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Output Capacitor Selection

I

1 *

Vout

Vin

L

ƒ

1

2

3

(3)

DVout + Vout

1 *

Vout

Vin

L

ƒ

1

8

Cout

ƒ )

ESR

(4)

Input Capacitor Selection

LAYOUT CONSIDERATIONS

TPS62290, TPS62291, TPS62293

SLVS764C – JUNE 2007 – REVISED MARCH 2008

Table 2. List of Inductors

INDUCTOR TYPE

SUPPLIER

3

× 3 × 1.5

LPS3015

Coilcraft

3 x 3 x 1.5

LQH3NPN2R2NM0

MURATA

3.2 x 2.6 x 1.2

MIPSA3226D2R2

FDK

The advanced fast-response voltage mode control scheme of the TPS6229x allows the use of tiny ceramic

capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are

recommended. The output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric capacitors,

aside from their wide variation in capacitance over temperature, become resistive at high frequencies.

At nominal load current, the device operates in PWM mode and the RMS ripple current is calculated as:

At nominal load current, the device operates in PWM mode and the overall output voltage ripple is the sum of the

voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging the

output capacitor:

At light load currents the converter operates in Power Save Mode and the output voltage ripple is dependent on

the output capacitor and inductor value. Larger output capacitor and inductor values minimize the voltage ripple

in PFM mode and tighten dc output accuracy in PFM mode.

The buck converter has a natural pulsating input current; therefore, a low ESR input capacitor is required for best

input voltage filtering and minimizing the interference with other circuits caused by high input voltage spikes. For

most applications, a 10-

µF ceramic capacitor is recommended. The input capacitor can be increased without any

limit for better input voltage filtering.

Take care when using only small ceramic input capacitors. When a ceramic capacitor is used at the input and the

instability or could even damage the part by exceeding the maximum ratings.

Table 3. List of Capacitor

CAPACITANCE

TYPE

SIZE

SUPPLIER

10

µF

GRM188R60J106M69D

0603 1.6x0.8x0.8mm3

Murata

As for all switching power supplies, the layout is an important step in the design. Proper function of the device

demands careful attention to PCB layout. Care must be taken in board layout to get the specified performance. If

the layout is not carefully done, the regulator could show poor line and/or load regulation, stability issues as well

as EMI problems. It is critical to provide a low inductance, impedance ground path. Therefore, use wide and

short traces for the main current paths. The input capacitor should be placed as close as possible to the IC pins

as well as the inductor and output capacitor.

Connect the GND Pin of the device to the Power Pad of the PCB and use this Pad as a star point. Use a

16

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