Datasheet

Remote Sense
Note: The Sense and VOUT lines are internally connected through low-value
resistors. Nevertheless, if the sense function is not used for remote regula-
tion the user should connect the +Sense to +VOUT and –Sense to –VOUT at
the DC-DC converter pins. UCQ series converters employ a sense feature to
provide point of use regulation, thereby overcoming moderate IR drops in PCB
conductors 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 imped-
ance 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.
[VOUT(+)-VOUT(–)] – [Sense(+)-Sense(–)] ≤ 10%VOUT
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
VOUT and Sense in conjunction with trim adjustment of the output voltage can
cause the over-voltage protection circuitry to activate (see Performance Speci-
fications for over-voltage limits). Power derating is based on maximum output
current and voltage at the converter’s output pins. Use of trim and sense func-
tions can cause output voltages to increase, thereby increasing output power
beyond the converter’s specified rating, or cause output voltages to climb into
the output over-voltage region. Therefore, the designer must ensure:
(VOUT at pins) x (IOUT) ≤ rated output power
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
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.
Thermal Shutdown
Trimming Output Voltage
UCQ converters have a trim capability (pin 6) that enables users to adjust the
output voltage (refer to the trim equations and trim graphs that follow). Adjust-
ments to the output voltage can be accomplished via a trim pot (Figure 5) or
a single fixed resistor as shown in Figures 6 and 7. A single fixed 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.
LOAD
+VOUT
+VIN
Sense Current
Contact and PCB resistance
losses due to IR drops
Contact and PCB resistance
losses due to IR drops
Sense Return
–VIN
ON/OFF
CONTROL
TRIM
+SENSE
–VOUT
–SENSE
4
5
1
2
6
8
I
OUT Return
I
OUT
7
3
Figure 4. Remote Sense Circuit Configuration
C1
C1 = 1µF
C2 = 10µF
LOAD 2-3 INCHES (51-76mm) FROM MODULE
C2
R
LOAD
7
8
4
5
SCOPE
+VOUT
–VOUT
+SENSE
–SENSE
Figure 3. Measuring Output Ripple/Noise (PARD)
www.murata-ps.com/support
Single Output UCQ Models
Low-Profile, Quarter Brick, 8.3-40 Amp
Isolated DC-DC Converters
MDC_UCQ Models.E02 Page 22 of 24