L =

D =

LM1771

SNVS446D – JUNE 2006 – REVISED JANUARY 2016

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Typical Applications (continued)

(8)

8.2.1.2.1.1

Frequency Selection

The LM1771 is available with three preset timing options that select the ON-time and hence determine the

switching frequency of the application. Increasing the switching frequency has the effect of reducing the inductor

size needed for the application while requiring a slight trade-off in efficiency. Table 4 shows the same frequency

due to the maximum duty-cycle limitations of the device coupled with the internal start-up.

500 ns

1000 ns

2000 ns

0.8

485

242

-

1

606

303

-

1.2

727

364

-

1.5

909

455

2237

1.8

-

545

273

2.5

-

-

379

3.3

-

-

500

(1)

Recommended switching frequency (kHz) based on output voltage

and timing option.

8.2.1.2.1.2

Inductor Selection

The inductor selection is an iterative process likely requiring several passes before settling on a final value. The

reason for this is because it influences the amount of ripple seen at the output, a critical component to ensure

general stability of an adaptive ON-time circuit. For the first pass at inductor selection the value can be obtained

by targeting a maximum peak-to-peak ripple current equal to 30% of the maximum load current. The inductor

current ripple (

(9)

Therefore, L can be initially set by applying the 30% rule in Equation 10:

(10)

The other features of the inductor that can be selected besides inductance value are saturation current and core

material. Because the LM1771 does not have a current limit, TI recommends having a saturation current higher

than the maximum output current to handle any ripple or momentary overcurrent events. The core material also

influences the saturation characteristics as ferrite materials have a hard saturation curve and take care that they

never saturate during normal use. A shielded inductor or low-profile unshielded inductor is recommended to

reduce EMI. This also helps prevent any spurious noise from picking up on the feedback node resulting in

unexpected tripping of the feedback comparator.

8.2.1.2.1.3

Output Capacitor

One of the most important components to select with the LM1771 is the output capacitor. This is because its size

and ESR have a direct effect on the stability of the loop. A constant ON-time control scheme works by sensing

the output voltage ripple and switching the FETs appropriately. The output voltage ripple on a buck converter can

be approximated by stating that the AC inductor ripple flows entirely into the output capacitor and is created by

the ESR of the capacitor. This can be expressed in Equation 11:

(11)

14

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