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1 *

Vout

Vin

L

ƒ

(1)

I

2

(2)

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)

TPS62290

SLVS764 – JUNE 2007

APPLICATION INFORMATION (continued)

Equation 1 calculates the maximum inductor current under static load conditions. The saturation current of the

inductor should be rated higher than the maximum inductor current as calculated with Equation 2. This is

recommended because during heavy load transient the inductor current will rise above the calculated value.

With:

f = Switching Frequency (2.25MHz typical)

L = Inductor Value

∆I

I

A more conservative approach is to select the inductor current rating just for the maximum switch current of the

corresponding converter.

Accepting larger values of ripple current allows the use of low inductance values, but results in higher output

voltage ripple, greater core losses, and lower output current capability.

The total losses of the coil have a strong impact on the efficiency of the dc/dc conversion and consist of both the

• The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)

• Additional losses in the conductor from the skin effect (current displacement at high frequencies)

• Magnetic field losses of the neighboring windings (proximity effect)

• Radiation losses

Table 2. List of Inductors

INDUCTOR TYPE

SUPPLIER

3

× 3 × 1.5

LPS3015

Coilcraft

The advanced fast-response voltage mode control scheme of the TPS62290 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.

14

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