Datasheet LT1938 (Analog Devices) - 10

LT1938
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, is the catch diode drop (~0.5V), VSW is the internal between the output and the FB pin. Choose the 1% resis- switch drop (~0.5V at max load). This equation shows tors according to: that slower switching frequency is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown in ⎛ V the next section, lower frequency allows a lower dropout R1 OUT ⎞ = R2 – 1 voltage. The reason input voltage range depends on the ⎝⎜ 1.265 ⎠⎟ switching frequency is because the LT1938 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. This means that the minimum and maximum The LT1938 uses a constant frequency PWM architecture duty cycles are: that can be programmed to switch from 300kHz to 2.8MHz DC = f t by using a resistor tied from the RT pin to ground. A table MIN SW ON MI ( N) 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 0.2 267 the minimum switch off time (~150ns). These equations 0.3 187 show that duty cycle range increases when switching 0.4 133 0.6 84.5 frequency is decreased. 0.8 60.4 1.0 45.3 A good choice of switching frequency should allow ad- 1.2 36.5 equate input voltage range (see next section) and keep 1.4 29.4 1.6 23.7 the inductor and capacitor values small. 1.8 20.5 2.0 16.9
Input Voltage Range
2.2 14.3 2.4 12.1 The maximum input voltage for LT1938 applications de- 2.6 10.2 2.8 8.66 pends on switching frequency, the Absolute Maximum Rat- ings on VIN and BOOST pins, and on operating mode.
Figure 1. Switching Frequency vs R T Value
If the output is in start-up or short-circuit operating modes,
Operating Frequency Tradeoffs
then VIN must be below 25V and below the result of the Selection of the operating frequency is a tradeoff between 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 voltage, highest acceptable switching frequency (f VOUT is the output voltage, VD is the catch diode drop SW(MAX)) for a given application can be calculated as follows: (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and V + V t f D OUT ON(MIN) is the minimum switch on time (~150ns). Note that SW MA ( X) = t (V +V –V ) a higher switching frequency will depress the maximum ON MI ( N) D IN SW operating input voltage. Conversely, a lower switching 1938fa 10 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 TYPICAL APPLICATION RELATED PARTS