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LT1777IS Datasheet(PDF) 9 Page - Linear Technology |
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LT1777IS Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  9 LT1777 APPLICATIONS INFORMATION external sense inductor to set a maximum allowed dI/dt rate. This attenuates the highest frequency components of generated B field RFI. Minimal lead length in the path is also essential to minimize generated RFI. A second potential source of magnetic RFI is the main (power) inductor. Fortunately, the natural triangular be- havior of the current waveform in the main inductor tends to generate magnetic field energy concentrated in the fundamental and lower harmonics. Nevertheless, the rela- tively intense magnetic field present in the main inductor can cause coupling problems, especially if the main induc- tor is of an open construction type. So called rod or barrel inductors may be the physically smallest and most effec- tive types, but their magnetic field extends far beyond the device itself. Closed type inductors, toroids for example, contain the magnetic field nearly completely. These are generally preferred for low noise behavior. The sense inductor sees a much more rapid current slew rate than does the main inductor. However the sense inductor is physically smaller and of much lower induc- tance than the main inductor. These factors tend to reduce its propensity to generate magnetic interference prob- lems. Nevertheless, more sensitive applications can opt for a closed type magnetic construction on the sense inductor. Basics of Low Noise Operation Switching power supply circuits are often preferred over linear topologies for their improved efficiency (POUT/ PIN). However, their typically rapid voltage and current slew rates often cause “radio frequency” interference prob- lems, commonly referred to as “RFI”. The LT1777 is designed to provide a less aggressive voltage slew rate and a user-programmable current slew rate to eliminate the highest frequency harmonics of RFI emissions. These highest frequency components are typically the most troublesome. Optimum behavior is obtained by a combi- nation of proper circuit design, which includes passive component selection, and proper printed circuit board layout technique. There are two types of RFI emissions, i.e., conducted and radiated. Conducted interference travels directly through “wires”, as opposed to radiated interference, which travels through the air. Conducted RFI can be created by a switching power supply at its input voltage supply node, its output node(s) or both. It is typically caused by pulsatile current flow through the residual high frequency imped- ance (ESR) of bypass capacitors. Radiated interference can be of two types: electric (E field) or magnetic (B field). E field interference is caused by stray capacitance coupling of the node(s) which swing rapidly over a large voltage excursion. In the LT1777, this in- cludes the VSW and VD nodes. E field radiation is kept low by minimizing the length and area of all traces connected to these nodes. A ground plane should always be used under the switcher circuitry to prevent interplane cou- pling. Although these nodes swing over a voltage range roughly equal to the input voltage, the limited dV/dt rate of the LT1777 reduces the highest frequency components of the generated E field RFI. B field RFI is simply coupling of high frequency magnetic fields generated by the offending circuitry. High frequency magnetic fields are created by relatively rapidly changing currents, and the high speed current switching path in the LT1777 is shown schematically in Figure 1. This includes the input capacitor, output switch, sense inductor and output diode. Normal switching supply operation requires a rapid switching of current back and forth between the output switch and output diode. The LT1777 uses the + + LT1777 VIN C1 D1 VOUT 1777 F01 C2 LSENSE LMAIN Figure 1. High Speed Current Switching Paths Selecting Sense Inductor The LT1777 uses an external sense inductor to set a theoretical limit for current ramp rate according to the formula: Max dI dt V L BE SENSE / = 2 |
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