TLF50211EL_15 Datasheet, PDF (23/27 Pages) Marl International Limited – 2.2 MHz Step-Down Regulator 500 mA, 5 V, low quiescent current

TLF50211EL
8.1
General Layout recommendations
Application Information
Introduction:
A switch mode step down converter is a potential source of electromagnetic disturbances which may affect the
environment as well as the device itself and cause sporadic malfunction up to damages depending on the amount
of noise.
In principal we may consider the following basic effects:
• radiated magnetic fields caused by circular currents, occurring mostly with the switching frequency and their
harmonics;
• radiated electric fields, often caused by (voltage) oscillations;
• conducted disturbances (voltage spikes or oscillations) on the lines, mostly input and output lines.
Radiated magnetic fields:
Radiated magnetic fields are caused by circular currents occurring in so called “current windows”. These circular
currents are alternating currents which are driven by the switching transistor. The alternating current in these
windows are driving magnetic fields. The amount of magnetic emissions is mainly depending on the amplitude of
the alternating current and the size of the so-called “window” (this is the area, which is defined by the circular
current paths.
We can divide into two windows:
• the input current “window” (path consisting of C_IN2, C_IN3, L_OUT and C_OUT): Only the alternate content of the input
current I_S is considered;
• the output current “window” (path consisting of D_CATCH , L_OUT and C_OUT): Output current ripple ΔI.
The area of these “windows” has to be kept as small as possible, with the relating elements placed next to each
others as close as possible. It is highly recommended to use a ground plane as a single layer which covers the
complete regulator area with all components shown in the application diagram. All connections to ground shall be
as short as possible.
Radiated electric fields:
Radiated electric fields are caused by voltage oscillations occurring by stray inductances and stray capacitances
at the connection between internal power stage (pin SWO), freewheeling diode D_CATCH, and output capacitor C_OUT.
They are also of course influenced by the commutation of the current from the internal power stage to the
freewheeling diode D_CATCH. Their frequencies might be above 100 MHz. Therefore, it is recommended to use a
fast Schottky diode and to keep the connections in this area as low inductive as possible. This can be achieved
by using short and broad connections and by arranging the related parts as close as possible. Following the
recommendation of using a ground layer these low inductive connections will form together with the ground layer
small capacitances which are desirable to damp the slope of these oscillations. The oscillations use connections
or wires as antennas, this effect can also be minimized by the short and broad connections.
Data Sheet
23
Rev. 1.0, 2013-06-19