 For more information www.linear.com/LT3480 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts