Datasheet LTC3607 (Analog Devices) - 9

LTC3607
applicaTions inForMaTion
An important design consideration is that the RDS(ON) V  V of the P-channel switch increases with decreasing input L = OUT • 1– OUT  supply voltage (see Typical Performance Characteristics). fO • I ∆ L  VIN(MAX) Therefore, the user should calculate the power dissipation when the LTC3607 is used at 100% duty cycle with low The inductor value will also have an effect on Burst Mode input voltage (see Thermal Considerations in the Applica- operation. The transition from low current operation tions Information section). begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in
Low/High Supply Operation
higher ripple current which causes this transition to occur at lower load currents. This causes a dip in efficiency in The LTC3607 incorporates an undervoltage lockout circuit the upper range of low current operation. In Burst Mode which shuts down the part when the input voltage drops operation, lower inductance values will cause the burst below about 3.7V to prevent unstable operation. frequency to increase. A general LTC3607 application circuit is shown in Figure 1. External component selection is driven by the load require-
Inductor Core Selection
ment, and begins with the selection of the inductor L. Once Different core materials and shapes will change the size/ the inductor is chosen, CIN and COUT can be selected. current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permal oy materials are
Inductor Selection
small and don’t radiate much energy, but general y cost The operating frequency directly effects both the inductor more than powdered iron core inductors with similar elec- value, and the ripple current. The inductor ripple current trical characteristics. The choice of which style inductor to ΔIL decreases with higher frequency and/or inductance use often depends more on the price vs size requirements and increases with higher VIN: and any radiated field/EMI requirements than on what the LTC3607 requires to operate. Table 1 shows the websites V  V of several surface mount inductor manufacturers. ∆I OUT OUT  L = • 1–   fO•L VIN
Table 1. Inductor Manufacturer
Coilcraft http:/ www.coilcraft.com/powersel_lowl.html Accepting larger values of ΔIL al ows the use of low inductances, but results in higher output voltage ripple, Cooper Bussmann http:/ www.cooperindustries.com/content/public/ en/bussmann/electronics/products/coiltronics_ greater core losses, and lower output current capability. inductorandtransformermagnetics.html A reasonable starting point for setting ripple current is Würth Electronic http:/ katalog.we-online.com/en/pbs/browse/ ΔI Power-Magnetics/Speicherdrosseln L = 0.4 • IO(MAX), where IO(MAX) is the maximum rated output current. The largest ripple current ΔI Murata http:/ www.murata.com/products/inductor/index. L occurs at html the maximum input voltage. To guarantee that the ripple TDK http:/ www.tdk.co.jp/tefe02/coil.htm current stays below a specified maximum, the inductor Vishay http://www.vishay.com/inductors/power- value should be chosen according to the fol owing equation: inductors/ Sumida http:/ www.sumida.com/en/products/ power_main.php 3607fb For more information www.linear.com/LTC3607 9 Document Outline Features Description Applications 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