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

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 refer-

ences, IC’s supply pins, etc.

APPLICATION IDEAS

Here follows a series of ideas/suggestionsaimed at

either improving performance or solving common

application problems of L4990-basedsupplies.

D97IN504

L4990

PGND

ISEN

OUT

SGND

ISOLATION

BOUNDARY

10

9

13

11

12

Figure 33. Isolated MOSFET Drive & Current Transformer Sensing in 2-switch Topologies.

L4990

T

STD1NB50-1

22V

D97IN505A

2.2M

Ω

33K

Ω

SELF-SUPPLY

WINDING

8

4

12

11

47K

Ω

Figure 34. Low consumption start-up

L4990 - L4990A

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