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LT1936EMS8E-TR Datasheet(PDF) 10 Page - Linear Technology |
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LT1936EMS8E-TR Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LT1936 10 1936fd APPLICATIONS INFORMATION This is the minimum output capacitance required, not the nominal capacitor value. For example, a 3.3V output requires 20μF of output capacitance. If a small 22μF, 6.3V ceramic capacitor is used, the circuit may be unstable be- cause the effective capacitance is lower than the nominal capacitance when biased at 3.3V. Look carefully at the capacitor’s data sheet to find out what the actual capaci- tance is under operating conditions (applied voltage and temperature). A physically larger capacitor, or one with a higher voltage rating, may be required. High performance electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier, and should be 0.05Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors. Frequency Compensation The LT1936 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT1936 does not require the ESR of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency compensation is provided by the components tied to the VC pin, as shown in Figure 1. Generally a capaci- tor (CC) and a resistor (RC) in series to ground are used. In addition, there may be lower value capacitor in parallel. This capacitor (CF) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR. An alternative to using external compensation components is to use the internal RC network by tying the COMP pin to the VC pin. This re- duces component count but does not provide the optimum transient response when the output capacitor value is high, and the circuit may not be stable when the output capacitor value is low. If the internal compensation network is not used, tie COMP to ground or leave it floating. Loop compensation determines the stability and transient performance. Designing the compensation network is a bit Figure 1. Model for Loop Response Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMENTS Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP Murata (404) 436-1300 www.murata.com Ceramic AVX www.avxcorp.com Ceramic, Tantalum TPS Series Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic 1.2V SW VC COMP GND 50k 600k 150pF LT1936 1936 F01 R1 OUTPUT ESR CF CC RC ERROR AMPLIFIER FB R2 C1 C1 CURRENT MODE POWER STAGE gm = 2mho gm = 250μmho + POLYMER OR TANTALUM CERAMIC CPL |
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