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NX2154 Datasheet(PDF) 8 Page - Microsemi Corporation |
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NX2154 Datasheet(HTML) 8 Page - Microsemi Corporation |
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8 / 17 page  NX2154/2154A 8 Rev.1.2 02/26/07 It should be considered that the proposed equa- tion is based on ideal case, in reality, the droop or over- shoot is typically more than the calculation. The equa- tion gives a good start. For more margin, more capaci- tors have to be chosen after the test. Typically, for high frequency capacitor such as high quality POSCAP es- pecially ceramic capacitor, 20% to 100% (for ceramic) more capacitors have to be chosen since the ESR of capacitors is so low that the PCB parasitic can affect the results tremendously. More capacitors have to be selected to compensate these parasitic parameters. Compensator Design Due to the double pole generated by LC filter of the power stage, the power system has 180o phase shift , and therefore, is unstable by itself. In order to achieve accurate output voltage and fast transient response,compensator is employed to provide highest possible bandwidth and enough phase margin.Ideally,the Bode plot of the closed loop system has crossover fre- quency between1/10 and 1/5 of the switching frequency, phase margin greater than 50o and the gain crossing 0dB with -20dB/decade. Power stage output capacitors usually decide the compensator type. If electrolytic capacitors are chosen as output capacitors, type II com- pensator can be used to compensate the system, be- cause the zero caused by output capacitor ESR is lower than crossover frequency. Otherwise type III compensa- tor should be chosen. A. Type III compensator design For low ESR output capacitors, typically such as Sanyo oscap and poscap, the frequency of ESR zero caused by output capacitors is higher than the cross- over frequency. In this case, it is necessary to compen- sate the system with type III compensator. The follow- ing figures and equations show how to realize the type III compensator by transconductance amplifier. Z1 42 Z2 2 33 P1 33 P2 12 4 12 1 F ...(11) 2 RC 1 F ...(12) 2 (R R )C 1 F ...(13) 2 RC 1 F ...(14) CC 2R CC = ×π ×× = ×π ×+× = ×π ×× = × ×π ×× + where FZ1,FZ2,FP1 and FP2 are poles and zeros in the compensator. Their locations are shown in figure 4. The transfer function of type III compensator for transconductance amplifier is given by: e mf OUT m in in1 V 1 gZ V 1 gZ Z /R −× = +×+ For the voltage amplifier, the transfer function of compensator is e f OUT in V Z VZ − = To achieve the same effect as voltage amplifier, the compensator of transconductance amplifier must satisfy this condition: R 4>>2/gm. And it would be desir- able if R 1||R2||R3>>1/gm can be met at the same time. Zin Zf Vout Vref Fb R2 R1 R3 R4 C3 C1 C2 Ve gm Figure 3 - Type III compensator using transconductance amplifier |
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