LM2676
Application Hints
(Continued)
maximum ripple current of 30% of the maximum load cur-
rent. If the ripple current exceeds this 30% limit the next
larger value is selected.
The inductors offered have been specifically manufactured
to provide proper operation under all operating conditions of
input and output voltage and load current. Several part types
are offered for a given amount of inductance. Both surface
mount and through-hole devices are available. The inductors
from each of the three manufacturers have unique charac-
teristics.
Renco: ferrite stick core inductors; benefits are typically
lowest cost and can withstand ripple and transient peak
currents above the rated value. These inductors have an
external magnetic field, which may generate EMI.
Pulse Engineering: powdered iron toroid core inductors;
these also can withstand higher than rated currents and,
being toroid inductors, will have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest
physical size inductors and are available only as surface
mount components. These inductors also generate EMI but
less than stick inductors.
OUTPUT CAPACITOR
The output capacitor acts to smooth the dc output voltage
and also provides energy storage. Selection of an output
capacitor, with an associated equivalent series resistance
(ESR), impacts both the amount of output ripple voltage and
stability of the control loop.
The output ripple voltage of the power supply is the product
of the capacitor ESR and the inductor ripple current. The
capacitor types recommended in the tables were selected
for having low ESR ratings.
In addition, both surface mount tantalum capacitors and
through-hole aluminum electrolytic capacitors are offered as
solutions.
Impacting frequency stability of the overall control loop, the
output capacitance, in conjunction with the inductor, creates
a double pole inside the feedback loop. In addition the
capacitance and the ESR value create a zero. These fre-
quency response effects together with the internal frequency
compensation circuitry of the LM2676 modify the gain and
phase shift of the closed loop system.
As a general rule for stable switching regulator circuits it is
desired to have the unity gain bandwidth of the circuit to be
limited to no more than one-sixth of the controller switching
frequency. With the fixed 260KHz switching frequency of the
LM2676, the output capacitor is selected to provide a unity
gain bandwidth of 40KHz maximum. Each recommended
capacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to
reduce the ESR of the output capacitor, to minimize output
ripple (a ripple voltage of 1% of Vout or less is the assumed
performance condition), or to increase the output capaci-
tance to reduce the closed loop unity gain bandwidth (to less
than 40KHz). When parallel combinations of capacitors are
required it has been assumed that each capacitor is the
exact same part type.
The RMS current and working voltage (WV) ratings of the
output capacitor are also important considerations. In a typi-
cal step-down switching regulator, the inductor ripple current
(set to be no more than 30% of the maximum load current by
the inductor selection) is the current that flows through the
output capacitor. The capacitor RMS current rating must be
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greater than this ripple current. The voltage rating of the
output capacitor should be greater than 1.3 times the maxi-
mum output voltage of the power supply. If operation of the
system at elevated temperatures is required, the capacitor
voltage rating may be de-rated to less than the nominal room
temperature rating. Careful inspection of the manufacturer’s
specification for de-rating of working voltage with tempera-
ture is important.
INPUT CAPACITOR
Fast changing currents in high current switching regulators
place a significant dynamic load on the unregulated power
source. An input capacitor helps to provide additional current
to the power supply as well as smooth out input voltage
variations.
Like the output capacitor, the key specifications for the input
capacitor are RMS current rating and working voltage. The
RMS current flowing through the input capacitor is equal to
one-half of the maximum dc load current so the capacitor
should be rated to handle this. Paralleling multiple capacitors
proportionally increases the current rating of the total capaci-
tance. The voltage rating should also be selected to be 1.3
times the maximum input voltage. Depending on the unregu-
lated input power source, under light load conditions the
maximum input voltage could be significantly higher than
normal operation and should be considered when selecting
an input capacitor.
The input capacitor should be placed very close to the input
pin of the LM2676. Due to relative high current operation
with fast transient changes, the series inductance of input
connecting wires or PCB traces can create ringing signals at
the input terminal which could possibly propagate to the
output or other parts of the circuitry. It may be necessary in
some designs to add a small valued (0.1µF to 0.47µF)
ceramic type capacitor in parallel with the input capacitor to
prevent or minimize any ringing.
CATCH DIODE
When the power switch in the LM2676 turns OFF, the current
through the inductor continues to flow. The path for this
current is through the diode connected between the switch
output and ground. This forward biased diode clamps the
switch output to a voltage less than ground. This negative
voltage must be greater than −1V so a low voltage drop
(particularly at high current levels) Schottky diode is recom-
mended. Total efficiency of the entire power supply is signifi-
cantly impacted by the power lost in the output catch diode.
The average current through the catch diode is dependent
on the switch duty cycle (D) and is equal to the load current
times (1-D). Use of a diode rated for much higher current
than is required by the actual application helps to minimize
the voltage drop and power loss in the diode.
During the switch ON time the diode will be reversed biased
by the input voltage. The reverse voltage rating of the diode
should be at least 1.3 times greater than the maximum input
voltage.
BOOST CAPACITOR
The boost capacitor creates a voltage used to overdrive the
gate of the internal power MOSFET. This improves efficiency
by minimizing the on resistance of the switch and associated
power loss. For all applications it is recommended to use a
0.01µF/50V ceramic capacitor.
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