Datasheet LT3685 (Analog Devices) - 9

LT3685
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 LT3685 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 LT3685 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 M ( IN) by using a resistor tied from the RT pin to ground. A table DC = – 1 f t showing the necessary R MAX SW OFF MIN ( ) T value for a desired switching frequency is in Figure 1. where fSW is the switching frequency, the tON(MIN) is the minimum switch on time (~150ns), and the t
SWITCHING FREQUENCY (MHz) R
OFF(MIN) is
T VALUE (kΩ)
the minimum switch off time (~150ns). These equations 0.2 187 0.3 121 show that duty cycle range increases when switching 0.4 88.7 frequency is decreased. 0.5 68.1 0.6 56.2 A good choice of switching frequency should allow ad- 0.7 46.4 0.8 40.2 equate input voltage range (see next section) and keep 0.9 34 the inductor and capacitor values small. 1.0 29.4 1.2 23.7 1.4 19.1
Input Voltage Range
1.6 16.2 1.8 13.3 The maximum input voltage for LT3685 applications 2.0 11.5 depends on switching frequency, the Absolute Maximum 2.2 9.76 Ratings of the V 2.4 8.66 IN and BOOST pins, and the operating mode.
Figure 1. Switching Frequency vs. RT Value
The LT3685 can operate from input voltages up to 38V,
Operating Frequency Tradeoffs
and safely withstand input voltages up 60V. Note that while VIN > 38V (typical), the LT3685 will stop switching, Selection of the operating frequency is a tradeoff between allowing the output to fall out of regulation. effi ciency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency While the output is in start-up, short-circuit, or other operation is that smaller inductor and capacitor values may overload conditions, the switching frequency should be be used. The disadvantages are lower effi ciency, lower chosen according to the following discussion. maximum input voltage, and higher dropout voltage. The For safe operation at inputs up to 60V the switching fre- highest acceptable switching frequency (fSW(MAX)) for a quency must be set low enough to satisfy VIN(MAX) ≥ 40V given application can be calculated as follows: according to the following equation. If lower VIN(MAX) is V + V desired, this equation can be used directly. f D OUT SW M ( AX) = t V ( +V –V ) ON MIN ( ) D IN SW 3685fb 9