Datasheet

UEI25 Series
Single Output Isolated 25-Watt DC/DC Converters
MDC_UEI25W.B09 Page 21 of 24
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There are two CAUTIONs to observe for the Trim input:
CAUTION: To avoid unplanned power down cycles, do not exceed EITHER the
maximum output voltage OR the maximum output power when setting the trim.
If the output voltage is excessive, the OVP circuit may inadvertantly shut down
the converter. If the maximum power is exceeded, the converter may enter
current limiting. If the power is exceeded for an extended period, the converter
may overheat and encounter overtemperature shut down.
CAUTION: Be careful of external electrical noise. The Trim input is a senstive
input to the converter’s feedback control loop. Excessive electrical noise may
cause instability or oscillation. Keep external connections short to the Trim
input. Use shielding if needed.
Trim Equations
Trim Up Trim Down
<Connect trim resistor
between Trim and –Vout>
<Connect trim resistor
between Trim and +Vout>
Where Vo = Desired output voltage. Adjustment accuracy is subject to resis-
tor tolerances and factory-adjusted output accuracy. Mount trim resistor close
to converter. Use short leads.
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:
Positive: Models equipped with Positive Logic are enabled when the On/Off
pin is left open or is pulled high to +15V
DC with respect to –VIN. An internal bias
current causes the open pin to rise to +V
IN. Positive-logic devices are disabled
when the On/Off is grounded or brought to within a low voltage (see Specifi ca-
tions) with respect to –V
IN.
Negative: Models with negative logic 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 +15V
DC Max. with respect to –VIN.
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. +15V (open pin) for the ON state,
assuming positive logic.
CAUTION: Do not apply voltages to the On/Off pin when there is no input
power voltage. Otherwise the converter may be permanently damaged.
Figure 4. Trim adjustments to decrease Output Voltage using a Fixed Resistor
LOAD
RTRIM DOWN
+VOUT
TRIM
ON/OFF
CONTROL
−VIN
−VOUT
+VIN
– 2050
V
O – 3.3
12775
– 2050
3.3 – V
O
5110 x (Vo –2.5)
UEI25-033-D48
RT
UP
(Ω) = RT
DOWN
(Ω) =
– 2050
V
O – 5
12775
– 2050
5 – V
O
5110 x (Vo –2.5)
UEI25-050-D48
RT
UP
(Ω) = RT
DOWN
(Ω) =
– 5110
V
O – 12
25000
– 5110
12 – V
O
10000 (Vo-2.5)
UEI25-120-D48
RT
UP
(Ω) = RT
DOWN
(Ω) =
Figure 5. Trim adjustments to increase Output Voltage using a Fixed Resistor
+VOUT
TRIM
ON/OFF
CONTROL
−VIN
LOAD
R TRIM UP
−VOUT
+VIN
Figure 6. Driving the On/Off Control Pin (suggested circuit)
ON/OFF
CONTROL
-VIN
+VCC