Compatible with the ModuMate interconnect system for socketing and surface mounting
Not intended for socket or Surfmate mounting
MicroRAM™
Page 1 of 19
Rev 2.4
09/2018
MicroRAM™
Electrical Characteristics
Electrical characteristics apply over the full operating range of input voltage, output power and baseplate temperature, unless otherwise specified.
All temperatures refer to the operating temperature at the center of the baseplate.
µRAM MODULE SPECIFICATIONS
(–20 to +100°C baseplate temperature)
Parameter
Operating current range
μRAM2XXX
Operating current range
μRAM3XXX
Operating input voltage
Transient output response
Load current step < 1A/μsec
Transient output response
Load current step < 1A/μsec
(C
TRAN
= 820μF)
Recommended headroom voltage
range (V
HR
) @ 1A load
[c]
Output ripple
Input V
P-P
= 100mV
Output ripple
Input V
P-P
= 500mV
SC output voltage
[d]
OR’ing threshold
μRAM bias current
Power dissipation
μRAM2XXX V
HR
= 380mV @ 1A
Power dissipation
μμRAM3XXX V
HR
= 380mV @ 1A
[c]
Min
0.02
0.02
3.0
Typ
Max
20
30
30
50
50
Unit
A
A
V
mV
P-P
mV
P-P
Notes
No internal current limiting. Converter input must be properly fused
such that the μRAM output current does not exceed the maximum
operating current rating by more than 30% under a steady state
condition.
Continuous
Step load change; see Figures 20, 23, & 26, pp. 16 – 17
Optional capacitance C
TRAN
can be used to increase transient current
capability; See Figures 21, 24 & 27, pp. 16 – 17
See Figures 4 and 5, p. 5 for detailed explanation. See Table 1 for
typical headroom setting resistor values.
Ripple frequency of 60Hz to 100kHz; optional CHR capacitor of
100μF required to increase low frequency attenuation as shown in
Figure 2, p. 3
Ripple frequency of 100kHz to 2MHz; as shown in Figure 2, p. 3
See table 1 for typical R
SC
values, note 2 for calculation.
V
IN
– V
OUT
325
425
10
5
10
5
mV
mV
P-P
MV
RMS
mV
P-P
MV
RMS
V
DC
1.23
–10
60
7.5
11.5
mV
mA
W
W
V
IN
= 28V; I
OUT
= 20A
V
IN
= 28V; I
OUT
= 30A
The headroom voltage V
HR
is the voltage difference between the V
IN+
and the V
OUT+
pins of the µRAM.
R
HR
=
[d]
V
OUT+
V
HR
• 2.3k
(See Table 1 for example R
HR
values)
The SC resistor is used to trim the converter’s output voltage (Vnom) to compensate for the headroom voltage drop of the µRAM when remote sense is not
used. This feature can only be used with converter’s that have a trim reference range between 1.21 and 1.25V.
R
SC
=
(
V
NOM
• 1k
)
1.23V
– 2k
(See Table 1 for example R
SC
values)
µRAM Output Voltage
3V
5V
12V
15V
24V
28V
V
HR
@ 1A
375mV
375mV
375mV
375mV
375mV
375mV
R
HR
Value (Ω)
18.2k
30.9k
73.2k
90.9k
147.0k
174.0k
R
SC
Value (Ω)
442
2.05k
7.68k
10.20k
17.40k
21.00k
Table 1 —
Calculated values of R
SC
and R
HR
for a headroom voltage of 375mV.
Use notes 1 and 2 to compute R
SC
and R
HR
values for different headroom voltages.
MicroRAM™
Page 2 of 19
Rev 2.4
09/2018
MicroRAM™
MicroRAM Theory of Operation
PARD Attenuation
Vicor’s MicroRAM uses both active and passive filtering to
attenuate PARD (Periodic and Random Deviations), typically
associated with a DC to DC converter’s output voltage. The
passive filter provides effective attenuation in the 50kHz to 20MHz
range. The low frequency range of the passive filter (ie; resonant
frequency) can be lowered by adding capacitance to the CTRAN
pin to ground and will improve the transient load capability, as is
shown in Figure 7. The active filter provides attenuation from lower
frequencies up to 2MHz. The lower frequency range of the active
filter can be extended down by adding an external by-pass cap
across the R
HR
resistor.
V
IN+
CTRAN
57µF
2.3k
Passive
Active
V
HR
VDIODE
V
OUT+
V
IN-
9.4µF
V
REF
C
HR
(Optional)
R
HR
V
OUT-
Figure 1 —
Simplified MicroRAM block diagram
Figure 2 —
MicroRAM attenuation with and without an additional C
HR
capacitor
The plots in Figure 2 show the increase in attenuation range that can be realized by adding an additional capacitor, CHR, across the RHR resistor, as shown in
Figure 1. These plots represent the total attenuation, due to both the active and passive filtering, before and after adding an additional 100µF of capacitance
for C
HR
. There are practical limitations to the amount of capacitance that can be added, which is explained in more detail under the VREF section.
MicroRAM™
Page 3 of 19
Rev 2.4
09/2018
MicroRAM™
PARD Attenuation (Cont.)
Active attenuation is achieved by using power MOSFETs as a
variable resistor that can dynamically change its impedance in
order to maintain a constant output voltage, equal to the voltage
programmed on its reference pin. When the input is lower, the
active loop reduces the FET’s resistance, lowering the overall
voltage drop across the MicroRAM. When the input is higher,
the resistance is increased, increasing the voltage drop across the
MicroRAM. The bandwidth of the active loop must be sufficiently
higher than the converters control loop so it does not introduce
significant phase shift to the sense loop of the converter.
There are both upper and lower limits to the range of resistance
variations. The lower limit is based on the path resistance between
V
IN+
and V
OUT+
and the amount of current passing through the
MicroRAM. On the high end, the resistance of the FET, and
therefore the maximum voltage drop, is limited to the voltage
when the body diode of the FET starts to conduct and ripple
passes through it to the output, exhibiting positive peaks of
ripple at the load.
The waveforms in Figure 3 are representative of a typical ripple
signal, riding on a DC voltage. The headroom voltage across the
MicroRAM (V
HR
) is the difference in DC voltage between Vin and
Vout. This headroom is programmed via R
HR
, shown in Figure 1.
The headroom voltage should be selected such that the headroom
voltage minus half the peak to peak ripple does not cross the
minimum headroom limit, or that the headroom voltage plus
half the peak to peak ripple does not exceed the voltage drop of
the FET’s intrinsic body diode voltage drop, that is current and
temperature dependent. The headroom must be properly set
below the point of diode conduction. In either of these two cases
if the headroom is depleted or the diode conducts, the ripple at
the CTRAN node will be exhibited as peaks of the ripple voltage
amplitude at the load, negating the active attenuation.
If the fundamental switching frequency of the converter is above
the resonant frequency of the passive LC filter (see Figure 8) the
fundamental switching and harmonic frequencies will be reduced
at the rate of 40dB per decade in frequency. The active filter will be
presented with lower peak to peak ripple and will have sufficient
dynamic range to attenuate the ripple. If the fundamental is below
the resonant frequency of the LC filter, then the active circuit will
attenuate the full noise signal.
The plot in Figure 4 illustrates the “effective” headroom voltage
over the full operating current range of the MicroRAM. The
reduction in headroom voltage, seen across the MicroRAM over
the full 30A load current range, is due to two factors; the effects
of the slope adjust and the insertion resistance of the MicroRAM.
The two green shaded areas represent the minimum and maximum
recommended headroom voltages listed in the MicroRAM’s
specification table. The gray area is the voltage drop due to the
MicroRAM’s insertion resistance, from the positive input to the
positive output, of the MicroRAM, multiplied by the load current.
This insertion resistance is typically 5mΩ at 25°C and can increase
to 6.5mΩ at 100°C.
V
IN
+ VDIODE
V
IN
[p-p]
V
IN
[DC]
V
HR1
V
IN
+ ( I
IN
* R
µRAM
)
V
OUT
V
HR2
Figure 3 —
Active attenuation and the effects of headroom
MicroRAM™
Page 4 of 19
Rev 2.4
09/2018
MicroRAM™
0.45
0.40
0.35
Headroom Voltage
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
30
Load Current
V
HR
, 30A, Min HR
V
HR
, 30A, Max HR
Insertion Loss
Figure 4 —
MicroRAM headroom voltage reduction over full load current range.
As the load current is increased, the internal slope adjust of the
MicroRAM will reduce the headroom voltage across the MicroRAM
at a rate of about 2mV/A for the 30A version (4mV/A for 20A
version) in an effort to reduce the power loss across the MicroRAM.
This headroom reduction, in conjunction with the increased voltage
drop across the MicroRAM due to its resistance, reduces the
effective headroom voltage and therefore the MicroRAM’s ability
FPGA/CPLD
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