PI3542-00-LGIZ Datasheet, PDF (27/37 Pages) Vicor Corporation – 36 Vin to 60 Vin Cool-Power ZVS Buck Regulator & LED Driver

discontinuous and continuous modes is reached, the
powertrain changes frequency to remain in critical
conduction mode. This mode of operation allows the
PI354x-00 product family to have a very simple
compensation scheme, as the control to output transfer
function always has a slope of -1. In addition, when critical
conduction is reached, the voltage controlled current source
becomes nearly ideal with a high output
equivalent resistance.
gMOD
Vout
rEQ
Cout
Rload
Veao
Figure 54 — PI354X Small Signal Model Control-Output
The control to output transfer function of the PI354x-00
product family is defined as the gain from the output of the
error amplifier , through the modulator and to the output
voltage. The transfer function equation is shown in
Equation (3), where gMOD is assumed to be 7S, rEQ = 0.4
Ohms, C_OUT = 600 μF and Rload = 1 Ohm:
gMOD
Gco(s) =
(3)
1
1
+
+ s (C )
Rload rEQ
OUT
The Control-Output transfer function (also known as the
small signal modulator gain) has a single pole response
determined by the parallel combination of Rload and rEQ
and the output capacitor Cout. Equation (4) determines the
frequency of the modulator pole:
1
Fpmod =
(4)
Rload • rEQ
2 • π • _{Rload + rEQ} • C_OUT
Figure 55 depicts the small signal response of the modulator
when perturbing EAO and measuring the differential gain
and phase from EAO to V_OUT.
Cool-Power^®
Page 27 of 37
Rev 1.3
02/2016
PI354x-00
20
0
20
40
60
1
0
Gain - dBV
Phase-Degrees
20
40
60
80
100
10
100
1000
10000
100000
Frequency- Hz
Figure 55 — PI354X Control-Output Gain/Phase Example
Error Amplifier
The small signal model of the error amplifier and
compensator is shown in Figure 56. The error amplifier is a
trans-conductance amplifier (TCA). The transfer function is
shown in Equation (5), where in this example R1 = 2.3k, R2 =
1k, GMeao = 5.1mS, R_OUT = 1Meg, Chf = 56 pF, Ccomp = 4.7 nF
and Rzi = 5 k. Here it is important to note that the external
components are Ccomp, R1 and R2. The other components
are internal to each specific model. See the data tables
section “Soft Start, Tracking And Error Amplifier” for details.
Vout
GMeao
R1
Rzi
Chf
Rout
Ccomp
Veao
R2
Figure 56 — PI354X Error Amplifier Model
80
0
Gain - dBV
Phase-Degrees
20
60
40
60
40
80
20
100
1
10
100
1000
10000
100000
1000000
Frequency- Hz
Figure 57 — PI354X Input-Control Gain/Phase
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