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 15 of 17
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Temperature/power derating is based on maximum output current and voltage
at the converter’s output pins. Use of the trim and sense functions can cause
output voltages to increase, thereby increasing output power beyond the UWQ’s
specifi ed rating, or cause output voltages to climb into the output overvoltage
region. Therefore:
(V
OUT at pins) x (IOUT)  rated output power
The Trim pin (pin 6) is a relatively high impedance node that can be susceptible
to noise pickup when connected to long conductors in noisy environments.
Remote Sense Input
Use the Sense inputs with caution. Sense is normally connected at the load.
Sense inputs compensate for output voltage inaccuracy delivered at the load.
This is done by correcting IR voltage drops along the output wiring 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. Consider using heavier wire if this drop is excessive. Sense inputs also
improve the stability of the converter and load system by optimizing the control
loop phase margin.
Note: The Sense input and power Vout lines are internally connected through
low value resistors to their respective polarities so that the converter can
operate without external connection to the Sense. Nevertheless, if the Sense
function is not used for remote regulation, the user should connect +Sense to
+Vout and –Sense to –Vout at the converter pins.
The remote Sense lines carry very little current. They are also capacitively
coupled to the output lines and therefore are in the feedback control loop to
regulate and stabilize the output. As such, they are not low impedance inputs
and must be treated with care in PC board layouts. Sense lines on the PCB
should run adjacent to DC signals, preferably Ground. In cables and discrete
wiring, use twisted pair, shielded tubing or similar techniques.
Any long, distributed wiring and/or signifi cant 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. You
may also adjust the output trim slightly to compensate for voltage loss in any
external fi lter elements. Do not exceed maximum power ratings.
Remote On/Off Control
On the input side, a remote On/Off Control can be specifi ed with either positive
or negative logic as follows:
Models are on (enabled) when the On/Off is grounded or brought to within
a low voltage (see Specifi cations) with respect to –V
IN. The device is off
(disabled) when the On/Off is left open or is pulled high to +13.5V
DC Max. with
respect to –V
IN.
Dynamic control of the On/Off function should be able to sink the specifi ed
signal current when brought low and withstand specifi ed voltage when brought
high. Be aware too that there is a fi nite time in milliseconds (see Specifi cations)
between the time of On/Off Control activation and stable, regulated output. This
time will vary slightly with output load type and current and input conditions.
There are two CAUTIONs for the On/Off Control:
CAUTION: While it is possible to control the On/Off with external logic if you
carefully observe the voltage levels, the preferred circuit is either an open
drain/open collector transistor or a relay (which can thereupon be controlled
by logic). The On/Off prefers to be set at approx. +13.5V (open pin) for the ON
state, assuming positive logic.
Figure 6. Remote Sense Circuit Confi guration
LOAD
Contact and PCB resistance
losses due to IR drops
Contact and PCB resistance
losses due to IR drops
+VOUT
+SENSE
TRIM
− SENSE
-VOUT
+ VIN
ON/OFF
CONTROL
–VIN
Sense Current
I OUT
Sense Return
I OUT Return
LOAD
R
TRIM UP
+VOUT
+VIN
–VIN
ON/OFF
CONTROL
TRIM
+SENSE
–VOUT
–SENSE
Figure 4. Trim Connections To Increase Output Voltages Using Fixed Resistors
LOAD
R
TRIM DOWN
+VOUT
+VIN
–VIN
ON/OFF
CONTROL
TRIM
+SENSE
–VOUT
–SENSE
Figure 5. Trim Connections To Decrease Output Voltages Using Fixed Resistors
UP
VO – 12
R
T (kΩ) =
–10.2
49.6(VO – 1.226)
12 – V
O
RT (kΩ) =
–10.2
60.45
DOWN
Trim Up* Trim Down*
*Vo = Desirable output voltage in Volts