www.ti.com

Inductor Selection

DIL + VO

1–

V

O

V

I

L

ƒ

(6)

I

L

max + IO max )

DIL

2

(7)

Output Capacitor Selection

I

RMSCout +

V

O

1–

V

O

V

I

L

ƒ

1

2

3

(8)

TPS62020, TPS62021

SLVS076B – JUNE 2003 – REVISED APRIL 2004

APPLICATION INFORMATION (continued)

The TPS62020 uses typically a 10-µH output inductor. Larger or smaller inductor values can be used to optimize

the performance of the device for specific operation conditions. When changing inductor values, the product of

the inductor value times output-capacitor value (L

×C) should stay constant. For example, when reducing the

inductor value, increase the output capacitor accordingly. See the application circuits in Figure 17, Figure 18, and

Figure 19. The selected inductor has to be rated for its dc resistance and saturation current. The dc resistance of

the inductance directly influences the efficiency of the converter. Therefore an inductor with the lowest dc

resistance should be selected for highest efficiency. Formula Equation 7 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 formula Equation 7. This is needed because during heavy load transient the

inductor current rises above the value calculated under Equation 7.

with:

• 7 = Switching frequency (1.25 MHz typical)

• L = Inductor value

• ∆I

• I

Open core inductors have a soft saturation characteristic and they can usually handle higher inductor currents

versus a comparable shielded inductor. A more conservative approach is to select the inductor current rating just

for the maximum switch current of 1.3 A for the TPS62020. Keep in mind that the core material from inductor to

inductor differs and has an impact on the efficiency, especially at high switching frequencies. Refer to Table 1

and the typical applications and inductors selection.

Table 1. Inductor Selection

INDUCTOR VALUE

DIMENSIONS

COMPONENT SUPPLIER

10 µH

6,6 mm

× 4,75 mm × 2,92 mm

Coilcraft DO1608C-103

10 µH

5,0 mm

× 5,0 mm × 3,0 mm

Sumida CDRH4D28-100

3.3 µH

5,0 mm

× 5,0 mm × 2,4 mm

Sumida CDRH4D22 3R3

6.8 µH

5,8 mm

× 7,4 mm × 1,5 mm

Sumida CMD5D13 6R8

The advanced fast response voltage mode control scheme of the TPS62020 allows the use of small ceramic

capacitors with a typical value of 10 µF and 22 µF without having large output voltage under and overshoots

during heavy load transients. Ceramic capacitors having low ESR values have the lowest output voltage ripple

and are recommended. If required, tantalum capacitors may be used as well. Refer to Table 2 for component

selection. If ceramic output capacitors are used, the capacitor RMS ripple current rating always meets the

application requirements. Just for completeness 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:

13