PI3542-00-LGIZ Datasheet, PDF (31/37 Pages) Vicor Corporation – 36 Vin to 60 Vin Cool-Power ZVS Buck Regulator & LED Driver

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Figure 66 — GMlgh(s) Gain/Phase Plot Compensated
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Figure 67 — Lighting Application Loop Gain/Phase Plot
Filter Considerations
The PI354x-00 requires low impedance ceramic input
capacitors (X7R/X5R or equivalent) to ensure proper start up
and high frequency decoupling for the power stage. The
PI354x-00 will draw nearly all of the high frequency current
from the low impedance ceramic capacitors when the main
high side MOSFET(s) are conducting. During the time the
MOSFET(s) are off, the input capacitors are replenished from
the source. Table 4 shows the recommended input and
output capacitors to be used for the PI354x-00 as well as per
capacitor RMS ripple current and the input and output ripple
voltages. Table 5 includes the recommended input and
output ceramic capacitors.
It is very important to verify that the voltage supply source
as well as the interconnecting lines are stable and do
not oscillate.
PI354x-00
Input Filter Case 1; Inductive source and local, external,
input decoupling capacitance with negligible ESR (i.e.:
ceramic type):
The voltage source impedance can be modeled as a series
R(line) L(line) circuit. The high performance ceramic
decoupling capacitors will not significantly damp the
network because of their low ESR; therefore in order to
guarantee stability the following conditions must
be verified:
L_line
R_line > _{(Cin + Cin ) • |r_EQin|}
(16)
int
ext
R << |r_EQin|
line
(17)
Where r_EQin can be calculated by dividing the lowest line
voltage by the full load input current. It is critical that the
line source impedance be at least an octave lower than the
converter’s dynamic input resistance, Equation (17).
However, R_line cannot be made arbitrarily low otherwise
Equation (16) is violated and the system will show instability,
due to an under-damped RLC input network.
Input Filter case 2; Inductive source and local, external
input decoupling capacitance with significant RCIN_EXT
ESR (i.e.: electrolytic type):
In order to simplify the analysis in this case, the voltage
source impedance can be modeled as a simple inductor Lline.
|r_EQin| > R_cinext
(18)
L
line
> |r_EQin|
(19)
(Cin • R )
int
cinext
Notice that, the high performance ceramic capacitors
CIN_INT within the PI354x-00 should be included in the
external electrolytic capacitance value for this purpose. The
stability criteria will be:
Equation (19) shows that if the aggregate ESR is too small –
for example by using very high quality input capacitors
(C_{IN_EXT}) – the system will be under-damped and may even
become destabilized. As noted, an octave of design margin in
satisfying Equation (18) should be considered the minimum.
When applying an electrolytic capacitor for input filter
damping the ESR value must be chosen to avoid loss of
converter efficiency and excessive power dissipation in the
electrolytic capacitor.
Cool-Power^®
Page 31 of 37
Rev 1.3
02/2016
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