TPS62870 datasheet(45/66 Pages) TI2

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Table 9-1. Design Parameters (continued)

SYMBOL

PARAMETER

VALUE

TOLτ

Tolerance of the internal timing parameter

±30%

kBW

Ratio of switching frequency to converter bandwidth (must be ≥ 4)

Nɸ

Number of phases

Preliminary Calculations

The maximum allowable deviation of the power supply is ±3.3%. The DC accuracy of the TPS6287x is specified

as ±1%, therefore, the maximum output voltage variation during a transient is given by:

∆VOUT=±VOUT× 3.3% –1% =±17.25mV

(6)

9.2.2 Detailed Design Procedure

The following subsections describe how to calculate the external components required to meet the specified

transient requirements of a given application. The calculations include the worst-case variation of components

and use the RMS method to combine the variation of uncorrelated parameters.

9.2.2.1 Selecting the Inductor

TPS6287x devices are optimized for inductors in the range 50nH to 300nH. If the transient response of the

converter is limited by the slew rate of the current in the inductor, using a smaller inductor can improve

performance. However, the output ripple current increases as the value of the inductor decreases, and higher

output current ripple generates higher output voltage ripple, which adds to the transient overshoot or undershoot.

The optimal configuration for a given application is always a trade-off between a number of parameters. TI

recommends a starting value of 110nH for typical applications.

The peak-to-peak inductor current ripple is given by:

ILPP =VOUTVIN VIN – VOUT

Nɸ×L×fsw

(7)

ILPP =0.753.3 3.3 –0.75

1×110×10–9×2.25×106 =2.342A

(8)

Table 9-2 lists a number of inductors designed for use with this application. This list is not exhaustive and other

inductors from other manufacturers can also be an excellent choice.

Table 9-2. List of Recommended Inductors

INDUCTANCE

CURRENT RATING

DIMENSIONS

DC RESISTANCE

PART NUMBER(1)

(ISAT AT 25°C)

(L × W × H)

92nH

24A

4 × 4 × 1.2mm

5.2mΩ (typical)

Coilcraft, XEL4012-920NE

100nH

30A

4 × 4 × 3.2mm

1.5mΩ (typical)

Coilcraft, XEL4030-101ME

110nH

29A

4 × 4 × 2.1mm

1.4mΩ (typical)

Coilcraft, XGL4020-111ME

110nH

29A

3.2 × 2.5 × 2.5mm

1.9mΩ (typical)

TDK, CLT32-R11

55nH

39.5A

3.2 × 2.5 × 2.5mm

1.0mΩ (typical)

TDK, CLT32-55N

110nH

17.0A

3.2 × 2.5 × 2.5mm

3.0mΩ (typical)

Cyntec, VCTA32252E-R11MS6

100nH

25A

4.2 × 4.0 × 2.1mm

1.9mΩ (typical)

Cyntec, VCHA042A-R10MS62M

100nH

44A

5.45 × 5.25 × 2.8mm

0.8mΩ (typical)

Cyntec, VCHW053T-R10NMS5

(1)

See Section 10.1.1.

9.2.2.2 Selecting the Input Capacitors

As with all buck converters, the input current of the TPS6287x devices is discontinuous. The input capacitors

provide a low-impedance energy source for the device, and the value, type, and location are critical for correct

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