Datasheet LT3980 (Analog Devices) - 9

LT3980
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) and 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  in the next section, lower frequency allows a lower dropout R =R OUT 1 2 – 1 voltage. The reason input voltage range depends on the  0 79V  . switching frequency is because the LT3980 switch has Reference designators refer to the Block Diagram. finite minimum on and off times. The switch can turn on for a minimum of ~200ns and turn off for a minimum of
Setting the Switching Frequency
~200ns. This means that the minimum and maximum duty cycles are: The LT3980 uses a constant frequency PWM architecture that can be programmed to switch from 100kHz to 2.4MHz DC = f t MIN SW ON MI ( N) by using a resistor tied from the RT pin to ground. A table DC = 1– f t showing the necessary RT 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 (~200ns), and the tOFF(MIN) is 0.1 432 the minimum switch off time (~200ns). These equations 0.2 215 0.3 137 show that duty cycle range increases when switching 0.4 97.6 frequency is decreased. 0.5 76.8 0.6 60.4 A good choice of switching frequency should allow ade- 0.7 51.1 quate input voltage range (see next section) and keep the 0.8 43.2 0.9 35.7 inductor and capacitor values small. 1.0 32.4 1.2 24.9
Input Voltage Range
1.4 20 1.6 16.2 The maximum input voltage for LT3980 applications de- 1.8 14 2.0 11 pends on switching frequency, Absolute Maximum Ratings of the VIN and BOOST pins, and the operating mode.
Figure 1. Switching Frequency vs RT Value
The LT3980 can operate from input voltages of up to 58V,
Operating Frequency Trade-Offs
and withstand voltages up to 80V. Note that while VIN is Selection of the operating frequency is a trade-off between above 61V typical (58V minimum and 64V maximum) efficiency, component size, minimum dropout voltage, and the part will keep the switch off and the output will not maximum input voltage. The advantage of high frequency be in regulation. operation is that smaller inductor and capacitor values may The switching frequency should be chosen according to be used. The disadvantages are lower efficiency, lower the following equation: maximum input voltage, and higher dropout voltage. The V + V highest acceptable switching frequency (f OUT D SW(MAX)) for a V = – V + V IN MA ( X) D SW given application can be calculated as follows: f t SW ON MI ( N) V + V f D OUT where V SW MA ( X) = IN(MAX) is the maximum typical operating input t (V +V –V ) voltage, V ON MI ( N) D IN SW OUT is the output voltage, VD is the catch diode 3980fa For more information www.linear.com/LT3980 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts