Datasheet

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ULT Series
Thirty-Second-Brick Isolated DC/DC Converters
with 2:1 Wide Input Range
MDC_ULT Series.F05 Page 24 of 26
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
magnetically 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 ap-
propriate 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 tem-
peratures from rising to excessive levels. The ULT Series is capable of enduring
an indefi nite short circuit output condition.
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 regulation
the user should connect the +Sense to +V
OUT and –Sense to –VOUT at the DC/
DC converter pins. ULT 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.
[V
OUT(+)-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
V
OUT and Sense in conjunction with trim adjustment of the output voltage can
cause the over-voltage protection circuitry to activate (see Performance Speci-
fi cations 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 specifi ed rating, or cause output voltages to climb into
the output over-voltage region. Therefore, the designer must ensure:
(V
OUT at pins) x (IOUT)  rated output power
Thermal Shutdown
The ULT 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 inside
the PWM (see U1 in fi gure 4) 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 Specifi cations.
Output Over-Voltage Protection
The ULT output voltage is monitored for an over-voltage condition using a
comparator. The signal is optically coupled to the primary side and if the output
voltage 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 de-
crease. Following a time-out period the PWM will restart, causing the output volt-
age 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 defi ned
as the point at which the full-power output voltage falls below the specifi ed
tolerance. See Performance/Functional Specifi cations. If the load current, being
drawn from the converter, is signifi cant enough, the unit will go into a short
circuit condition as described below.
C1–C4 = 100μF CERAMIC*
LOAD 2-3 INCHES (51-76mm) FROM MODULE
*The ULT-12/2.5-D48 model is tested with 47μF output caps.
R
LOAD
C1 C2 C3 C4
SCOPE
+OUTPUT
–OUTPUT
+SENSE
–SENSE
Figure 3. Measuring Output Ripple/Noise (PARD)
Figure 4. Thermal Shutdown
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
I
OUT Return
I
OUT
Figure 5. Remote Sense Circuit Confi guration