Datasheet LT1940, LT1940L (Analog Devices) - 10

LT1940/LT1940L
U U W U APPLICATIO S I FOR ATIO
is likely to see high surge currents when the input source where ∆IL is the peak-to-peak ripple current in the induc- is applied, tantalum capacitors should be surge rated. The tor. The RMS content of this ripple is very low, and the manufacturer may also recommend operation below the RMS current rating of the output capacitor is usually not rated voltage of the capacitor. Be sure to place the 1µF of concern. ceramic as close as possible to the VIN and GND pins on Another constraint on the output capacitor is that it must the IC for optimal noise immunity. have greater energy storage than the inductor; if the stored A final caution is in order regarding the use of ceramic energy in the inductor is transferred to the output, you capacitors at the input. A ceramic input capacitor can would like the resulting voltage step to be small compared combine with stray inductance to form a resonant tank to the regulation voltage. For a 5% overshoot, this require- circuit. If power is applied quickly (for example by plug- ment becomes COUT > 10L(ILIM/VOUT)^2. ging the circuit into a live power source) this tank can ring, Finally, there must be enough capacitance for good tran- doubling the input voltage and damaging the LT1940. The sient performance. The last equation gives a good starting solution is to either clamp the input voltage or dampen the point. Alternatively, you can start with one of the designs tank circuit by adding a lossy capacitor in parallel with the in this data sheet and experiment to get the desired ceramic capacitor. For details, see AN88. performance. This topic is covered more thoroughly in the section on loop compensation.
Output Capacitor Selection
The high performance (low ESR), small size and robust- For 5V and 3.3V outputs with greater than 1A output, a ness of ceramic capacitors make them the preferred type 10µF 6.3V ceramic capacitor (X5R or X7R) at the output for LT1940 applications. However, all ceramic capacitors results in very low output voltage ripple and good transient are not the same. As mentioned above, many of the higher response. For lower voltages, 10µF is adequate but in- value capacitors use poor dielectrics with high tempera- creasing COUT to 15µF or 22µF will improve transient ture and voltage coefficients. In particular, Y5V and Z5U performance. Other types and values can be used; the types lose a large fraction of their capacitance with applied following discusses tradeoffs in output ripple and tran- voltage and temperature extremes. Because the loop sient performance. stability and transient response depend on the value of The output capacitor filters the inductor current to gener- COUT, you may not be able to tolerate this loss. Use X7R ate an output with low voltage ripple. It also stores energy and X5R types. in order satisfy transient loads and to stabilize the LT1940’s You can also use electrolytic capacitors. The ESRs of most control loop. Because the LT1940 operates at a high aluminum electrolytics are too large to deliver low output frequency, you don’t need much output capacitance. Also, ripple. Tantalum and newer, lower ESR organic electro- the current mode control loop doesn’t require the pres- lytic capacitors intended for power supply use are suit- ence of output capacitor series resistance (ESR). For these able, and the manufacturers will specify the ESR. The reasons, you are free to use ceramic capacitors to achieve choice of capacitor value will be based on the ESR required very low output ripple and small circuit size. for low ripple. Because the volume of the capacitor deter- Estimate output ripple with the following equations: mines its ESR, both the size and the value will be larger V than a ceramic capacitor that would give you similar ripple RIPPLE = ∆IL/(8f COUT) for ceramic capacitors, and performance. One benefit is that the larger capacitance VRIPPLE = ∆IL ESR for electrolytic capacitors (tantalum may give better transient response for large changes in and aluminum); load current. Table 2 lists several capacitor vendors. 1940fa 10