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.
Design Guide (Continued)
peak-to-peak ripple current equal to 30% of the maximum
load current. The inductor current ripple (∆IL) can be calcu-
lated by:
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.
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.
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 (RSNS
should be added. This can be seen below in the following
circuit.
)
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:
∆VOUT = ∆IL x RESR
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:
www.national.com
10
NSC [ National Semiconductor ]
