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LT3506EFE-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LT3506EFE-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LT3506/LT3506A 0 3506afb tor must have low impedance at the switching frequency to do this effectively, and it must have an adequate ripple current rating. With two switchers operating at the same frequency but with different phases and duty cycles, cal- culating the input capacitor RMS current is not simple. However, a conservative value is the RMS input current for the channel that is delivering most power (VOUT • IOUT). This is given by: I I V V V V I INRMS OUT OUT IN OUT IN OUT = − ( ) < • 2 and is largest when VIN = 2VOUT (50% duty cycle). As the second, lower power channel draws input current, the input capacitor’s RMS current actually decreases as the out-of-phase current cancels the current drawn by the higher power channel. Considering that the maximum load current from a single channel is ~1.6A, RMS ripple current will always be less than 0.8A. The high frequency of the LT3506 reduces the energy storage requirements of the input capacitor, so that the capacitance required is less than 22μF (less than 10μF for the LT3506A). The combination of small size and low impedance (low equivalent series resistance or ESR) of ceramic capacitors makes them the preferred choice. The low ESR results in very low voltage ripple and the capacitorscanhandleplentyofripplecurrent.Theyarealso comparatively robust and can be used in this application at their rated voltage. X5R and X7R types are stable over temperature and applied voltage, and give dependable service. Other types (Y5V and Z5U) have very large tem- perature and voltage coefficients of capacitance, so they mayhaveonlyasmallfractionoftheirnominalcapacitance in your application. While they will still handle the RMS ripple current, the input voltage ripple may become fairly large, and the ripple current may end up flowing from your input supply or from other bypass capacitors in your system, as opposed to being fully sourced from the local input capacitor. An alternative to a high value ceramic capacitor is a lower value along with a larger electrolytic capacitor, for example a 1μF ceramic capacitor in parallel with a low ESR tantalum capacitor. For the electrolytic capacitor, a value larger than 22 mF (10mF for the LT3506A) will be required to meet the ESR and ripple current requirements. Because the input capacitorislikelytoseehighsurgecurrentswhentheinput source is applied, tantalum capacitors should be surge rated. The manufacturer may also recommend operation below the rated voltage of the capacitor. Be sure to place the 1μF ceramic as close as possible to the VIN and GND pins on the IC for optimal noise immunity. A final caution is in order regarding the use of ceramic capacitors at the input. A ceramic input capacitor can combine with stray inductance to form a resonant tank circuit. If power is applied quickly (for example by plug- ging the circuit into a live power source) this tank can ring, doubling the input voltage and damaging the LT3506. The solution is to either clamp the input voltage or dampen the tank circuit by adding a lossy capacitor in parallel with the ceramic capacitor. For details, see Application Note 88. Output Capacitor Selection The output capacitor filters the inductor current to gen- erate an output with low voltage ripple. It also stores energy in order satisfy transient loads and to stabilize the LT3506’s control loop. Because the LT3506 operates at a high frequency, you don’t need much output capacitance. Also, the current mode control loop doesn’t require the presence of output capacitor series resistance (ESR). For these reasons, you are free to use ceramic capacitors to achieve very low output ripple and small circuit size. Estimate output ripple with the following equations: VRIPPLE = ΔIL/(8 • f • COUT) for ceramic capacitors, and VRIPPLE = ΔIL • ESR for electrolytic capacitors (tantalum and aluminum); where ΔIL is the peak-to-peak ripple current in the induc- tor. The RMS content of this ripple is very low, and the RMS current rating of the output capacitor is usually not of concern. Another constraint on the output capacitor is that it must have greater energy storage than the induc- tor; if the stored energy in the inductor is transferred to the output, you would like the resulting voltage step to be small compared to the regulation volt- age. For a 5% overshoot, this requirement becomes COUT > 10L(ILIM/VOUT)2. APPLICATIO S I FOR ATIO |
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