NB4N441

http://onsemi.com

8

Figure 4. Power Supply Filter

0.01 mF

47 mF

L=1000 mH

R=15 W

0.01 mF

3.3 V or

5.0 V

3.3 V or

5.0 V

Power Supply Filtering

The NB4N441 is a mixed analog/digital product and as

such, it exhibits some sensitivities that would not necessarily

be seen on a fully digital product. Analog circuitry is

naturally susceptible to random noise, especially if this noise

is seen on the power supply pins. The NB4N441 provides

this design technique is to try and isolate the high switching

noise of the digital outputs from the relatively sensitive

internal analog phase−locked loop. In a controlled

environment such as an evaluation board, this level of

isolation is sufficient. However, in a digital system

environment where it is more difficult to minimize noise on

the power supplies, a second level of isolation may be

required. The simplest form of isolation is a power supply

illustrates a typical power supply filter scheme. The

NB4N441 is most susceptible to noise with spectral content

in the 1 kHz to 1 MHz range. Therefore, the filter should be

designed to target this range. The key parameter that needs

to be met in the final filter design is the DC voltage drop that

26 mA. Assuming that a minimum of 2.9 V must be

resistor shown in Figure 4 must have a resistance of 5

W

Max to meet the voltage drop criteria. The RC filter pictured

will provide a broadband filter with approximately 100:1

attenuation for noise whose spectral content is above

20 kHz. As the noise frequency crosses the series resonant

point of an individual capacitor, it’s overall impedance

begins to look inductive and thus increases with increasing

frequency. The parallel capacitor combination shown

ensures that a low impedance path to ground exists for

frequencies well above the bandwidth of the PLL. The level

of required filtering is subject to further optimization and

same GND plane.