Datasheet LT8365 (Analog Devices) - 14

link to page 10 link to page 10 link to page 15 LT8365
APPLICATIONS INFORMATION
not short-circuit protected. Under a shorted output condi- output current capability. Choosing smaller values of tion, the inductor current is limited only by the input supply ∆ISW increases output current capability, but requires capability. For applications requiring a step-up converter large inductances and reduces the current loop gain (the that is short-circuit protected, please refer to the Applica- converter will approach voltage mode). Accepting larger tions Information section covering SEPIC converters. values of ∆ISW provides fast transient response and The conversion ratio as a function of duty cycle is: allows the use of low inductances, but results in higher input current ripple and greater core losses, and reduces VOUT output current capability. It is recommended to choose a = 1 VIN 1 − D ∆ISW of approximately 0.60A. in continuous conduction mode (CCM). Given an operating input voltage range, and having chosen the operating frequency and ripple current in the inductor, For a boost converter operating in CCM, the duty cycle the inductor value of the boost converter can be determined of the main switch can be calculated based on the output using the following equation: voltage (VOUT) and the input voltage (VIN). The maximum duty cycle (D VIN(MIN) MAX) occurs when the converter has the • DMAX minimum input voltage: L = ΔISW • fOSC V The peak inductor current is the switch current limit D OUT − VIN(MIN) MAX = (maximum 2.7A), and the RMS inductor current is ap- VOUT proximately equal to IL(MAX)(AVG). Discontinuous conduction mode (DCM) provides higher Choose an inductor that can handle at least 2.7A without conversion ratios at a given frequency at the cost of re- saturating, and ensure that the inductor has a low DCR duced efficiencies, higher switching currents, and lower (copper-wire resistance) to minimize I2R power losses. Note available output power. that in some applications, the current handling requirements of the inductor can be lower, such as in the SEPIC topology
Boost Converter: Maximum Output Current Capability
where each inductor only carries one-half of the total switch
and Inductor Selection
current. For better efficiency, use similar valued inductors For the boost topology, the maximum average inductor with a larger volume. Many different sizes and shapes are current is: available from various manufacturers (see Table 2). Choose a core material that has low losses at the programmed switch- I • 1 ing frequency, such as a ferrite core. The final value chosen L(MAX)(AVG)= IO(MAX) • 1 1 − DMAX η for the inductor should not allow peak inductor currents to where η (< 1.0) is the converter efficiency. exceed 1.5A in steady state at maximum load. Due to toler- Due to the current limit of its internal power switch, the ances, be sure to account for minimum possible inductance LT8365 should be used in a boost converter whose maxi- value, switching frequency and converter efficiency. mum output current (IO(MAX)) is: For inductor current operation in CCM and duty cycles above 50%, the LT8365's internal slope compensa- V I IN(MIN) O(MAX) ≤ • 1.5 ( A − 0.5 • ΔISW) • η tion prevents sub-harmonic oscillations provided the VOUT inductor value exceeds a minimum value given by: Minimum possible inductor value and switching frequency VIN (2 •D – 1) should also be considered since they will increase inductor L > • –5 •D2 +10 •D – 1 ( )• f( ) 1(–D) ripple current ∆I OSC SW. The inductor ripple current ∆ISW has a direct effect on the Lower L values are allowed if the inductor current operates choice of the inductor value and the converter’s maximum in DCM or duty cycle operation is below 50%. Rev. 0 14 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts