Datasheet

HPQ-12/25-D48 Series
Isolated 300-Watt Quarter Brick DC-DC Converters
MDC_HPQ-12/25-D48 Series.B07 Page 14 of 16
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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
Where Vout = 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. Note that “∆” is given as a small fraction, not a
percentage.
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 +13.5V
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.
On positive-logic models, to reduce noise coupling on the external on/off
control, use the circuit shown in fi gure 6.
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 +13.5V
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. +13.5V (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 Increase Output Voltage using a Fixed Resistor
LOAD
R TRIM UP
+VOUT
+SENSE
TRIM
-SENSE
–VOUT
+VIN
ON/OFF
CONTROL
–VIN
Figure 5. Trim adjustments to Decrease Output Voltage using a Fixed Resistor
+VOUT
+SENSE
TRIM
-SENSE
–VOUT
+VIN
ON/OFF
CONTROL
–VIN
LOAD
R TRIM DOWN
Figure 7. Driving the On/Off Control Pin (suggested circuit)
ON/OFF
CONTROL
-VIN
+VCC
Figure 6. On/Off Control Filter
ON/OFF
-INPUT
316 K
47 K
0.1 F
2.5V CIRCUIT
+INPUT
R
adj_up
(
in kΩ
)
= 5.11 x - - 2
12V x (1+∆) 1
∆
1.225 x ∆
where ∆ =
12V -V
out
12V
R
adj_down
(
in kΩ
)
= 5.11 x - 2
1
∆
where ∆ =
V
out
-12V
12V
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