9
LT1777
APPLICATIONS INFORMATION
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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 (P
OUT
/ P
IN
).
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 V
SW
and V
D
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
V
IN
C1
D1
V
OUT
1777 F01
C2
L
SENSE
L
MAIN
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
