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LTC3601IMSE Datasheet(PDF) 10 Page - Linear Technology |
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LTC3601IMSE Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LTC3601 10 3601f APPLICATIONS INFORMATION A general LTC3601 application circuit is shown on the first page of this data sheet. External component selection is largely driven by the load requirement and begins with the selection of the inductor L. Once the inductor is chosen, the input capacitor, CIN, the output capacitor, COUT, the internal regulatorcapacitor,CINTVCC,andtheboostcapacitor,CBOOST, can be selected. Next, the feedback resistors are selected to set the desired output voltage. Finally, the remaining option- al external components can be selected for functions such as external loop compensation, track/soft-start, externally programmed oscillator frequency and PGOOD. Operating Frequency Selection of the operating frequency is a trade-off between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage. The operating frequency, fO, of the LTC3601 is determined by an external resistor that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by using the following equation: R E f RT O = 32 11 . where RRT is in Ω and fO is in Hz. Connecting the RT pin to INTVCC will default the converter to fO = 2MHz; however, this switching frequency will be more sensitive to process and temperature variations than when using a resistor on RT (see Typical Performance Characteristics). Inductor Selection For a given input and output voltage, the inductor value and operating frequency determine the inductor ripple current. More specifically, the inductor ripple current decreases with higher inductor value or higher operating frequency according to the following equation: IL = VOUT f• L 1– VOUT VIN where ΔIL = inductor ripple current, f = operating frequency and L = inductor value. A trade-off between component size, efficiency and operating frequency can be seen from this equation. Accepting larger values of ΔIL allows the use of lower value inductors but results in greater core loss in the inductor, greater ESR loss in the output capaci- tor, and larger output ripple. Generally, highest efficiency operation is obtained at low operating frequency with small ripple current. A reasonable starting point for setting the ripple current is about 40% of IOUT(MAX). Note that the largest ripple current occurs at the highest VIN. To guarantee the ripple current does not exceed a specified maximum the inductance should be chosen according to: L = VOUT f• IL(MAX) 1– VOUT VIN(MAX) Once the value for L is known the type of inductor must be selected. Actual core loss is independent of core size for a fixed inductor value but is very dependent on the inductance selected. As the inductance increases, core loss decreases. Unfortunately, increased inductance requires more turns of wire leading to increased copper loss. Ferrite designs exhibit very low core loss and are pre- ferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core materials saturate “hard,” meaning the induc- tance collapses abruptly when the peak design current is RT (kΩ) 0 0 1000 2000 3000 4000 6000 100 200 300 400 3601 F01 500 600 5000 Figure 1. Switching Frequency vs RT |
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