*Pin styles S & N are compatible with the ModuMate interconnect system for socketing and surface mounting.
Output Ripple Attenuation Module
Page 1 of 19
Rev. 1.8
3/2013
vicorpower.com
800 735.6200
ELECTRICAL CHARACTERISTICS
µRAM
MODULE SPECIFICATIONS
(-20°C TO +100°C baseplate temperature)
Min
Typ
Max
20
30
30
50
Unit
A
A
V
mVp-p
MicroRAM
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.
Parameter
Notes
Operating current range
µRAM2XXX
µRAM3XXX
0.02
0.02
3.0
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 CTRAN can be used
to increase transient current capability; See Figures
21, 24 & 27, pp. 16–17
See Figures 4 and 5, pp. 5 for detailed explanation.
See Table 1 for typical headroom setting resistor values.
Ripple frequency of 60 Hz to 100 kHz; optional CHR
capacitor of 100µF required to increase low frequency
attenuation as shown in Figure 2, pp. 3
Ripple frequency of 100 kHz to 2 MHz;
as shown in Figure 2, pp. 3
See table 1 for typical RSC values, note 2 for calculation.
V
IN
– V
OUT
Operating input voltage
Transient output response
Load current step < 1 A/µsec
Transient output response
Load current step < 1 A/µsec
(CTRAN = 820
µF)
50
mVp-p
Recommended headroom voltage
range (VHR) @ 1A load.1
Output ripple
Input Vp-p = 100 mV
Output ripple
Input Vp-p = 500 mV
SC output voltage
OR’ing threshold
2
325
425
10
5
10
5
mV
mVp-p
MVrms
mVp-p
MVrms
Vdc
1.23
–10
60
7.5
11.5
Power dissipation
µRAM2XXX
VHR = 380 mV @ 1 A
µRAM
bias current
mV
mA
W
W
µRAM3XXX
VHR = 380 mV @ 1 A
V
IN
= 28 V; I
OUT
= 20 A
V
IN
= 28 V; I
OUT
= 30 A
1
The headroom voltage VHR is the voltage difference between the V
IN
+ and the V
OUT
+ pins of the µRAM.
RHR =
V
OUT
+
*
2.3k
VHR
(See Table 1 for example RHR values)
2
The SC resistor is used to trim the converter’s output voltage (V
NOM
) 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.
RSC =
µRAM
output voltage
3V
5V
12 V
15 V
24 V
28 V
(
V
NOM
*
1
k)
–
2
k
1.23
V
V
HR
@ 1A
375 mV
375 mV
375 mV
375 mV
375 mV
375 mV
(See Table 1 for example RSC values)
R
HR
Value (Ω )
18.2 k
30.9 k
73.2 k
90.9 k
147.0 k
174.0 k
R
SC
Value (Ω )
442.00 k
2.05 k
7.68 k
10.20 k
17.40 k
21.00 k
Table 1
–
Calculated values of RSC and RHR for a headroom voltage of 375 mV.
Use notes 1 and 2 to compute RSC and RHR values for different headroom voltages.
Output Ripple Attenuation Module
Page 2 of 19
Rev. 1.8
3/2013
vicorpower.com
800 735.6200
PARD Attenuation
MICRORAM THEORY OF OPERATION:
Passive
VIN+
CTRAN
57 µF
Active
MicroRAM
VHR
VDIODE
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 50 KHz
to 20 MHz 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 2 MHz.
The lower frequency range of the active filter can be extended
down by adding an external by-pass cap across the R
HR
resistor.
VOUT+
2.3 k
VIN-
9.4 µF
VREF
CHR
(Optional)
RHR
VOUT-
Figure 1
—
Simplified MicroRAM Block Diagram
Figure 2
—
MicroRAM attenuation with and without an additional CHR 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 100 µF of capacitance
for CHR. There are practical limitations to the amount of capacitance that can be added, which is explained in more detail under the VREF section.
Output Ripple Attenuation Module
Page 3 of 19
Rev. 1.8
3/2013
vicorpower.com
800 735.6200
PARD Attenuation (Continued)
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 (VHR) is the difference in DC voltage
between V
IN
and V
OUT
. This headroom is programmed via
RHR, 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
MicroRAM
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 40 dB 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 30 A 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 5 mΩ at 25°C and can
increase to 6.5 mΩ at 100°C.
VIN + VDIODE
VIN[p-p]
VIN[DC]
VHR1
VIN + ( IIN * R[uRAM] )
VHR2
Figure 3
—
Active Attenuation and the Effects of Headroom
Output Ripple Attenuation Module
Page 4 of 19
VOUT
Rev. 1.8
3/2013
vicorpower.com
800 735.6200
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
Figure 4
—
MicroRAM headroom voltage reduction over full load current range.
VHR, 30 A, Min HR
VHR, 30 A, Max HR
Insertion Loss
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 2 mV/A for the 30 A version
(4 mV/A for 20 A 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
to attenuate PARD at higher load currents.
0.35
0.30
0.25
Headroom Voltage
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
30
Load Current
Figure 5
—
Slope adjust comparison of 20 A and 30 A MicroRAM.
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