Datasheet LT1931, LT1931A (Analog Devices) - 6

LT1931/LT1931A
U U W U APPLICATIO S I FOR ATIO
The inductors shown in Table 2 for use with the LT1931A zero to the system. For the tantalum and OS-CON capaci- were chosen for their small size. For better efficiency, use tors, this zero is located at a lower frequency due to the similar valued inductors with a larger volume. For in- higher value of the ESR, while the zero of a ceramic stance, the Sumida CR43 series, in values ranging from capacitor is at a much higher frequency and can generally 3.3µH to 10µH, will give a LT1931A application a few be ignored. percentage points increase in efficiency. A phase lead zero can be intentionally introduced by placing a capacitor (C4) in parallel with the resistor (R1)
CAPACITOR SELECTION
between VOUT and VNFB as shown in Figure 1. The Low ESR (equivalent series resistance) capacitors should frequency of the zero is determined by the following be used at the output to minimize the output ripple voltage. equation. Multilayer ceramic capacitors are an excellent choice, as 1 they have an extremely low ESR and are available in very ƒZ = small packages. X5R dielectrics are preferred, followed by 2π •R1•C4 X7R, as these materials retain their capacitance over wide By choosing the appropriate values for the resistor and voltage and temperature ranges. A 10µF to 22µF output capacitor, the zero frequency can be designed to improve capacitor is sufficient for most LT1931 applications while the phase margin of the overall converter. The typical a 4.7µF to 10µF capacitor will suffice for the LT1931A. target value for the zero frequency is between 20kHz to Solid tantalum or OS-CON capacitors can be used, but 60kHz. Figure 3 shows the transient response of the they will occupy more board area than a ceramic and will inverting converter from Figure 1 without the phase lead have a higher ESR. Always use a capacitor with a sufficient capacitor C4. The phase margin is reduced as evidenced voltage rating. by more ringing in both the output voltage and inductor Ceramic capacitors also make a good choice for the input current. A 220pF capacitor for C4 results in better phase decoupling capacitor, which should be placed as close as margin, which is revealed in Figure 4 as a more damped possible to the LT1931/LT1931A. A 1µF to 4.7µF input response and less overshoot. Figure 5 shows the transient capacitor is sufficient for most applications. Table 3 shows response when a 22µF tantalum capacitor with no phase a list of several ceramic capacitor manufacturers. Consult lead capacitor is used on the output. The higher output the manufacturers for detailed information on their entire voltage ripple is revealed in the upper waveform as a selection of ceramic parts. thicker line. The transient response is adequate which implies that the ESR zero is improving the phase margin.
Table 3. Ceramic Capacitor Manufacturers
Taiyo Yuden (408) 573-4150 www.t-yuden.com AVX (803) 448-9411 VOUT 20mV/DIV www.avxcorp.com AC COUPLED Murata (714) 852-2001 I www.murata.com L1A + IL1B 0.5A/DIV AC COUPLED LOAD 200mA The decision to use either low ESR (ceramic) capacitors or CURRENT 100mA the higher ESR (tantalum or OS-CON) capacitors can 100µs/DIV 1931 F03 effect the stability of the overall system. The ESR of any
Figure 3. Transient Response of Inverting Converter
capacitor, along with the capacitance itself, contributes a
Without Phase Lead Capacitor
1931fa 6