Datasheet LT3680 (Analog Devices) - 9

link to page 9 link to page 10 link to page 9 LT3680
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 R in the next section, lower frequency allows a lower drop- = R OUT 1 2 – 1 out voltage. The reason input voltage range depends on 0 7 . 9V the switching frequency is because the LT3680 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. Typical minimum on time at 25°C is 80ns. This The LT3680 uses a constant frequency PWM architecture means that the minimum and maximum duty cycles are: that can be programmed to switch from 200kHz to 2.4MHz DC = f t MIN SW by using a resistor tied from the RT pin to ground. A table ON MI ( N) 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 (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 fre- 0.5 78.7 quency is decreased. 0.6 63.4 0.7 53.6 A good choice of switching frequency should allow ade- 0.8 45.3 0.9 39.2 quate input voltage range (see next section) and keep the 1.0 34 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 LT3680 applications 2.2 10.7 depends on switching frequency and Absolute Maximum 2.4 9.09 Ratings of the VIN and BOOST pins (36V and 56V
Figure 1. Switching Frequency vs. R
respectively).
T 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: efficiency, component size, minimum dropout voltage, and V + V maximum input voltage. The advantage of high frequency V OUT D = – V + V IN MA ( X) D SW operation is that smaller inductor and capacitor values may f t SW ON MI ( N) be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The where VIN(MAX) is the maximum operating input volt- highest acceptable switching frequency (f age, V SW(MAX)) for a OUT is the output voltage, VD is the catch diode given application can be calculated as follows: drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), f V SW is the switching frequency (set by RT), + V f D OUT and t SW MA ( X) = ON(MIN) is the minimum switch on time (~100ns). t (V + V – V ) Note that a higher switching frequency will depress the ON MI ( N) D IN SW Rev C For more information www.analog.com 9 Document Outline Features Applications Description 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