Datasheet LTC3601 (Analog Devices) - 10

LTC3601
APPLICATIONS INFORMATION
A general LTC3601 application circuit is shown on the first Connecting the RT pin to INTVCC will default the converter page of this data sheet. External component selection is to fO = 2MHz; however, this switching frequency will be largely driven by the load requirement and begins with the more sensitive to process and temperature variations than selection of the inductor L. Once the inductor is chosen, when using a resistor on RT (see Typical Performance the input capacitor, CIN, the output capacitor, COUT , the Characteristics). internal regulator capacitor, CINTVCC, and the boost capaci- tor, CBOOST, can be selected. Next, the feedback resistors
Inductor Selection
are selected to set the desired output voltage. Finally, the For a given input and output voltage, the inductor value and remaining optional external components can be selected operating frequency determine the inductor ripple current. for functions such as external loop compensation, track/ More specifically, the inductor ripple current decreases soft-start, externally programmed oscillator frequency with higher inductor value or higher operating frequency and PGOOD. according to the following equation:
Operating Frequency
 V  V  ΔI OUT  1– OUT  Selection of the operating frequency is a trade-off between L =  f • L  VIN  efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. where ∆IL = inductor ripple current, f = operating frequency Operation at lower frequencies improves efficiency by and L = inductor value. A trade-off between component reducing internal gate charge losses but requires larger size, efficiency and operating frequency can be seen from inductance values and/or capacitance to maintain low this equation. Accepting larger values of ∆IL allows the use output ripple voltage. of lower value inductors but results in greater core loss in the inductor, greater ESR loss in the output capacitor, The operating frequency, fO, of the LTC3601 is determined and larger output ripple. Generally, highest efficiency op- by an external resistor that is connected between the RT eration is obtained at low operating frequency with small pin and ground. The value of the resistor sets the ramp ripple current. current that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated A reasonable starting point for setting the ripple current is by using the following equation: about 40% of IOUT(MAX). Note that the largest ripple current occurs at the highest VIN. To guarantee the ripple current R does not exceed a specified maximum the inductance RT = 3.2 E11 fO should be chosen according to: where RRT is in Ω and fO is in Hz.  V  V  L = OUT  1– OUT  6000  f • ΔI  V   L(MAX)  IN(MAX)  5000 However, the inductor ripple current must not be so large 4000 that its valley current level exceeds the negative current limit of –1.2A (typical) when the circuit is operating in 3000 forced continuous mode. If the inductor current trough reaches the negative current limit while the part is in FREQUENCY (kHz) 2000 forced continuous mode operation, VOUT may charge up 1000 to above its target regulation voltage. In such instances, choose a larger inductor value to reduce the ripple current. 0 0 100 200 300 400 500 600 The alternative is to reduce the inductor ripple current by RT (kΩ) decreasing the RT resistor value, which will increase the 3601 F01 switching frequency.
Figure 1. Switching Frequency vs RT
3601fc 10 For more information www.linear.com/LTC3601 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Related Parts