Datasheet LTC3616 (Analog Devices) - 14

LTC3616
applicaTions inForMaTion Frequency Synchronization Inductor Selection
The LTC3616’s internal oscillator can be synchronized to For a given input and output voltage, the inductor value an external frequency by applying a square wave clock and operating frequency determine the ripple current. The signal to the RT/SYNC pin. During synchronization, the top ripple current ∆IL increases with higher VIN and decreases switch turn-on is locked to the falling edge of the external with higher inductance: frequency source. The synchronization frequency range is 300kHz to 4MHz. During synchronization all operation ⎛ V ⎞ ⎛ V ⎞ ∆I OUT ⎜ ⎟ • ⎜1– OUT ⎟ modes can be selected. L = ⎝f ⎝ V SW • L ⎠ IN ⎠ It is recommended that the regulator is powered down Having a lower ripple current reduces the core losses (RUN pin to ground) before removing the clock signal on in the inductor, the ESR losses in the output capacitors the RT/SYNC pin in order to reduce inductor current ripple. and the output voltage ripple. A reasonable starting point AC coupling should be used if the external clock generator for selecting the ripple current is ∆IL = 0.3 • IOUT(MAX). cannot provide a continuous clock signal throughout start- The largest ripple current occurs at the highest VIN. To up, operation and shutdown of the LTC3616. The size of guarantee that the ripple current stays below a specified capacitor CSYNC depends on parasitic capacitance on the maximum, the inductor value should be chosen according RT/SYNC pin and is typically in the range of 10pF to 22pF. to the following equation: ⎛ V ⎞ ⎛ V ⎞ V L = OUT ⎜ OUT IN ⎜ ⎟ LTC3616 f ⎟ • ⎜1– ⎟ ⎝ V SVIN fOSC ⎝ SW • ∆IL(MAX) ⎠ IN ⎠ RT/SYNC 2.25MHz The inductor value will also have an effect on Burst Mode V operation. The transition to low current operation begins IN LTC3616 when the peak inductor current falls below a level set by the SVIN 0.4V f RT/SYNC OSC ∝1/RT burst clamp. Lower inductor values result in higher ripple RT SGND current which causes this to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower induc- VIN LTC3616 tance values will cause the burst frequency to increase. SVIN fOSC RT/SYNC 1/TP SGND
Inductor Core Selection
1.2V 0.3V Once the value for L is known, the type of inductor must be TP selected. Actual core loss is independent of core size for V fixed inductor value, but it is very dependent on the induc- IN LTC3616 CSYNC SV tance selected. As the inductance increases, core losses de- IN fOSC RT/SYNC 1/TP crease. Unfortunately, increased inductance requires more SGND turns of wire and therefore, copper losses will increase. RT 3616 F02
Figure 2. Setting the Switching Frequency
3616fc 14 For more information www.linear.com/LTC3616 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts