peak-to-peak ripple current equal to 30% of the maximum

load current. The inductor current ripple (

∆I

lated by:

Therefore, L can be initially set to the following by applying

the 30% rule:

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,

it is recommended to have a saturation current higher than

the maximum output current to handle any ripple or momen-

tary over-current events. The core material also influences

the saturation characteristics as ferrite materials have a hard

saturation curve and care should be taken such 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.

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 out-

put voltage ripple and switching the FETs appropriately. The

output voltage ripple on a buck converter can be approxi-

mated 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 the following equation:

∆V

∆I

To ensure stability, two constraints need to be met. The first

is that there is sufficient ESR to create enough voltage ripple

at the feedback pin. The recommendation is to have at least

10mV of ripple seen at the feedback pin. This can be calcu-

lated by multiplying the output voltage ripple by the gain

seen through the feedback resistors. This gain, H, can be

calculated below:

If the output voltage is fairly high, causing significant attenu-

ation through the feedback resistors, a feed-forward capaci-

tor can be used. This is actually recommended for most

circuits as it improves performance. See the feed-forward

capacitor section for more details.

The second criteria is to ensure that there is sufficient ripple

at the output that is in-phase with the switch. The problem

exists that there is actually ripple caused by the capacitor

charging and discharging, not only the ESR ripple. Since

these are effectively out of phase, problems can exist. To

avoid this issue it is recommended that the ratio of the two

ripples (

β) is always greater than 5. To calculate the mini-

mum ESR value needed, the following equation can be

used.

In general the best capacitors to use are chemistries that

have a known and consistent ESR across the entire operat-

ing temperature range. Tantalum capacitors or similar chem-

istries such as Niobium Oxide perform well along with certain

families of Aluminum Electrolytics. Small value POSCAPs

and SP CAPs also work as they have sufficient ESR. When

used in conjunction with a low value inductor it is possible to

have an extremely stable design. The only capacitors that

require modification to the circuit are ceramic capacitors.

Ceramic capacitors cause problems meeting both criteria

because they have low ESR and low capacitance. There-

fore, if they are to be used, an external ESR resistor (R

should be added. This can be seen below in the following

circuit.

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