Datasheet LT3990, LT3990-3.3, LT3990-5 (Analog Devices) - 10

LT3990/LT3990-3.3/LT3990-5
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.  1.21  The input voltage range depends on the switching fre- Reference designators refer to the Block Diagram. Note quency because the LT3990 switch has finite minimum that choosing larger resistors will decrease the quiescent on and off times. The switch can turn off for a minimum current of the application circuit. of ~160ns but the minimum on-time is a strong function of temperature. Use the minimum switch on-time curve
Setting the Switching Frequency
(see Typical Performance Characteristics) to design for an application’s maximum temperature, while adding The LT3990 uses a constant frequency PWM architecture about 30% for part-to-part variation. The minimum and that can be programmed to switch from 200kHz to 2.2MHz maximum duty cycles that can be achieved taking these by using a resistor tied from the RT pin to ground. A table on and off times into account are: showing the necessary RT value for a desired switching frequency is in Table 1. DCMIN = fSW • tON(MIN) DC
Table 1. Switching Frequency vs R
MAX = 1 – fSW • tOFF(MIN)
T Value SWITCHING FREQUENCY (MHz) RT VALUE (kΩ)
where fSW is the switching frequency, the tON(MIN) is the 0.2 787 minimum switch on-time, and the tOFF(MIN) is the minimum 0.3 511 switch off-time (~160ns). These equations show that 0.4 374 0.5 287 duty cycle range increases when switching frequency is 0.6 232 decreased. 0.8 169 1.0 127 A good choice of switching frequency should allow ad- 1.2 102 equate input voltage range (see next section) and keep 1.4 84.5 1.6 69.8 the inductor and capacitor values small. 1.8 59 2.0 51.1
Input Voltage Range
2.2 44.2 The minimum input voltage is determined by either the
Operating Frequency Trade-Offs
LT3990’s minimum operating voltage of 4.2V or by its Selection of the operating frequency is a trade-off between maximum duty cycle (as explained in previous section). efficiency, component size, minimum dropout voltage and The minimum input voltage due to duty cycle is: maximum input voltage. The advantage of high frequency V operation is that smaller inductor and capacitor values may V OUT + VD IN(MIN) = – VD + VSW be used. The disadvantages are lower efficiency, lower 1– f SW • tOFF(MIN) maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (f where VIN(MIN) is the minimum input voltage, VOUT is the SW(MAX)) for a given application can be calculated as follows: output voltage, VD is the catch diode drop (~0.7V), VSW is the internal switch drop (~0.5V at max load), fSW is V f OUT + VD the switching frequency (set by RT), and tOFF(MIN) is the SW(MAX) = t ( ) minimum switch off-time (160ns). Note that higher switch- ON(MIN) VIN – VSW + VD ing frequency will increase the minimum input voltage. 3990fa 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 Typical Application Related Parts