TL494_15 Datasheet, PDF (20/33 Pages) Unisonic Technologies – VOLTAGE MODE PWM CONTROL CIRCUIT

TL494
SLVS074G – JANUARY 1983 – REVISED JANUARY 2015
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11 Power Supply Recommendations
The TL494 is designed to operate from an input voltage supply range between 7 V and 40 V. This input supply
should be well regulated. If the input supply is located more than a few inches from the device, additional bulk
capacitance may be required in addition to the ceramic bypass capacitors. A tantalum capacitor with a value of
47 μF is a typical choice, however this may vary depending upon the output power being delivered.
12 Layout
12.1 Layout Guidelines
Always try to use a low EMI inductor with a ferrite type closed core. Some examples would be toroid and
encased E core inductors. Open core can be used if they have low EMI characteristics and are located a bit
more away from the low power traces and components. Make the poles perpendicular to the PCB as well if using
an open core. Stick cores usually emit the most unwanted noise.
12.1.1 Feedback Traces
Try to run the feedback trace as far from the inductor and noisy power traces as possible. You would also like
the feedback trace to be as direct as possible and somewhat thick. These two sometimes involve a trade-off, but
keeping it away from inductor EMI and other noise sources is the more critical of the two. Run the feedback trace
on the side of the PCB opposite of the inductor with a ground plane separating the two.
12.1.2 Input/Output Capacitors
When using a low value ceramic input filter capacitor, it should be located as close to the VCC pin of the IC as
possible. This will eliminate as much trace inductance effects as possible and give the internal IC rail a cleaner
voltage supply. Some designs require the use of a feed-forward capacitor connected from the output to the
feedback pin as well, usually for stability reasons. In this case it should also be positioned as close to the IC as
possible. Using surface mount capacitors also reduces lead length and lessens the chance of noise coupling into
the effective antenna created by through-hole components.
12.1.3 Compensation Components
External compensation components for stability should also be placed close to the IC. Surface mount
components are recommended here as well for the same reasons discussed for the filter capacitors. These
should not be located very close to the inductor either.
12.1.4 Traces and Ground Planes
Make all of the power (high current) traces as short, direct, and thick as possible. It is good practice on a
standard PCB board to make the traces an absolute minimum of 15 mils (0.381 mm) per Ampere. The inductor,
output capacitors, and output diode should be as close to each other possible. This helps reduce the EMI
radiated by the power traces due to the high switching currents through them. This will also reduce lead
inductance and resistance as well, which in turn reduces noise spikes, ringing, and resistive losses that produce
voltage errors. The grounds of the IC, input capacitors, output capacitors, and output diode (if applicable) should
be connected close together directly to a ground plane. It would also be a good idea to have a ground plane on
both sides of the PCB. This will reduce noise as well by reducing ground loop errors as well as by absorbing
more of the EMI radiated by the inductor. For multi-layer boards with more than two layers, a ground plane can
be used to separate the power plane (where the power traces and components are) and the signal plane (where
the feedback and compensation and components are) for improved performance. On multi-layer boards the use
of vias will be required to connect traces and different planes. It is good practice to use one standard via per 200
mA of current if the trace will need to conduct a significant amount of current from one plane to the other.
Arrange the components so that the switching current loops curl in the same direction. Due to the way switching
regulators operate, there are two power states. One state when the switch is on and one when the switch is off.
During each state there will be a current loop made by the power components that are currently conducting.
Place the power components so that during each of the two states the current loop is conducting in the same
direction. This prevents magnetic field reversal caused by the traces between the two half-cycles and reduces
radiated EMI.
20
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