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dc1903af

DEMO MANUAL DC1903A

operating principles

Table 1

DEMO CIRCUIT

INPUT VOLTAGE

DEVICE

DC1903A-A

3V to 3.6V

LTM2889-3

DC1903A-B

4.5V to 5.5V

LTM2889-5

Demo circuit 1903A is available in two configurations,

supporting both versions of the LTM2889. Table 1 details

the demo circuit configurations.

3. Board/ground plane size has been minimized. This

reduces the dipole antenna formed between the logic

side and isolated side ground planes.

4. Top signal routing and ground floods have been op-

timized to reduce signal loops, minimizing differential

mode radiation.

5. Common mode filtering is integrated into the input and

output pin headers. Filtering helps to reduce emissions

caused by conducted noise and minimizes the effects

of cabling to common mode emissions.

6. A combination of low ESL and high ESR decoupling is

used. A low ESL ceramic capacitor is located close to

themoduleminimizinghighfrequencynoiseconduction.

A high ESR tantalum capacitor is included to minimize

board resonances and prevent voltage spikes due to

hot plugging of the supply voltage.

EMI performance is shown in Figure 1, measured using a

GHz transverse electromagnetic (GTEM) cell and method

detailed in IEC 61000-4-20, testing and measurement

techniques – emission and immunity testing in transverse

electromagnetic waveguides.

The demo circuit has been designed and optimized for low

RFemissions.SomefeaturesoftheLTM2889arenotavail-

able for evaluation on the demo circuit. The logic supply

voltage VL is tied to VCC, and the ON and RE pins are not

available on the input pin header, but may be controlled

by jumpers JP1 and JP3 respectively. EMI mitigation

techniques used include the following:

1. Four layer PCB, allowing for isolated side to logic side

bridgecapacitor.Thebridgecapacitorisformedbetween

aninnerlayeroffloatingcopperwhichoverlapsthelogic

side and isolated side ground planes. This structure

creates two series capacitors, each with approximately

0.008” of insulation, supporting the full dielectric with-

standratingof2500VRMS.Thebridgecapacitorprovides

a low impedance return path for injected currents due

to parasitic capacitances of the LTM2889’s signal and

power isolating elements.

2. Discrete bridge capacitors (C3, C4) mounted between

GND2 and GND. The discrete capacitors provide ad-

ditional attenuation at frequencies below 400MHz.

Capacitors are safety rated type Y2, manufactured by

Murata, part number GA342QR7GF471KW01L, rated

for 250VRMS.

Figure 1. DC1903A Radiated Emissions

DETECTOR = QPEAK

RBW = 120kHz

VBW = 300kHz

SWEEP TIME = 17s

# OF POINTS = 501

CISPR 22 CLASS B LIMIT

DC1903A–A

DC1903A–B

FREQUENCY (MHz)

0 100 200 300 400 500 600 700 800 900 1000

–30

–20

–10

0

10

20

30

40

50

60

DC2889a F01