Datasheet LT3681 (Analog Devices) - 9

LT3681
APPLICATIONS INFORMATION FB Resistor Network
where VIN is the typical input voltage, VOUT is the out- put voltage, V The output voltage is programmed with a resistor divider D is the power Schottky catch diode drop (~0.55V), V between the output and the FB pin. Choose the 1% resis- SW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency tors according to: is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown in the next section, lower frequency al- V ⎞ R = ⎛ R OUT 1 2 – 1 lows a lower dropout voltage. The reason input voltage ⎝⎜ 1 265 ⎠⎟ . range depends on the switching frequency is because the LT3681 switch has fi nite minimum on and off times. The Reference designators refer to the Block Diagram. switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. This means that the minimum
Setting the Switching Frequency
and maximum duty cycles are: The LT3681 uses a constant frequency PWM architecture DC = f t that can be programmed to switch from 300kHz to 2.8MHz MIN SW ON MIN ( ) by using a resistor tied from the RT pin to ground. A table DC = 1– f t MAX SW OFF M ( IN) showing the necessary RT value for a desired switching frequency is in Figure 1. where fSW is the switching frequency, the tON(MIN) is the
SWITCHING FREQUENCY (MHz) R
minimum switch on time (~150ns), and the t
T VALUE (k
Ω
)
OFF(MIN) is 0.3 187 the minimum switch off time (~150ns). These equations 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 LT3681 applications de- 2.8 8.66 pends on switching frequency, the Absolute Maximum Rat-
Figure 1. Switching Frequency vs. RT Value
ings on VIN and BOOST pins, and on operating mode.
Operating Frequency Tradeoffs
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 tradeoff between following equation: effi ciency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency V + V V OUT D = – V + V D SW ( ) operation is that smaller inductor and capacitor values may IN MAX 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 (f VOUT is the output voltage, VD is the catch diode drop SW(MAX)) for a given application can be calculated as follows: (~0.55V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and V + V tON(MIN) is the minimum switch on time (~150ns). Note that f D OUT = SW M ( AX) a higher switching frequency will depress the maximum t V ( + V – V ( ) ON MIN D IN SW operating input voltage. Conversely, a lower switching 3681f 9