Datasheet LT3693 (Analog Devices) - 9

LT3693
APPLICATIONS INFORMATION FB Resistor Network
where VIN is the typical input voltage, VOUT is the output voltage, V The output voltage is programmed with a resistor divider D is the catch diode drop (~0.5V) and VSW is the internal switch drop (~0.5V at max load). This equation between the output and the FB pin. Choose the 1% resis- shows that slower switching frequency is necessary to tors according to: safely accommodate high VIN/VOUT ratio. Also, as shown V ⎞ in the next section, lower frequency allows a lower dropout R = ⎛ R OUT 1 2 – 1 ⎝⎜ 0 79V ⎠⎟ . voltage. The reason input voltage range depends on the switching frequency is because the LT3693 switch has fi nite Reference designators refer to the Block Diagram. minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns.
Setting the Switching Frequency
Typical minimum on time at 25°C is 80ns. This means that The LT3693 uses a constant frequency PWM architecture the minimum and maximum duty cycles are: that can be programmed to switch from 200kHz to 2.4MHz DC = f t MIN SW ON MIN ( ) by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching DC = 1– f t 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 0.2 215 the minimum switch off time (~150ns). These equations 0.3 140 0.4 100 show that duty cycle range increases when switching 0.5 78.7 frequency is decreased. 0.6 63.4 0.7 53.6 A good choice of switching frequency should allow ad- 0.8 45.3 0.9 39.2 equate input voltage range (see next section) and keep 1.0 34 the inductor and capacitor values small. 1.2 26.7 1.4 22.1 1.6 18.2
Input Voltage Range
1.8 15 2.0 12.7 The maximum input voltage for LT3693 applications 2.2 10.7 depends on switching frequency and Absolute Maxi- 2.4 9.09 mum Ratings of the VIN and BOOST pins (36V and 56V respectively).
Figure 1. Switching Frequency vs. RT Value
While the output is in start-up, short-circuit, or other
Operating Frequency Tradeoffs
overload conditions, the switching frequency should be Selection of the operating frequency is a tradeoff between chosen according to the 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 (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), f V + V SW is the switching frequency (set by RT), and f D OUT = tON(MIN) is the minimum switch on time (~100ns). Note that SW M ( AX) t V ( + V – V ( ) a higher switching frequency will depress the maximum ON MIN D IN SW 3693f 9