Datasheet LT1946A (Linear Technology) - 7

LT1946A
U U W U APPLICATIO S I FOR ATIO
capacitor is sufficient for most applications, but systems with very low output currents may need only a 1µF or smaller output capacitor. Solid tantalum or OSCON ca- VOUT 200mV/DIV pacitors can be used, but they will occupy more board area AC COUPLED than a ceramic and will have a higher ESR. Always use a capacitor with a sufficient voltage rating. I Ceramic capacitors also make a good choice for the input L1 0.5A/DIV decoupling capacitor, which should be placed as close as possible to the LT1946A. A 2.2µF to 4.7µF input capacitor R is sufficient for most applications. Table 2 shows a list of C = 2.5k 1946A F03a 50µs/DIV several ceramic capacitor manufacturers. Consult the
Figure 3a. Transient Response Shows Excessive Ringing
manufacturers for detailed information on their entire selection of ceramic parts.
Table 2. Ceramic Capacitor Manufacturers
VOUT 200mV/DIV Taiyo Yuden (408) 573-4150 www.t-yuden.com AC COUPLED AVX (803) 448-9411 www.avxcorp.com Murata (714) 852-2001 www.murata.com IL1 0.5A/DIV
Compensation
To compensate the feedback loop of the LT1946A, a series RC = 6.5k 1946A F03b 50µs/DIV resistor-capacitor network should be connected from the
Figure 3b. Transient Response is Better
COMP pin to GND. For most applications, a capacitor in the range of 90pF to 470pF will suffice. A good starting value for the compensation capacitor, CC, is 270pF. The com- pensation resistor, RC, is usually in the range of 20k to VOUT 200mV/DIV 100k. A good technique to compensate a new application AC COUPLED is to use a 100k potentiometer in place of RC, and use a 270pF capacitor for CC. By adjusting the potentiometer while observing the transient response, the optimum IL1 value for R 0.5A/DIV C can be found. Figures 3a-3c illustrate this process for the circuit of Figure 1. Figure 3a shows the transient response with RC equal to 2.5k. The phase RC = 27.4k 1946A F03c 50µs/DIV margin is poor as evidenced by the excessive ringing in the
Figure 3c. Transient Response is Well Damped
output voltage and inductor current. In Figure 3b the value of RC is increased to 6.5k, which results in a more damped
Compensation-Theory
response. Figure 3c shows the results when RC is in- Like all other current mode switching regulators, the creased further to 27.4k. The transient response is nicely LT1946A needs to be compensated for stable and efficient damped and the compensation procedure is complete. The COMP pin provides access to an internal resistor operation. Two feedback loops are used in the LT1946A: a fast current loop which does not require compensation, (120k) and capacitor (90pF). For some applications, these and a slower voltage loop which does. Standard bode plot values will suffice and no external RC and CC will be needed. analysis can be used to understand and adjust the voltage feedback loop. sn1946a 1946afs 7