1.2MHz, Low-Cost,

High-Performance Chargers

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21

b) Minimize ground trace lengths in the high-current

paths.

c) Minimize other trace lengths in the high-current

paths.

d) Use > 5mm wide traces in the high-current

paths.

length).

f) Minimize the LX node (MOSFETs, rectifier cath-

ode, inductor (15mm max length)). Keep LX on

one side of the PCB to reduce EMI radiation.

Ideally, surface-mount power components are flush

against one another with their ground terminals

almost touching. These high-current grounds are

then connected to each other with a wide, filled

zone of top-layer copper, so they do not go through

vias. The resulting top-layer subground plane is

connected to the normal inner-layer ground plane

at the paddle. Other high-current paths should also

be minimized, but focusing primarily on short

ground and current-sense connections eliminates

about 90% of all PCB layout problems.

2) Place the IC and signal components. Keep the

main switching node (LX node) away from sensitive

capacitor). Important: the IC must be no further than

10mm from the current-sense resistors. Quiet con-

arate ground (GND) island. There is very little current

flowing in these traces, so the ground island need not

be very large. When placed on an inner layer, a siz-

able ground island can help simplify the layout

because the low-current connections can be made

through vias. The ground pad on the backside of the

package should also be connected to this quiet

ground island.

3) Keep the gate drive traces (DHI and DLO) as short

as possible (L < 20mm), and route them away from

should also be relatively wide (W > 1.25mm).

4) Place ceramic bypass capacitors close to the IC.

The bulk capacitors can be placed further away.

Place the current-sense input filter capacitors under

the part, connected directly to the GND pin.

5) Use a single-point star ground placed directly

below the part at the PGND pin. Connect the power

ground (ground plane) and the quiet ground island

at this location.