NCX10S SERIES
Isolated 10W Single Output DC/DC Converters
APPLICATION NOTES
SET VOLTAGE
The output voltage is set to 100mV above
nominal, to offset resistive losses and thus
assist with worst case error calculations. This
allowance can be altered with a single fixed
resistor, connected from the trimming pin to
one of the output pins.
OTHER TRIMMING FUNCTIONS
The voltage applied to the trim pin need not
be from a potentiometer. The trim facility may
be driven electronically to provide
“margining”, remote sense, active load
sharing, current control or remote
programming (There is a minimum output
voltage below which control is not possible).
SHUTDOWN & UNDER VOLTAGE TRIM
When the shutdown pin (Pin 11) is shorted to
the negative input, the converter will stop. Its
current consumption will then be less than
1mA at nominal supply voltage. The
shutdown/UV adjust pin serves the dual
function of shutting the converter down into a
low power state, and allowing adjustment of
the under voltage start threshold.
To shut the unit down this pin should be taken
to below 1.5V, using either an isolating
diode (figure 2) with the anode connected to
the shutdown pin or using an open collector
pulldown (figure 3).
The UV threshold can be controlled either via
a resistor connected between the pin and 0V,
or by connecting a series 1MΩ resistor to a
voltage of between 0V and 2.5V.
The variation of startup voltage with either
parallel resistor or applied voltage is shown
Applied Voltage (V)
in figures 4 & 5. Note that voltage should not
be applied directly to the pin without a series
resistor or diode.
If the shutdown pin is to be connected to a
long wire, it is recommended that a capacitor
decouples the pin to the supply common in
order to avoid the risk of injecting noise into
the converter circuit. A series resistor may
also be helpful. Values of 10nF and 1kΩ may
be used.
Many converters may be switched together
simply by linking the primary control pins via
a schottky signal diode anode to the control
pin. The primary common pins must also be
linked, see figure 6.
FILTERING
The module includes a basic level of filtering,
sufficient for many applications. Where lower
noise levels are desired, filters can easily be
added to achieve any required noise
performance.
A DC/DC converter generates noise in two
principle forms: that which is radiated from
its body and that conducted on its external
connections. There are three separate modes
of conducted noise: input differential, output
differential and input-output.
This last appears as common mode at the
input and the output, and cannot therefore be
removed by filtering at the input or output
alone. The first level of filtering is to connect
a capacitor between input and output returns,
to reduce this form of noise. It typically
contains high harmonics of the switching
frequency, which tend to appear as spikes on
surrounding circuits. The voltage rating of this
capacitor must match the required isolation
voltage. (Due to the great variety in isolation
voltage and required noise performance, this
capacitor has not been included within the
converter.)
Input ripple is a voltage developed across the
internal input decoupling capacitor. It is
therefore measured with a defined supply
source impedance. Although simple series
inductance will provide filtering, on its own it
can degrade the stability. A shunt capacitor is
therefore recommended across the converter
input terminals, so that it is fed from a low
impedance.
If no filtering is required, the inductance of
long supply wiring could also cause a
problem, requiring an input decoupling
capacitor for stability. An electrolytic will
perform well in these situations.
See figure 7 for a recommended
configuration to reduce all three conducted
modes.
The component values and ratings will
depend on the converter rating and voltage,
and the required noise performance. The
input-output filtering is performed by the
common-mode choke on the primary. This
could be placed on the output, but would
then degrade the regulation and produce less
2.5
Shutdown Resistor (MΩ)
17.5
18 18.5 19
Startup Voltage (V)
19.5
20
15
13
11
9
7
5
3
1
17
17.5
18 18.5 19
Startup Voltage (V)
19.5
20
e.g. BAT42
DC
SD
2.0
1.5
Less than 0.8V
for Shutdown
DC
–V
IN
1.0
17
Figure 2: Shutdown with Isolating Diode
Figure 4: NCX10S24XXX Startup Voltage Vs Startup Pin Resistor/Voltage
2.5
15
Shutdown Resistor (MΩ)
33.5 34 34.5 35 35.5
Startup Voltage (V)
36
13
11
9
7
5
3
1
32
33
34
35
Startup Voltage (V)
36
e.g. BC546
10K 0.25W
Greater than 1.5V
for Shutdown
Applied Voltage (V)
SD
DC
DC
–V
IN
2.0
1.5
1.0
32.5 33
Figure 3: Shutdown with Open Collector
Pulldown
Figure 5: NCX10S48XXX Startup Voltage Vs Startup Pin Resistor/Voltage
CANDD [ C&D TECHNOLOGIES ]
