RT8167B datasheet(37/38 Pages) RICHTEK

37 / 38 page

RT8167B

DS8167B-00

October 2011

www.richtek.com

Output Capacitor Selection

Output capacitors are used to obtain high bandwidth for

the output voltage beyond the bandwidth of the converter

itself. Usually, the CPU manufacturer recommends a

capacitor configuration. Two different kinds of output

capacitors can be found, bulk capacitors closely located

to the inductors and ceramic output capacitors in close

proximity to the load. Latter ones are for mid-frequency

decoupling with very small ESR and ESL values while the

bulk capacitors have to provide enough stored energy to

overcome the low-frequency bandwidth gap between the

regulator and the CPU.

Layout Consideration

Careful PC board layout is critical to achieving low

switching losses and clean, stable operation. The

switching power stage requires particular attention. If

possible, mount all of the power components on the top

side of the board with their ground terminals flushed

against one another. Follow these guidelines for optimum

PC board layout :

Keep the high current paths short, especially at the

ground terminals.

Keep the power traces and load connections short. This

is essential for high efficiency.

When trade-offs in trace lengths must be made, it's

preferable to allow the inductor charging path to be made

longer than the discharging path.

Place the current sense component close to the

controller. ISENxP and ISENxN connections for current

limit and voltage positioning must be made using Kelvin

sense connections to guarantee the current sense

accuracy. The PCB trace from the sense nodes should

be parallel to the controller.

Route high-speed switching nodes away from sensitive

analog areas (COMPx, FBx, ISENxP, ISENxN, etc...)

Special attention should be paid in placing the DCR

current sensing components. The DCR current sensing

capacitor and resistors must be placed close to the

controller.

The capacitor connected to the ISEN1N/ISENAN for noise

decoupling is optional and it should also be placed close

to the ISEN1N/ISENAN pin.

The NTC thermistor should be placed physically close

to the inductor for better DCR thermal compensation.

RT8167B Datasheet(PDF) 37 Page - Richtek Technology Corporation

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