PI3545-00-LGIZ Datasheet, PDF (30/37 Pages) Vicor Corporation – 36 Vin to 60 Vin Cool-Power ZVS Buck Regulator & LED Driver

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Figure 62 — Gled(s) Gain/Phase Plot
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).
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Figure 63 — Eint(s) Gain/Phase Plot Rlgh = 100k
Cool-Power®
Page 30 of 37
Rev 1.3
2/2016
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Figure 64 — GMlgh(s) Gain/Phase Plot Voltage Loop Open
The GMlgh(s) plot is from integrator to EAO with the voltage
loop open and sourcing 400uA of current.
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Figure 65 — Glgheao(s) Gain/Phase Plot Rlgh = 100k
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.
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