PCB layout issue

LX pin

The LX pin of the device is a fast switching node, so PCB tracks should be

kept as short as possible. To minimize ground 'bounce', the ground pin of the

device should be soldered directly to the ground plane.

Coil,

decoupling

capacitors,

current sense

resistor

It is particularly important to mount the coil and the input decoupling

capacitor as close to the device pins as possible to minimize parasitic

resistance and inductance, which will degrade efficiency. It is also important

to minimize any track resistance in series with current sense resistor RS. Its

best to connect VIN directly to one end of RS and Isense directly to the

opposite end of RS with no other currents flowing in these tracks. It is

important that the cathode current of the Schottky diode does not flow in a

track between RS and VIN as this may give an apparent higher measure of

current than is actual because of track resistance.

ADJ pin

The ADJ pin is a high impedance input for voltages up to 1.35V so, when left

floating, PCB tracks to this pin should be as short as possible to reduce noise

pickup. A 100nF capacitor from the ADJ pin to ground will reduce frequency

modulation of the output under these conditions. An additional series 10k

resistor can also be used when driving the ADJ pin from an external circuit .

This resistor will provide filtering for low frequency noise and provide

protection against high voltage transients.

High voltage

tracks

Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk

of leakage currents due to board contamination. The ADJ pin is soft-clamped

for voltages above 1.35V to desensitize it to leakage that might raise the ADJ

pin voltage and cause excessive output current. However, a ground ring

placed around the ADJ pin is recommended to minimize changes in output

current under these conditions.

FC1360

2009. 07. 10

6/7

Revision No : 0