Technical Notes

I/O Filtering and Noise Reduction

The RBC is tested and specified 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 specified for low ESR and are fully

rated to handle the units' input ripple currents. Similarly, the internal output

capacitors are specified for low ESR and full-range frequency response.

In critical applications, input/output ripple/noise may be further reduced using

filtering 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 specific system

configuration 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 specified absolute maximum ratings and input polarity rever-

sals of longer than "instantaneous" duration can cause permanent damage to

these devices.

Start-Up Time

input voltage crosses the lower limit of the specified input voltage range

@ 100kHz

@ 100kHz

+INPUT

COMMON

CURRENT

PROBE

TO

OSCILLOSCOPE

+

–

Figure 2. Measuring Input Ripple Current

and the fully loaded output voltage enters and remains within its specified

regulation band. Actual measured times will vary with input source imped-

ance, external input capacitance, and the slew rate and final value of the input

voltage as it appears to the converter.

On/Off Control with the nominal input voltage already applied to the converter.

The specification defines the interval between the time at which the converter

is turned on and the fully loaded output voltage enters and remains within its

specified 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-flow conditions.

Similarly, the curves indicate how much air flow is required to reliably deliver

a specific output current at known temperatures.

The highest temperatures in RBC's occur at their output inductor, whose heat

using thermocouples to monitor the inductor temperature and varying the load

to keep that temperature below +110°C under the assorted conditions of air

flow 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 indefinitely. 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 airflow.

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”

configuration 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.B02 Page 9 of 10

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