Datasheet LTC3417A (Analog Devices) - 9

LTC3417A
applicaTions inForMaTion
A general LTC3417A application circuit is shown in The maximum operating frequency is also constrained Figure 4. External component selection is driven by the by the minimum on-time and duty cycle. This can be load requirement, and begins with the selection of the calculated as: inductors L1 and L2. Once L1 and L2 are chosen, CIN, C ⎛ V ⎞ OUT1 and COUT2 can be selected. f OUT O(MAX) ≈ 6.67⎜⎜ ⎟ ( ) V ⎟ MHz ⎝ IN(MAX)
Operating Frequency
⎠ Selection of the operating frequency is a tradeoff between The minimum frequency is limited by leakage and noise efficiency and component size. High frequency operation coupling due to the large resistance of RT. allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by
Inductor Selection
reducing internal gate charge losses but requires larger Although the inductor does not influence the operating inductance values and/or capacitance to maintain low frequency, the inductor value has a direct effect on ripple output ripple voltage. current. The inductor ripple current, ∆IL, decreases with The operating frequency, f higher inductance and increases with higher V O, of the LTC3417A is determined IN or by pulling the FREQ pin to V V IN for 1.5MHz operation or OUT. by connecting an external resistor from FREQ to ground. V ⎛ V ⎞ The value of the resistor sets the ramp current that is ∆I OUT OUT L = ⎜1– f ⎟ ⎝ V used to charge and discharge an internal timing capacitor O • L IN ⎠ within the oscillator and can be calculated by using the Accepting larger values of ∆I following equation: L allows the use of low induc- tances, but results in higher output voltage ripple, greater core losses and lower output current capability. R 1.61• 1011 T ≈ (Ω) – 16.586kΩ f A reasonable starting point for setting ripple current is O ∆IL = 0.35ILOAD(MAX), where ILOAD(MAX) is the maximum for 0.6MHz ≤ fO ≤ 4MHz. Alternatively, use Figure 1 to current output. The largest ripple, ∆IL, occurs at the maxi- select the value for RT. mum input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value 160 should be chosen according to the following equation: 140 V ⎛ V ⎞ 120 L = OUT ⎜1– OUT ⎟ f ⎜ V ⎟ 100 O • ∆IL ⎝ IN(MAX) ⎠ (kΩ) 80 R T The inductor value will also have an effect on Burst Mode 60 operation. The transition from low current operation begins 40 when the peak inductor current falls below a level set by the 20 burst clamp. Lower inductor values result in higher ripple 0 current which causes this to occur at lower load currents. 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 This causes a dip in efficiency in the upper range of low FREQUENCY (MHz) 3417 F01 current operation. In Burst Mode operation, lower inductor values will cause the burst frequency to increase.
Figure 1. Frequency vs RT
3417afc 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Package Description Revision History Related Parts