PI3545-00-LGIZ Datasheet, PDF (29/37 Pages) Vicor Corporation – 36 Vin to 60 Vin Cool-Power ZVS Buck Regulator & LED Driver

LGH Amplifier Small Signal Model
A small signal model of the LGH amplifier is shown in
Figure 60.
Vlgh
Rzi
Cint
Eint
GMlgh
ELS
400 u
leao
Chf
Rzi
Rout
Ccomp
Veao
PI354x-00
gMOD
Veao
Vout
RLED
rEQ
Cout
Rshunt
Vlgh
Figure 60 — LGH Amplifier Small Signal Model
Figure 61 — Lighting Application Modulator Gain Model
The LGH amplifier consists of three distinct stages. The first is
a wide bandwidth integrator stage, followed by a fixed gain
level shift circuit. Finally, the level shift circuit drives a trans-
conductance (TCA) amplifier with an open collector sink
only output stage. Since the LGH output is internally
connected to the output of the voltage error amplifier, the
compensation components show up in the model and are
used by both stages, depending on which one is in use. Only
one stage should be in use at a time. When using LGH or if
the LGH input rises above the internal reference, the voltage
error amplifier acts as a 400uA current source pull up for the
EAO pin.
Figure 61 shows a small signal model of the modulator gain
when using the application circuit shown in Figure 59 with
two 3.4 V high current LED’s in series. RLED is the series
combination of the AC resistance of each LED, which is 0.2
Ohms. Rshunt is used to sense the current through the LED
string. It has a value of 0.050 Ohms in this case. The other
component values were defined earlier and remain the same
values. Equation (11) defines the transfer function of the
modulator and Equation (12) defines the pole of transfer
function. The transfer function of the LGH amplifier is
defined in Equation (13). The open loop gain of Eint is 2500
and ELS = 4.4.
Gled(s) = gMOD • (rEQ • Rshunt)/((Rshunt + RLED + rEQ) + s (C • rEQ • RLED+Rshunt • rEQ • C ))
OUT
OUT
(11)
1
Fpled =
= 1.2 kHz
(12)
2 • π • ((RLED + Rshunt))//reQ) • C
OUT
Glgheao(s) = Eint (s) = • ELS • GMlgh •
R_OUT + s (Rzi • Ccomp • R_OUT)
1 + s • (Ccomp+Chf) + s² • (Chf • Ccomp • Rzi)
(13)
Chf + Ccomp
Fphf =
= 580 kHz (14)
2 • π • (Rzi // R ) • Ccomp • Chf
OUT
The integrator pole is determined by the external input resistor Rlgh and the internal Cint, which is 20 pF. Assuming Rlgh = 100 k
and Eint = 2500:
Cool-Power^®
Page 29 of 37
Rev 1.3
02/2016
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