commonly referred to as power limiting. Current limit inception is defined

as the point at which the full-power output voltage falls below the specified

tolerance. See Performance/Functional Specifications. If the load current, being

drawn from the converter, is significant enough, the unit will go into a short

circuit condition as described below.

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, the mag-

netically coupled voltage used to develop primary side voltages will also drop,

thereby shutting down the PWM controller. Following a time-out period, the

PWM will restart causing the output voltage to begin ramping to their appropri-

ate value. If the short-circuit condition persists, another shutdown cycle will be

initiated. This on/off cycling is referred to as “hiccup” mode. The hiccup cycling

reduces the average output current, thereby preventing internal temperatures

from rising to excessive levels. The UCQ Series is capable of enduring an

indefinite short circuit output condition.

Remote Sense

resistors. Nevertheless, if the sense function is not used for remote regulation

DC converter pins. UCQ series converters employ a sense feature to provide

point of use regulation, thereby overcoming moderate IR drops in PCB conduc-

tors or cabling. The remote sense lines carry very little current and therefore

require minimal cross-sectional-area conductors. The sense lines, which are

capacitively coupled to their respective output lines, are used by the feedback

control-loop to regulate the output. As such, they are not low impedance points

and must be treated with care in layouts and cabling. Sense lines on a PCB

should be run adjacent to dc signals, preferably ground.

In Figure 3, the two copper strips simulate real-world PCB impedances

between the power supply and its load. In order to minimize measurement

errors, scope measurements should be made using BNC connectors, or the

probe ground should be less than ½ inch and soldered directly to the fixture.

Figure 3. Measuring Output Ripple/Noise (PARD)

Floating Outputs

Since these are isolated DC/DC converters, their outputs are “floating” with

respect to their input. Designers will normally use the –Output (pin 4) as the

ground/return of the load circuit. You can however, use the +Output (pin 8) as

ground/return to effectively reverse the output polarity.

Minimum Output Loading Requirements

UCQ converters employ a synchronous-rectifier 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.

Thermal Shutdown

The UCQ converters are equipped with thermal-shutdown circuitry. If environ-

mental conditions cause the temperature of the DC/DC converter to rise above

the designed operating temperature, a precision temperature sensor will power

down the unit. When the internal temperature decreases below the threshold

of the temperature sensor, the unit will self start. See Performance/Functional

Specifications.

Output Over-Voltage Protection

The UCQ output voltage is monitored for an over-voltage condition using a com-

parator. The signal is optically coupled to the primary side and if the output volt-

age rises to a level which could be damaging to the load, the sensing circuitry

will power down the PWM controller causing the output voltage to decrease.

Following a time-out period the PWM will restart, causing the output voltage

to ramp to its appropriate value. If the fault condition persists, and the output

voltage again climbs to excessive levels, the over-voltage circuitry will initiate

another shutdown cycle. This on/off cycling is referred to as “hiccup” mode.

Current Limiting

As soon as the output current increases to approximately 130% of its rated

value, the DC/DC converter will go into a current-limiting mode. In this condi-

tion, the output voltage will decrease proportionately with increases in output

current, thereby maintaining somewhat constant power dissipation. This is

C1

C1 = 1µF CERAMIC

C2 = 10µF TANTALUM

LOAD 2-3 INCHES (51-76mm) FROM MODULE

C2

7

8

COPPER STRIP

4

5

COPPER STRIP

SCOPE

+OUTPUT

–OUTPUT

+SENSE

–SENSE

Figure 4. Remote Sense Circuit Configuration

In cables and discrete wiring applications, twisted pair or other techniques

should be used. Output over-voltage protection is monitored at the output volt-

age pin, not the Sense pin. Therefore, excessive voltage differences between

Single Output UCQ Models

Low-Profile, Quarter Brick, 8.3-40 Amp

Isolated DC/DC Converters

www.murata-ps.com

MDC_UCQ Models.B07 Page 10 of 16