Datasheet LT3481 (Analog Devices) - 9

LT3481
APPLICATIONS INFORMATION FB Resistor Network
where VIN is the typical input voltage, VOUT is the output voltage, is the catch diode drop (~0.5V), V The output voltage is programmed with a resistor divider SW is the internal switch drop (~0.5V at max load). This equation shows between the output and the FB pin. Choose the 1% resis- that slower switching frequency is necessary to safely tors according to: accommodate high VIN/VOUT ratio. Also, as shown in V ⎞ the next section, lower frequency allows a lower dropout R = ⎛ R OUT 1 2 – 1 ⎝⎜ 1 265 ⎠⎟ . voltage. The reason input voltage range depends on the switching frequency is because the LT3481 switch has Reference designators refer to the Block Diagram. fi nite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of
Setting the Switching Frequency
~150ns. This means that the minimum and maximum The LT3481 uses a constant frequency PWM architecture duty cycles are: that can be programmed to switch from 300kHz to 2.8MHz DC = f t MIN SW ON MIN ( ) by using a resistor tied from the RT pin to ground. A table showing the necessary R DC = 1– f t T value for a desired switching MAX SW OFF M ( IN) frequency is in Figure 1. where fSW is the switching frequency, the tON(MIN) is the
SWITCHING FREQUENCY (MHz) RT VALUE (k
Ω
)
minimum switch on-time (~150ns), and the tOFF(MIN) is 0.2 267 the minimum switch off-time (~150ns). These equations 0.3 187 0.4 133 show that duty cycle range increases when switching 0.6 84.5 frequency is decreased. 0.8 60.4 1.0 45.3 A good choice of switching frequency should allow ad- 1.2 36.5 1.4 29.4 equate input voltage range (see next section) and keep 1.6 23.7 the inductor and capacitor values small. 1.8 20.5 2.0 16.9 2.2 14.3
Input Voltage Range
2.4 12.1 2.6 10.2 The maximum input voltage for LT3481 applications 2.8 8.66 depends on switching frequency, the absolute maximum ratings on VIN and BOOST pins, and on operating mode.
Figure 1. Switching Frequency vs RT Value Operating Frequency Trade-Offs
If the output is in start-up or short-circuit operating modes, then VIN must be below 34V and below the result of the Selection of the operating frequency is a trade-off between following equation: effi ciency, component size, minimum dropout voltage, and V + V maximum input voltage. The advantage of high frequency V OUT D = – V + V IN MAX D SW ( ) operation is that smaller inductor and capacitor values may f t SW ( ) ON MIN be used. The disadvantages are lower effi ciency, lower maximum input voltage, and higher dropout voltage. The where VIN(MAX) is the maximum operating input voltage, highest acceptable switching frequency (fSW(MAX)) for a VOUT is the output voltage, VD is the catch diode drop given application can be calculated as follows: (~0.5V), VSW is the internal switch drop (~0.5V at max V + V load), fSW is the switching frequency (set by RT), and f D OUT = t SW M ( AX) ON(MIN) is the minimum switch on-time (~150ns). Note that t V ( +V –V ( ) ON MIN D IN SW a higher switching frequency will depress the maximum operating input voltage. Conversely, a lower switching 3481fc 9