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Understanding power issues in SDI products (Part 2)

Posted: 22 Aug 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Power-supply? serial digital interface? low drop-out regulator?

Power-supply design for high-speed devices can usually drive the need for low-noise, low-ripple voltage rails. Just like other high-speed devices, serial digital interface (SDI) components, require the control of output voltage (VCC) ripple and noise in order to fully maximize their performance.

Each SDI component within the signal path can have differing VCC ripple and noise requirements ranging from 5-100mV. There are numerous options for converting power for VCC, which can result in a low ripple, low noise voltage rail. This article will discuss various solutions for meeting the power requirements of SDI video components.

If specified, the manufacturer's datasheet often lists the output-voltage ripple for power regulators at 20MHz measurement bandwidth. This is fine for comparative purposes, but not when evaluating power components for SDI applications. Doing so misses the high-frequency switching artifacts that are typically seen on an output-voltage measurement as spikes or noise. Those higher-frequency switching-related artifacts cannot be ignored when considering the power requirements for SDI components which are, by their function, high bandwidth devices.

Figure 1 shows the difference between an output-voltage ripple measurement of a buck converter (24V to 3.3V@ 2A) with 20MHz measurement bandwidth (BW) and with full BW. While the full bandwidth ripple/noise amplitude of about 18mV p-p is good, it may not be sufficient for some SDI applications and could limit their performance. (For discussion purposes in this article, all ripple and noise references will be full BW unless noted otherwise.)

Switch-mode solutions
There are many options for converting power for SDI and other devices in a product. As a result, many designers are unsure about which power-supply topology is most efficient or, in the case of powering SDI, which option has the lowest VCC ripple and noise.

Power converters like National Semiconductor's Simple Switcher Power Modules could be option for powering SDI and other circuits requiring low VCC ripple and noise. The modules shown in Table 1 include an internal inductor and MOSFETs and have output ripple/noise performance suitable for many of the SDI components, without the need for additional filtering.

However, the ripple and noise requirements for 3G reclockers and cable drivers can be low drop-out regulator (LDO) or a Pi filter, which will be discussed later in this article.

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Figure 1: 20MHz BW (top trace) and 300MHz BW (lower trace).

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Table 1: Some of National Semiconductor power modules.

When designing any switch-mode power supply (SMPS), including a power module, it's important to select output capacitors for minimizing voltage ripple. For many SMPS, this means selecting a sufficient amount of output capacitance with low ESR (effective series resistance). Voltage ripple is a function of the inductor ripple current, the switching frequency (FSW) and the output capacitor's ESR. Therefore, minimizing the ESR in the output capacitors will minimize the output voltage ripple.

The output-voltage noise, also known as switching artifacts, is another matter. This is typically related to a number of factors such as PCB layout of the power supply, turn-on and turn-off transitional times of power transistors, reverse recovery of diodes (as applicable) and the inter-winding capacitance of the switch-mode inductor.

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