Datasheet LT3580 (Analog Devices) - 10

LT3580
APPLICATIONS INFORMATION
these materials retain their capacitance over wider voltage
Compensation—Adjustment
and temperature ranges. A 4.7μF to 20μF output capaci- To compensate the feedback loop of the LT3580, a series tor is sufficient for most applications, but systems with resistor-capacitor network in parallel with a single capacitor very low output currents may need only a 1μF or 2.2μF should be connected from the VC pin to GND. For most output capacitor. Always use a capacitor with a sufficient applications, the series capacitor should be in the range voltage rating. Many capacitors rated at 2.2μF to 20μF, of 470pF to 2.2nF with 1nF being a good starting value. particularly 0805 or 0603 case sizes, have greatly reduced The parallel capacitor should range in value from 10pF to capacitance at the desired output voltage. Solid tantalum 100pF with 47pF a good starting value. The compensation or OS-CON capacitors can be used, but they will occupy resistor, R more board area than a ceramic and will have a higher C, is usually in the range of 5k to 50k. A good technique to compensate a new application is to use a ESR with greater output ripple. 100kΩ potentiometer in place of series resistor RC. With Ceramic capacitors also make a good choice for the input the series capacitor and parallel capacitor at 1nF and 47pF decoupling capacitor, which should be placed as closely as respectively, adjust the potentiometer while observing the possible to the LT3580. A 2.2μF to 4.7μF input capacitor transient response and the optimum value for RC can be is sufficient for most applications. found. Figures 3a to 3c illustrate this process for the circuit Table 2 shows a list of several ceramic capacitor manufac- of Figure 14 with a load current stepped between 400mA turers. Consult the manufacturers for detailed information and 500mA. Figure 3a shows the transient response with on their entire selection of ceramic parts. RC equal to 1k. The phase margin is poor, as evidenced by the excessive ringing in the output voltage and inductor
Table 2. Ceramic Capacitor Manufacturers
current. In Figure 3b, the value of RC is increased to 3k, Kemet www.kemet.com which results in a more damped response. Figure 3c Murata www.murata.com shows the results when RC is increased further to 10k. The Taiyo Yuden www.t-yuden.com transient response is nicely damped and the compensation procedure is complete. VOUT VOUT 200mV/DIV 200mV/DIV AC COUPLED AC COUPLED IL IL 0.5A/DIV 0.5A/DIV R 200μs/DIV C = 1k 3580 F03a R 200μs/DIV C = 3k 3580 F03b
Figure 3a. Transient Response Shows Excessive Ringing Figure 3b. Transient Response Is Better
VOUT 200mV/DIV AC COUPLED IL 0.5A/DIV R 200μs/DIV C = 10k 3580 F03c
Figure 3c. Transient Response Is Well Damped
3580fg 10