L4990 - L4990A
Layout hints. Generally speaking a proper cir-
cuitboard layout is vital for correct operation but is
not an easy task. Careful component placing, cor-
rect traces routing, appropriate traces widths and,
in case of high voltages, compliance with isolation
distances are the major issues. The L4990 eases
this task by putting two pins at disposal for sepa-
rate current returns of bias (SGND) and switch
drive currents (PGND) The matter is complex and
only few important 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
2) Noise coupling can be reduced by minimizing
the area circumscribed by current loops. This
applies particularly to loops where high pulsed
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 refer-
ences, IC’s supply pins, etc.
Here follows a series of ideas/suggestionsaimed at
either improving performance or solving common
application problems of L4990-basedsupplies.
Figure 33. Isolated MOSFET Drive & Current Transformer Sensing in 2-switch Topologies.
Figure 34. Low consumption start-up
22V VREF 4 8