Datasheet LT3970 (Analog Devices) - 10

LT3970 Series
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 integrated catch diode drop (~0.7V), between the output and the FB pin. Choose the 1% resis- and VSW is the internal switch drop (~0.5V at max load). tors according to: This equation shows that slower switching frequency is necessary to accommodate high VIN/VOUT ratio.  V R1= R2 OUT – 1 Lower frequency also allows a lower dropout voltage. The   1.21 input voltage range depends on the switching frequency because the LT3970 switch has finite minimum on and off Reference designators refer to the Block Diagram. Note times. The switch can turn on for a minimum of ~150ns and that choosing larger resistors will decrease the quiescent turn off for a minimum of ~160ns (note that the minimum current of the application circuit. on-time is a strong function of temperature). This means
Setting the Switching Frequency
that the minimum and maximum duty cycles are: The LT3970 uses a constant frequency PWM architecture DCMIN = fSW • tON(MIN) that can be programmed to switch from 200kHz to 2.2MHz DCMAX = 1 – fSW • tOFF(MIN) by using a resistor tied from the RT pin to ground. A table where f showing the necessary R SW is the switching frequency, the tON(MIN) is the T value for a desired switching minimum switch on-time (~150ns), and the t frequency is in Table 1. OFF(MIN) is the minimum switch off-time (~160ns). These equations
Table 1. Switching Frequency vs R
show that duty cycle range increases when switching
T Value SWITCHING FREQUENCY (MHz) R
frequency is decreased.
T VALUE (kΩ)
0.2 768 A good choice of switching frequency should allow ad- 0.3 499 0.4 357 equate input voltage range (see next section) and keep 0.5 280 the inductor and capacitor values small. 0.6 226 0.8 158 1.0 124
Input Voltage Range
1.2 100 1.4 80.6 The minimum input voltage is determined by either the 1.6 68.1 LT3970’s minimum operating voltage of 4.2V or by its 1.8 57.6 maximum duty cycle (as explained in previous section). 2.0 49.9 2.2 42.2 The minimum input voltage due to duty cycle is:
Operating Frequency Trade-Offs
V V OUT + VD IN(MIN) = – V 1– f D + VSW Selection of the operating frequency is a trade-off between SW • tOFF(MIN) efficiency, component size, minimum dropout voltage and where V maximum input voltage. The advantage of high frequency IN(MIN) is the minimum input voltage, VOUT is the output voltage, V operation is that smaller inductor and capacitor values may D is the catch diode drop (~0.7V), VSW is the internal switch drop (~0.5V at max load), f be used. The disadvantages are lower efficiency, lower SW is the switching frequency (set by RT), and t maximum input voltage, and higher dropout voltage. The OFF(MIN) is the minimum switch off-time (160ns). Note that higher switch- highest acceptable switching frequency (fSW(MAX)) for a ing frequency will increase the minimum input voltage. given application can be calculated as follows: If a lower dropout voltage is desired, a lower switching V f OUT + VD frequency should be used. SW(MAX) = t ( ) ON(MIN) VIN – VSW + VD 3970fc 10 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 Related Parts