Datasheet LT3972 (Analog Devices) - 9

LT3972
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 LT3972 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 LT3972 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 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 (kΩ)
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 LT3972 applications de- 2.2 10.7 pends on switching frequency, Absolute Maximum Ratings 2.4 9.09 of the VIN and BOOST pins, and the operating mode.
Figure 1. Switching Frequency vs RT Value
The LT3972 can operate from input voltages of up to 33V, and withstand voltages up to 62V. Note that while V
Operating Frequency Trade-Offs
IN is above 35V typical (33V minimum and 37V maximum) Selection of the operating frequency is a trade-off between the part will keep the switch off and the output will not effi ciency, component size, minimum dropout voltage, and be in regulation. maximum input voltage. The advantage of high frequency To safely allow inputs of up to 62V, be sure to choose a operation is that smaller inductor and capacitor values may Schottky diode, inductor size, and switching frequency be used. The disadvantages are lower effi ciency, lower to allow safe operation at 37V according to the following maximum input voltage, and higher dropout voltage. The discussions. highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: While the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be V + V f D OUT = chosen according to the following equation: SW M ( AX) t V ( +V –V ( ) ON MIN D IN SW 3972fa 9