Datasheet

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ULT Series
Thirty-Second-Brick Isolated DC/DC Converters
with 2:1 Wide Input Range
MDC_ULT Series.F05 Page 23 of 26
circuits and layout as shown in the following two fi gures. External input capaci-
tors (CIN in Figure 2) serve primarily as energy-storage elements, minimiz-
ing line voltage variations caused by transient IR drops in conductors from
backplane to the DC/DC. Input caps should be selected for bulk capacitance
(at appropriate frequencies), low ESR, and high rms-ripple-current ratings. The
switching nature of DC/DC converters requires that dc voltage sources have
low ac impedance as highly inductive source impedance can affect system
stability. In Figure 2, CBUS and LBUS simulate a typical dc voltage bus. Your
specifi c system confi guration may necessitate additional considerations.
In critical applications, output ripple/noise (also referred to as periodic and
random deviations or PARD) may be reduced below specifi ed limits using fi lter-
ing techniques, the simplest of which is the installation of additional external
output capacitors. They function as true fi lter elements and should be selected
for bulk capacitance, low ESR and appropriate frequency response.
All external capacitors should have appropriate voltage ratings and be
located as close to the converter as possible. Temperature variations for all
relevant parameters should also be taken carefully into consideration. The most
effective combination of external I/O capacitors will be a function of line voltage
and source impedance, as well as particular load and layout conditions.
Floating Outputs
Since these are isolated DC/DC converters, their outputs are “fl oating” with
respect to their input. Designers will normally use the –Output as the ground/
return of the load circuit. You can however, use the +Output as ground/return to
effectively reverse the output polarity.
Minimum Output Loading Requirements
ULT converters employ a synchronous-rectifi er design topology and all models
regulate within spec and are stable under no-load to full load conditions.
Operation under no-load conditions however might slightly increase the output
ripple and noise.
Input Fusing
Certain applications and/or safety agencies may require the installation of
fuses at the inputs of power conversion components. Fuses should also be
used if the possibility of sustained, non-current-limited, input-voltage polarity
reversals exists. For Murata Power Solutions' ULT series DC/DC converters, we
recommend the use of a fast blow fuse, installed in the ungrounded input sup-
ply line with a typical value about twice the maximum input current, calculated
at low line with the converter’s minimum effi ciency.
All relevant national and international safety standards and regulations must
be observed by the installer. For system safety agency approvals, the convert-
ers must be installed in compliance with the requirements of the end- use
safety standard.
Input Reverse-Polarity Protection
If the input voltage polarity is accidentally reversed, an internal diode will be-
come forward biased and likely draw excessive current from the power source.
If this source is not current limited or the circuit appropriately fused, it could
cause permanent damage to the converter.
Input Under-Voltage Shutdown and Start-Up Threshold
Under normal start-up conditions, devices will not begin to regulate properly
until the ramping-up input voltage exceeds the Start-Up Threshold Voltage.
Once operating, devices will not turn off until the input voltage drops below the
Under-Voltage Shutdown limit. Subsequent re-start will not occur until the input
is brought back up to the Start-Up Threshold. This built in hysteresis prevents
any unstable on/off situations from occurring at a single input voltage.
Start-Up Time
The VIN to VOUT Start-Up Time is the time interval between the point at which
the ramping input voltage crosses the Start-Up Threshold and the fully loaded
output voltage enters and remains within its specifi ed accuracy band. Actual
measured times will vary with input source impedance, external input capaci-
tance, and the slew rate and fi nal value of the input voltage as it appears at the
converter. The ULT Series implements a soft start circuit to limit the duty cycle
of its PWM controller at power up, thereby limiting the input inrush current.
The On/Off Control to V
OUT start-up time assumes the converter has its
nominal input voltage applied but is turned off via the On/Off Control pin. The
specifi cation defi nes the interval between the point at which the converter is
turned on (released) and the fully loaded output voltage enters and remains
within its specifi ed accuracy band. Similar to the V
IN to VOUT start-up, the On/Off
Control to V
OUT start-up time is also governed by the internal soft start circuitry
and external load capacitance. The difference in start up time from V
IN to VOUT
and from On/Off Control to V
OUT is therefore insignifi cant.
Input Source Impedance
The input of ULT converters must be driven from a low ac-impedance source. The
DC/DC’s performance and stability can be compromised by the use of highly induc-
tive source impedances. The input circuit shown in Figure 2 is a practical solution
that can be used to minimize the effects of inductance in the input traces. For opti-
mum performance, components should be mounted close to the DC/DC converter.
I/O Filtering, Input Ripple Current, and Output Noise
All models in the ULT Series are tested/specifi ed for input refl ected ripple cur-
rent and output noise using the specifi ed external input/output components/
C IN V IN C BUS
L BUS
C IN = 33μF, ESR < 700mΩ @ 100kHz
C
BUS = 220μF, ESR < 100mΩ @ 100kHz
L
BUS = 12μH
+VIN
–VIN
CURRENT
PROBE
TO
OSCILLOSCOPE
+
–
Figure 2. Measuring Input Ripple Current
TECHNICAL NOTES
Model Tested with
Maximum
Capacitance
Loading
ULT-3.3/7.5-D48
Four 100µF output capacitors & Three 1µF and
33µF (low ESR) external input capacitors
5000µF
ULT-5/5-D48
Four 100µF output capacitors & Three 1µF and
33µF (low ESR) external input capacitors
5000µF
ULT-12/2.5-D48
Four 47µF output capacitors & three 1µF and 33µF
(low ESR) external input capacitors.
2200µF