Datasheet LT3506, LT3506A (Analog Devices) - 10

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