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PI3546-00-LGIZ Datasheet(PDF) 30 Page - Vicor Corporation |
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PI3546-00-LGIZ Datasheet(HTML) 30 Page - Vicor Corporation |
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30 / 37 page  Figure 62 is the Bode plot of the Gled(s) transfer function, which in LGH mode is what needs to be compensated for by the LGH amplifier and compensator. This transfer function defines the gain and phase from the error amplifier output (EAO) to the current shunt Rshunt. Figure 65 is a plot of the transfer function Glgheao(s), which defines the gain and phase from the LGH pin (voltage across current sensing Rshunt) to EAO. As shown in Equation (13), the output is dependent on the integrator stage and the following trans- conductance stage. Figures 63 and 64 show the two individual sections that make up Equation (13) which produces Glgheao(s). The GMlgh(s) plot is from integrator to EAO with the voltage loop open and sourcing 400uA of current. When combining Figure 63 with Figure 64, it becomes clear that additional compensation is needed to have enough phase and gain margin like can be seen with the voltage loop plot. We can remedy that easily, by adding a series R-C in parallel with Rlgh as shown in the lighting application diagram in Figure 59. The capacitor will be chosen to work with Rlgh to add a zero approximately 1.2 kHz before the zero provided by the GMlgh(s) transfer function (the trans- conductance stage of the LGH amplifier). This value will be chosen to be 270 pF. The external added resistor will form a high frequency pole to roll the gain off at higher frequency. This pole will be set at approximately 120 kHz so a common 4.99 k resistor will be used. The resulting Bode plot with the new compensator of Glgheao(s) can be seen in Figure 66. Figure 67 shows the final Bode plot of the loop gain when using a lighting application with LED’s operating in constant current mode. Note that it is very important to understand the AC resistance of the LED’s that are being used. Please consult the LED manufacturer for details. For a series string, you should add the individual LED resistances and combine them into one lumped value to simplify the analysis. Cool-Power® Rev 1.3 vicorpower.com Page 30 of 37 2/2016 800 927.9474 PI354x-00 Figure 65 — Glgheao(s) Gain/Phase Plot Rlgh = 100k 1 10 100 1000 10000 100000 1000000 50 0 50 100 150 150 100 50 0 Gain - dBV Phase-Degrees Frequency- Hz Figure 63 — Eint(s) Gain/Phase Plot Rlgh = 100k 1 10 100 1000 10000 100000 40 20 0 20 40 60 80 100 80 60 40 20 0 Gain - dBV Phase-Degrees Frequency- Hz Figure 64 — GMlgh(s) Gain/Phase Plot Voltage Loop Open 1 10 100 1000 10000 100000 1000000 0 20 40 60 80 100 80 60 40 20 0 Gain - dBV Phase-Degrees Frequency- Hz Figure 62 — Gled(s) Gain/Phase Plot 1 10 100 1000 10000 100000 80 60 40 20 0 100 80 60 40 20 0 Gain - dBV Phase-Degrees Frequency- Hz |
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