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

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

2.0

1.5

1.0

17

17.5

18

18.5

19

19.5

20

Startup Voltage (V)

15

13

11

9

7

5

3

1

17

17.5

18

18.5

19

19.5

20

Startup Voltage (V)

NCX10S SERIES

Isolated 10W Single Output DC/DC Converters

Figure 2: Shutdown with Isolating Diode

Figure 3: Shutdown with Open Collector

Pulldown

Figure 4: NCX10S24XXX Startup Voltage Vs Startup Pin Resistor/Voltage

2.5

2.0

1.5

1.0

32.5 33 33.5 34 34.5 35 35.5 36

Startup Voltage (V)

15

13

11

9

7

5

3

1

32

33

34

35

36

Startup Voltage (V)

Figure 5: NCX10S48XXX Startup Voltage Vs Startup Pin Resistor/Voltage

APPLICATION NOTES

DC

DC

–V

IN

SD

e.g. BAT42

Less than 0.8V

for Shutdown

DC

–V

IN

SD

e.g. BC546

Greater than 1.5V

for Shutdown

10K 0.25W

DC