LED7707TR datasheet(39/48 Pages) STMICROELECTRONICS

39 / 48 page

LED7707

Application information

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6.5

Layout consideration

A careful PCB layout is important for proper operation. In this section some guidelines

are provided in order to achieve a good layout.

The device has two different ground pins: signal ground (SGND) and power ground

(PGND). The PGND pin handles the switching current related to the boost section; for

this reason the PCB traces should be kept as short as possible and with adequate

width.

The signal ground is the return for the device supply and the current generators and

can be connected to the thermal pad.

The heat dissipation area (adequate to the application conditions) should be placed

backside respect to the device and with the lowest thermal impedance possible (i.e.

PCB traces in the backside should be avoided). The dissipation area is thermally and

electrically connected to the thermal pad by several vias (nine vias are recommended).

The signal and power grounds must be connected together in a single point as close as

possible to the PGND pin to reduce ground loops.

The R-C components of the compensation network should be placed as close as

possible to the COMP pin in order to avoid noise issue and instability of the

compensation.

Noise sensitive signals (i.e. feedbacks and compensation) should be routed as short as

possible to minimize noise collection. The LED7707 pinout makes it easy to separate

power components (e.g. inductor, diode) from signal ones.

The LX switching node should have and adequate width for high efficiency.

The critical power path inductor-LX-PGND must be as short as possible by mounting

the inductor, the diode and COUT as close as possible each other.

10. The capacitors of the compensated divider connected to the OVSEL pin should be

placed as close as possible to the OVSEL pin.

11. In order to assure good performance in terms of row current accuracy/mismatch, the

PCB traces from the rows pins to the LEDs should have similar length and width.

12. The capacitors of the filter connected to LDO5 and VIN pins should be mounted as

close as possible to the mentioned pins

Figure 22

and Figure 23 shows the demonstration board layout (top view and bottom view

respectively).

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