Datasheet LT3684 (Analog Devices) - 9

LT3684
APPLICATIONS INFORMATION FB Resistor Network
where VIN is the typical input voltage, VOUT is the output The output voltage is programmed with a resistor divider voltage VD, is the catch diode drop (~0.5V), VSW is the between the output and the FB pin. Choose the 1% resis- internal switch drop (~0.5V at max load). This equation tors according to: shows that slower switching frequency is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown  V  R1= R2 OUT  −1 in the next section, lower frequency allows a lower dropout 1.265  voltage. The reason input voltage range depends on the switching frequency is because the LT3684 switch has Reference designators refer to the Block Diagram. finite 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 LT3684 uses a constant frequency PWM architecture duty cycles are: that can be programmed to switch from 300kHz to 2.8MHz DC by using a resistor tied from the RT pin to ground. A table MIN = fSW tON(MIN) showing the necessary R DC T value for a desired switching MAX = 1− fSWtOFF(MIN) 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 equate input voltage range (see next section) and keep 1.4 29.4 1.6 23.7 the inductor and capacitor values small. 1.8 20.5 2.0 16.9
Input Voltage Range
2.2 14.3 2.4 12.1 The maximum input voltage for LT3684 applications 2.6 10.2 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 R T Value
If the output is in start-up or short-circuit operating modes,
Operating Frequency Tradeoffs
then VIN must be below 34V and below the result of the Selection of the operating frequency is a tradeoff between following equation: efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency VIN(MAX) = VOUT + VD − VD + VSW operation is that smaller inductor and capacitor values may 1− f SW tON(MIN) be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The where VIN(MAX) is the maximum operating input voltage, highest acceptable switching frequency (f V SW(MAX)) for a OUT 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 load), fSW is the switching frequency (set by RT), and f t SW(MAX) = VD + VOUT ON(MIN) is the minimum switch on time (~150ns). Note that t ( ) a higher switching frequency will depress the maximum ON(MIN) VD + VIN − VSW operating input voltage. Conversely, a lower switching 3684fa For more information www.linear.com/LT3684 9