L5993 datasheet(14/22 Pages) STMICROELECTRONICS

14 / 22 page

pends both on the mains voltage range and the

synchronization frequency range.

An additional ”fine tuning” can be achieved by

adding a small DC offset (in the ten mV) on the

current sense pin (13, ISEN).

For wide range mains applications it is anyway

recommended to compensate the propagationde-

lay of the current sense path (PWM comparator +

latch + driver) with the circuit shown in the ”Appli-

cation Ideas” section, fig. 41.

Layout hints

Generally speaking a proper circuitboard layout is

vital for correct operation but is not an easy task.

Careful component placing, correct traces routing,

appropriate traces widths and, in case of high

voltages, compliance with isolation distances are

the major issues. The L5993 eases this task by

putting two pins at disposal for separate current

returns of bias (SGND) and switch drive currents

(PGND) The matter is complex and only few im-

portant points will be here reminded.

1) All current returns (signal ground, power

ground, shielding, etc.) should be routed sepa-

rately and should be connected only at a single

ground point.

2) Noise coupling can be reduced by minimizing

the area circumscribed by current loops. This

applies particularly to loops where high pulsed

currents flow.

3) For high current paths, the traces should be

doubled on the other side of the PCB whenever

possible: this will reduce both the resistance

and the inductance of the wiring.

4) Magnetic field radiation (and stray inductance)

can be reduced by keeping all traces carrying

switched currents as short as possible.

5) In general, traces carrying signal currents

should run far from traces carrying pulsed cur-

rents or with quickly swinging voltages. From

this viewpoint, particular care should be taken

of the high impedance points (current sense in-

put, feedback input, ...). It could be a good idea

to route signal traces on one PCB side and

power traces on the other side.

6) Provide adequate filtering of some crucial

points of the circuit, such as voltage references,

IC’s supply pins, etc.

L5993

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