OKY
-T/10 & T/16-D12 Series
Programmable DOSA-SMT 10/16-Amp DC/DC Converters
Output Short Circuit Condition
When a converter is in current-limit mode, the output voltage will drop as the
output current demand increases. If the output voltage drops too low (approxi-
mately 98% of nominal output voltage for most models), the PWM controller
will shut down. Following a time-out period, the PWM will restart, causing the
output voltage to begin ramping up to its appropriate value. If the short-circuit
condition persists, another shutdown cycle will initiate. This rapid on/off cycling
is called “hiccup mode”. The hiccup cycling reduces the average output cur-
rent, thereby preventing excessive internal temperatures and/or component
damage. A short circuit can be tolerated indefinitely.
Remote Sense Input
The Sense input is normally connected at the load for the respective Sense
polarity (+Sense to the +Vout load). The sense input compensates for voltage
drops along the output wiring such as moderate IR drops and the current
carrying capacity of PC board etch. This output drop (the difference between
Sense and Vout when measured at the converter) should not exceed 0.5V. Use
heavier connections if this drop is excessive. The sense input also improves the
stability of the converter and load system by optimizing the control loop phase
margin.
If the Sense function is not used for remote regulation, the user should con-
nect the Sense to their respective Vout at the converter pins.
Sense lines on the PCB should run adjacent to DC signals, preferably
Ground. Any long, distributed wiring and/or significant inductance introduced
into the Sense control loop can adversely affect overall system stability. If in
doubt, test your applications by observing the converter’s output transient
response during step loads. There should not be any appreciable ringing or
oscillation.
Do not exceed maximum power ratings. Excessive voltage differences
between Vout and Sense together with trim adjustment of the output can cause
the overvoltage protection circuit to activate and shut down the output.
Power derating of the converter is based on the combination of maximum
output current and the highest output voltage at the ouput pins. Therefore the
designer must insure:
(Vout at pins) x (Iout) ≤ (Max. rated output power)
Remote On/Off Control
The remote On/Off Control can be ordered with either polarity. Please refer to
the Connection Diagram on page 1 for On/Off connections.
Positive polarity models are enabled when the On/Off pin is left open or is
pulled high to +Vin with respect to –Vin. Therefore, the On/Off control can be
disconnected if the converter should always be on. Positive-polarity devices are
disabled when the On/Off is grounded or brought to within a low voltage (see
Specifications) with respect to –Vin.
Negative polarity devices are on (enabled) when the On/Off pin is left open
or brought to within a low voltage (see Specifications) with respect to –Vin. The
device is off (disabled) when the On/Off is pulled high (see Specifications) with
respect to –Vin.
+SENSE
+OUTPUT
COPPER STRIP
C1
C2
SCOPE
R
LOAD
-OUTPUT
COPPER STRIP
C1 = 0.1μF CERAMIC
C2 = 10μF TANTALUM
LOAD 2-3 INCHES (51-76mm) FROM MODULE
Figure 3: Measuring Output Ripple and Noise (PARD)
up to the shutdown temperature, an on-board electronic temperature sensor
will power down the unit. When the temperature decreases below the turn-on
threshold, the converter will automatically restart. There is a small amount of
temperature hysteresis to prevent rapid on/off cycling.
CAUTION:
If you operate too close to the thermal limits, the converter may
shut down suddenly without warning. Be sure to thoroughly test your applica-
tion to avoid unplanned thermal shutdown.
Temperature Derating Curves
The graphs in the next section illustrate typical operation under a variety of
conditions. The Derating curves show the maximum continuous ambient air
temperature and decreasing maximum output current which is acceptable
under increasing forced airflow measured in Linear Feet per Minute (“LFM”).
Note that these are AVERAGE measurements. The converter will accept brief
increases in current or reduced airflow as long as the average is not exceeded.
Note that the temperatures are of the ambient airflow, not the converter
itself which is obviously running at higher temperature than the outside air.
Also note that very low flow rates (below about 25 LFM) are similar to “natural
convection”, that is, not using fan-forced airflow.
Murata Power Solutions makes Characterization measurements in a closed
cycle wind tunnel with calibrated airflow. We use both thermocouples and an
infrared camera system to observe thermal performance.
CAUTION:
If you routinely or accidentally exceed these Derating guidelines,
the converter may have an unplanned Over Temperature shut down. Also, these
graphs are all collected at slightly above Sea Level altitude. Be sure to reduce
the derating for higher density altitude.
Output Current Limiting
Current limiting inception is defined as the point at which full power falls
below the rated tolerance. See the Performance/Functional Specifications.
Note particularly that the output current may briefly rise above its rated value
in normal operation as long as the average output power is not exceeded. This
enhances reliability and continued operation of your application. If the output
current is too high, the converter will enter the short circuit condition.
www.murata-ps.com
20 Oct 2009
email: sales@murata-ps.com
MDC_OKY_T10T16.D12.A05_long
Page 7 of 17
MURATA-PS [ MURATA POWER SOLUTIONS INC. ]
