Datasheet LT1777 (Analog Devices) - 9

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