Datasheet

UWQ-12/17-Q48T-C Series
Wide Input, Isolated DOSA Quarter Brick
DC-DC Converters with Trim and Sense
MDC_UWQ-12-17-Q48T-C.A01 Page 14 of 17
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These converters employ a synchronous rectifi er design topology. All models
regulate within specifi cation and are stable under no load to full load conditions.
Operation under no load might however slightly increase output ripple and noise.
Thermal Shutdown
To protect against thermal over-stress, these converters include thermal shut-
down circuitry. If environmental or application conditions cause the tempera-
ture of the DC-DC’s to rise above the Operating Temperature Range 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 hysteresis
to prevent rapid on/off cycling. CAUTION: If the product is operated too close
to the thermal limits, it may shut down suddenly without warning. Be sure to
thoroughly test your application to avoid unplanned thermal shutdown.
Temperature Derating Curves
The graphs in this data sheet illustrate typical operation under a variety of condi-
tions. The Derating curves show the maximum continuous ambient air temperature
and decreasing maximum output current which is acceptable under increasing
forced airfl ow measured in Linear Feet per Minute (“LFM”) or meters per second
(M/S). Note that these are AVERAGE, steady state measurements. The converter will
accept brief increases in temperature and/or current or reduced airfl ow as long as
the average is not exceeded.
Note that the temperatures are of the ambient airfl ow, not the converter itself
which is obviously running at higher temperature than the outside air. Also note
that “natural convection” is defi ned as very low fl ow rates which are not using
fan-forced airfl ow. Depending on the application, “natural convection” is usu-
ally about 30-65 LFM but is not equal to still air (0 LFM).
Murata Power Solutions makes Characterization measurements in a closed
cycle wind tunnel with calibrated airfl ow. Both thermocouples and an infrared
camera system are used to observe thermal performance.
CAUTION: If these Derating guidelines are exceeded, the converter may have
an unplanned Over Temperature shut down. Also, these graphs are all collected
near Sea Level altitude. Be sure to reduce the derating for higher altitude.
Output Overvoltage Protection (OVP)
This converter monitors its output voltage for an over-voltage condition using
magnetic feedback circuitry. If the output exceeds OVP limits, the sensing
circuit will power down the unit, and the output voltage will decrease. After
a time-out period, the PWM will automatically attempt to restart, causing the
output voltage to ramp up to its rated value. It is not necessary to power down
and reset the converter for this automatic OVP-recovery restart.
If the fault condition persists and the output voltage climbs to excessive levels,
the OVP circuitry will initiate another shutdown cycle. This on/off cycling is
referred to as “hiccup” mode.
Output Fusing
The converter is extensively protected against current, voltage and temperature
extremes. However, your application circuit may need additional protection. In the
extremely unlikely event of output circuit failure, excessive voltage could be applied
to the application circuit. Consider using an appropriate external protection.
Current Limiting
As power demand increases on the output and enters the specifi ed “limit
inception range” limiting circuitry activates in the DC-DC converter to limit/
restrict the maximum current or total power available. Once the current
reaches a certain range the output voltage will start to decrease while the
output current continues to increase, thereby maintaining constant power, until
a maximum peak current is reached and the converter enters a “hiccup” (on
off cycling) mode of operation until the load is reduced below the threshold
level, whereupon it will return to a normal mode of operation. Current limit
inception is defi ned as the point where the output voltage has decreased by a
pre-specifi ed percentage (usually a 2% decrease from nominal).
Short Circuit Condition
The short circuit condition is an extension of the “Current Limiting” condition.
When the monitored peak current signal reaches a certain range, the PWM
controller’s outputs are shut off thereby turning the converter “off.” This is
followed by an extended time out period. This period can vary depending on
other conditions such as the input voltage level. Following this time out period,
the PWM controller will attempt to re-start the converter by initiating a “normal
start cycle” which includes softstart. If the “fault condition” persists, another
“hiccup” cycle is initiated. This “cycle” can and will continue indefi nitely until
such time as the “fault condition” is removed, at which time the converter will
resume “normal operation.” Operating in the “hiccup” mode during a fault
condition is advantageous in that average input and output power levels are
held low preventing excessive internal increases in temperature.
Trimming Output Voltage
UWQ converters have a trim capability (pin 6) that enables users to adjust the
output voltage from +10% to –10% (refer to the trim equations in the table
below). Adjustments to the output voltage can be accomplished with a single
fi xed resistor as shown in Figures 4 and 5. A single fi xed resistor can increase
or decrease the output voltage depending on its connection. Resistors should
be located close to the converter and have TCR’s less than 100ppm/°C to
minimize sensitivity to changes in temperature. If the trim function is not used,
leave the trim pin open.
Standard UWQs have a “positive trim” where a single resistor connected from
the Trim pin (pin 6) to the +Sense (pin 7) will increase the output voltage. A
resistor connected from the Trim Pin (pin 6) to the –Sense (pin 5) will decrease
the output voltage.
Trim adjustments greater than the specifi ed +10%/–10% can have an adverse
affect on the converter’s performance and are not recommended. Excessive
voltage differences between V
OUT and Sense, in conjunction with trim adjust-
ment of the output voltage, can cause the overvoltage protection circuitry to
activate (see Performance Specifi cations for overvoltage limits).
Floating Outputs
Since these are isolated DC-DC converters, their outputs are “fl oating” with
respect to their input. The essential feature of such isolation is ideal ZERO
CURRENT FLOW between input and output. Real-world converters however
do exhibit tiny leakage currents between input and output. These leakages
consist of both an AC stray capacitance coupling component and a DC leakage
resistance. When using the isolation feature, do not allow the isolation voltage
to exceed specifi cations. Otherwise the converter may be damaged. Designers
will normally use the negative output (-Output) as the ground return of the load
circuit. You can however use the positive output (+Output) as the ground return
to effectively reverse the output polarity.
Minimum Output Loading Requirements