Technical Notes
I/O Filtering and Noise Reduction
The RBC is tested and specifi ed with external output capacitors. These
capacitors are necessary to accommodate our test equipment and may not
be required to achieve desired performance in your application. The RBC is
designed with high-quality, high-performance internal I/O caps, and will oper-
ate within spec in most applications with no additional external components.
In particular, the RBC input capacitors are specifi ed for low ESR and are fully
rated to handle the units' input ripple currents. Similarly, the internal output
capacitors are specifi ed for low ESR and full-range frequency response.
In critical applications, input/output ripple/noise may be further reduced using
fi ltering techniques, the simplest being the installation of external I/O caps.
External input capacitors serve primarily as energy-storage devices. They
minimize high-frequency variations in input voltage (usually caused by IR
drops in conductors leading to the DC/DC) as the switching converter draws
pulses of current. Input capacitors should be selected for bulk capacitance
(at appropriate frequencies), low ESR, and high rms-ripple-current ratings.
The switching nature of modern DC/DC's requires that the dc input voltage
source have low ac impedance at the frequencies of interest. Highly inductive
source impedances can greatly affect system stability. Your specifi c system
confi guration may necessitate additional considerations.
Input Fusing
Most applications and or safety agencies require the installation of fuses at
the inputs of power conversion components. The RBC Series may have an
optional input fuse. Therefore, if input fusing is mandatory, either a normal-
blow or a fast-blow fuse with a value no greater than twice the maximum
input current should be installed within the ungrounded input path to the
converter.
Input Overvoltage and Reverse-Polarity Protection
The RBC does not incorporate input reverse-polarity protection. Input voltages
in excess of the specifi ed absolute maximum ratings and input polarity rever-
sals of longer than "instantaneous" duration can cause permanent damage to
these devices.
Start-Up Time
The VIN to VOUT Start-Up Time is the interval between the time at which a rising
input voltage crosses the lower limit of the specifi ed input voltage range
C
IN
V
IN
C
BUS
L
BUS
C
IN
= 33µF, ESR < 700m7 @ 100kHz
C
BUS
= 220µF, ESR < 100m7 @ 100kHz
L
BUS
= 12µH
+INPUT
COMMON
CURRENT
PROBE
TO
OSCILLOSCOPE
+
–
Figure 2. Measuring Input Ripple Current
and the fully loaded output voltage enters and remains within its specifi ed
regulation band. Actual measured times will vary with input source imped-
ance, external input capacitance, and the slew rate and fi nal value of the input
voltage as it appears to the converter.
The On/Off to V
OUT Start-Up Time assumes the converter is turned off via the
On/Off Control with the nominal input voltage already applied to the converter.
The specifi cation defi nes the interval between the time at which the converter
is turned on and the fully loaded output voltage enters and remains within its
specifi ed regulation band.
Thermal Considerations and Thermal Protection
The typical output-current thermal-derating curves shown below enable
designers to determine how much current they can reliably derive from each
model of the RBC under known ambient-temperature and air-fl ow conditions.
Similarly, the curves indicate how much air fl ow is required to reliably deliver
a specifi c output current at known temperatures.
The highest temperatures in RBC's occur at their output inductor, whose heat
is generated primarily by I
2
R losses. The derating curves were developed
using thermocouples to monitor the inductor temperature and varying the load
to keep that temperature below +110°C under the assorted conditions of air
fl ow and air temperature. Once the temperature exceeds +125°C (approx.),
the thermal protection will disable the converter using the hiccup shutdown
mode.
Undervoltage Shutdown
When the input voltage falls below the undervoltage threshold, the converter
will terminate its output. However, this is not a latching shutdown mode. As
soon as the input voltage rises above the Start-Up Threshold, the converter
will restore normal operation. This small amount of hysteresis prevents most
uncommanded power cycling. Since some input sources with higher output
impedance will increase their output voltage greater than this hysteresis as
soon as the load is removed, it is possible for this undervoltage shutdown to
cycle indefi nitely. To prevent this, be sure that the input supply always has
adequate voltage at full load.
Thermal Shutdown
Extended operation at excessive temperature will initiate overtemperature
shutdown triggered by a temperature sensor inside the PWM controller. This
operates similarly to overcurrent and short circuit mode. The inception point of
the overtemperature condition depends on the average power delivered, the
ambient temperature and the extent of forced cooling airfl ow.
Remote On/Off Control
The RBC may be turned off or on using the external remote on/off control. This
terminal consists of a digital input to the internal PWM controller through a
protective resistor and diode.
The on/off input circuit should be CMOS logic referred to the –Input power
terminal however TTL or TTL-LS logic will also work or a switch to ground. If
preferred, you can even run this using a bipolar transistor in “open collector”
confi guration or an “open drain” FET transistor. You may also leave this input
unconnected and the converter will run whenever input power is applied.
RBC-12/17-D48 Series
Quarter Brick, Regulated Bus Converters
MDC_RBC-12/17-D48.C02 Page 9 of 10
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