Rugged Environment
DC-DC Converters <40 W
IMX 7-Family
Edition 2/96 - Melcher AG
6 - 25
MELCHER
The Power Partners.
6.1
Supplementary Data
Inhibit Function
The output(s) of the converter may be enabled or disabled
by means of a logic signal (TTL, CMOS, etc.) applied to the
inhibit pin. No output voltage overshoot will occur when the
unit is turned on. If the inhibit function is not required the
inhibit pin should be connected to Vo– to enable the output
(active low logic, fail safe).
Converter operating:
–10 V...0.8 V
Converter inhibited
or inhibit pin left open:
2.4 V...
Ui max (<75 V for 70 IMX 7)
either: Between the R pin and Vo– to achieve an output
voltage adjustment range of approximately
Uo = 75...100 % Uo nom.
Uo
Rext ≈ 4 k –––––––––
Uo nom – Uo
or: Between the R pin and Vo+ to achieve an output volt-
age range of approximately
Uo = 100...105 % Uo nom.
(
Uo – 2.5 V)
R'ext ≈ 4 k ––––––––––––––––––––
2.5 V (
Uo/Uo nom – 1)
b) Adjustment by means of an external voltage
Uext be-
tween Vo– and R pins.
The control voltage range is 1.87...2.62 V and allows for
an adjustment in the range of approximately 75...105%.
Uo 2.5 V
Uext
≈ –––––––––
Uo nom
Attempting to adjust the output below this range will
cause the converter to shutdown (hiccup mode).
Note: Applying an external control voltage >2.75 V
may damage the converter.
Fig. 10
If the inhibit is not used the inhibit pin should be con-
nected to Vi–
Connection in Series or in Parallel
Connection in series:
The outputs of single or double output units can be con-
nected in series without any precautions, taking into con-
sideration that the highest output voltage should remain
below 60 V for SELV operation.
Connection in parallel:
The outputs of one or several double output units with equal
nominal voltage can be connected in parallel. Approximate
current sharing between 2 or several units is ensured by
their load dependent output characteristic.
Short Circuit Behaviour
The current limit characteristic shuts down the converter
whenever a short circuit is applied to its output. It acts self-
protecting and automatically recovers after removal of the
overload condition.
Vi+
Vi–
i
Adjustable Output Voltage (R-input)
As a standard feature, the IMX 7 single and double output
units offer adjustable output voltage(s) by using the control
input R pin. If the R pin is left open circuit the output voltage
is set to
Uo nom. The output voltage is adjustable in the range
75...105 % of
Uo nom.The circuit works for single and double
output units in a different way. For output voltages
Uo >
Uo nom, the minimum input voltage Ui min (see "Electrical In-
put Data") increases proportionally to
Uo/Uo nom.
Single output units:
The R input control pin is referenced to the secondary side
of the converter. Adjustment of the output voltage is possi-
ble by means of either an external resistor or a voltage
source.
a) Adjustment by means of an external resistor
Rext:
Depending upon the value of the required output voltage,
the resistor shall be connected
R
Vo+
Vo–
+
U
ext
–
4 k
Uref 2.5 V
control
circuit
Vi+
Vi–
R
ext
R'
ext
Fig. 11
Output voltage control for single output units by means of
the R-input
Double output units:
The R-pin is referenced to the primary side. The figure
below shows the circuit topology. Adjustment of the output
voltage is possible by means of either an external resistor in
the range of 100...105 % of
Uo nom, or an external voltage
source in the range of 75...105% of
Uo nom.
R
Vo1+
Vo2–
+
U
ext
–
Control
circuit
Uref 2.5 V
Vi+
Vi–
R
ext
Vo1–
Vo2+
Fig. 12
Output voltage control for double output units by means of
the R-input