ADP1829ACPZ-R7 datasheet(25/32 Pages) AD

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ADP1829

Rev. 0 | Page 25 of 32

PCB LAYOUT GUIDELINES

In any switching converter, some circuit paths carry high dI/dt,

which can create spikes and noise. Other circuit paths are

sensitive to noise. Still others carry high dc current and can

produce significant IR voltage drops. The key to proper PCB

layout of a switching converter is to identify these critical paths

and arrange the components and copper area accordingly.

When designing PCB layouts, be sure to keep high current

loops small. In addition, keep compensation and feedback

components away from the switch nodes and their associated

components.

The following is a list of recommended layout practices for the

ADP1829, arranged by decreasing order of importance:

• The current waveform in the top and bottom FETs is a pulse

with very high dI/dt, so the path to, through, and from each

individual FET should be as short as possible and the two

paths should be commoned as much as possible. In designs

that use a pair of D-Pak or SO-8 FETs on one side of the

PCB, it is best to counter-rotate the two so that the switch

node is on one side of the pair and the high-side drain can

be bypassed to the low-side source with a suitable ceramic

bypass capacitor, placed as close as possible to the FETs in

order to minimize inductance around this loop through the

FETs and capacitor. The recommended bypass ceramic

capacitor values range from 1 μF to 22 μF depending upon

the output current. This bypass capacitor is usually

connected to a larger value bulk filter capacitor and should

be grounded to the PGND plane.

• GND, VREG bypass, soft start capacitor, and the bottom

end of the output feedback divider resistors should be tied

to an (almost isolated) small AGND plane. All of these

connections should have connections from the pin to the

AGND plane that are as short as possible. No high current

or high dI/dt signals should be connected to this AGND

plane. The AGND area should be connected through one

wide trace to the negative terminal of the output filter

capacitors.

• The PGND pin handles high dI/dt gate drive current

returning from the source of the low-side MOSFET. The

voltage at this pin also establishes the 0 V reference for the

overcurrent limit protection (OCP) function and the CSL

pin. A small PGND plane should connect the PGND pin

and the PVCC bypass capacitor through a wide and direct

path to the source of the low-side MOSFET. The placement

of CIN is critical for controlling ground bounce. The negative

terminal of CIN needs to be placed very close to the source of

the low-side MOSFET.

• Avoid long traces or large copper areas at the FB and CSL

pins, which are low signal level inputs that are sensitive to

capacitive and inductive noise pickup. It is best to position

any series resistors and capacitors as closely as possible to

these pins. Avoid running these traces close and parallel to

high dI/dt traces.

• The switch node is the noisiest place in the switcher circuit

with large ac and dc voltage and current. This node should

be wide to keep resistive voltage drop down. However, to

minimize the generation of capacitively coupled noise, the

total area should be small. Place the FETs and inductor all

close together on a small copper plane in order to minimize

series resistance and keep the copper area small.

• Gate drive traces (DH and DL) handle high dI/dt, so tend to

produce noise and ringing. They should be as short and

direct as possible. If possible, avoid using feedthrough vias

in the gate drive traces. If vias are needed, it is best to use

two relatively large ones in parallel to reduce the peak

current density and the current in each via. If the overall

PCB layout is less than optimal, slowing down the gate drive

slightly can be very helpful to reduce noise and ringing. It is

occasionally helpful to place small value resistors (such as

5 Ω or 10 Ω) in series with the gate leads, mainly DH traces

to the high-side FET gates. These can be populated with 0 Ω

resistors if resistance is not needed. Note that the added gate

resistance increases the switching rise and fall times, and

that also increases the switching power loss in the MOSFET.

• The negative terminal of output filter capacitors should be

tied closely to the source of the low-side FET. Doing this

helps to minimize voltage difference between GND and

PGND at the ADP1829.

• Generally, be sure that all traces are sized according to the

current that will be handled as well as their sensitivity in the

circuit. Standard PCB layout guidelines mainly address

heating effects of current in a copper conductor. These are

completely valid, but they do not fully cover other concerns

such as stray inductance or dc voltage drop. Any dc voltage

differential in connections between ADP1829 GND and the

converter power output ground can cause a significant

output voltage error because it affects converter output

voltage according to the ratio with the 600 mV feedback

reference. For example, a 6 mV offset between ground on

the ADP1829 and the converter power output causes a 1%

error in the converter output voltage.

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