Technical Specification
Non-Isolated
SIP Converter
3.0 - 3.6V
15A
in
half of the trim-down allowance (e.g., further than -2.5% on a
0.9V, or -5% on a 1.2V).
denly. This can be caused by either a shutdown of the NiQor
from a fault or from the load itself, for example when a card is
hot-swapped out, suddenly dropping the load to zero. This is
further justification for keeping the source inductance low, as
mentioned above. When the power source is configured with
remote sensing, the series resistance of the filter inductor and
any other conductors or devices between the source and the
sense point will result in a voltage drop which, in the event of
a load current interruption, would add to the NiQor input volt-
age.
If no inductor is used to isolate the input ripple of the NiQor
converters from the source or from inputs of other NiQor con-
verters, then this external capacitance might be provided by the
DC/DC converter used as the power source. SynQor's
PowerQor series converters typically have tantalum and ceram-
ic output capacitors that would provide the damping.
An input inductor would help isolate the ripple currents and
voltages from the source or other NiQor style converters on the
voltage supply rail. If an input inductor is used, the recom-
mended capacitance should guarantee stability and control the
ripple current for up to 1.0µH of input inductance.
A TVS device could also be used to clamp the voltage level dur-
ing these conditions, but the relatively narrow range between
operating voltage and the absolute maximum voltage restrict
the use of these devices to lower source current levels that will
not drive the transient voltage suppressor above the voltage
limit when all the source current is flowing into the clamp. A
TVS would be a good supplemental control, in addition to care-
ful selection of inductance and capacitance values.
The input inductor need not have very high inductance.
A
value of 500 nanohenries would equate to almost one ohm of
series impedance at the switching frequency of 300 kHz. This
would be working against an assumed capacitive ESR of 30mΩ
on the supply side of the inductor, providing significant isola-
tion and ripple reduction.
Equivalent Model for Input Ripple: A simple but reason-
ably accurate model of input ripple is to treat the NiQor input
as a pulsed AC current source at 300 kHz.in parallel with a
very low ESR capacitor, see Figure B. The peak-to-peak current
of the source model is equal to the NiQor load current, repre-
senting the peak current in the NiQor's smoothing choke. The
capacitor represents the 40µF input ceramic capacitance of the
NiQor converter, with a nearly negligible ESR of less than 1
mΩ. A further refinement can be made by setting the duty cycle
of the pulsed source to the output voltage divided by the input
voltage.
No external capacitance is required at the output, however, the
ripple voltage can be further reduced if ceramic and tantalum
capacitors are added at the output. Since the internal output
capacitance is about 50µF, approximately that amount of
capacitance would be needed to produce a noticeable reduc-
tion in output ripple. The value of the tantalum capacitors is
both to provide a high capacitance for pulsed loads and to pro-
vide damping of the distribution network with their inherent
ESR, which is low, but higher than ceramics. Additional output
capacitance in the range of 300-500µF is beneficial for reduc-
ing the deviation in response to a fast load transient.
The only error in this simplified model is that it ignores the
inductive current in the choke, usually less than 20% of the load
current, and it ignores the resistive losses inside the NiQor con-
verter, which would alter the duty cycle very slightly.
Input Over-Voltage Prevention: The power system
designer must take precautions to prevent damaging the NiQor
converters by input overvoltage. This is another reason to be
careful about damping the input filter so that no ringing occurs
from an underdamped filter. The voltage must be prevented
from exceeding the absolute maximum voltage indicated in the
Electrical Specifications section of the data sheet under all con-
ditions of turn-on, turn-off and load transients and fault condi-
tions. The power source should have an over voltage shutdown
threshold as close as reasonably possible to the operating
point.
The model is a good guide for calculating the effects of exter-
nal input capacitors and other filter elements on ripple voltage
and ripple current stress on capacitors.
40
µF
INPUT
I
<1mΩ
p-p
Additional protection can come from additional input capaci-
tance, perhaps on the order of 1,000µF, but contingent on the
I
= I
Load
p-p
source inductance value. A large source inductance would
require more capacitance to keep the input voltage below the
absolute maximum, if the load current were interrupted sud-
Figure B: Equivalent model for input ripple
Product # NQ03xxxVMA15
Phone 1-888-567-9596
Doc.# 005-2NV3xxE Rev. E
6/24/04
Page 15
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