to reduce the closed loop unity gain bandwidth (to less than
40KHz). When parallel combinations of capacitors are re-
quired it has been assumed that each capacitor is the exact
same part type.
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.
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
greater than this ripple current. The voltage rating of the out-
put capacitor should be greater than 1.3 times the maximum
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 tem-
perature rating. Careful inspection of the manufacturer's
specification for de-rating of working voltage with temperature
is important.
ADDITIONAL APPLICATION INFORMATION
When the output voltage is greater than approximately 6V,
and the duty cycle at minimum input voltage is greater than
approximately 50%, the designer should exercise caution in
selection of the output filter components. When an application
designed to these specific operating conditions is subjected
to a current limit fault condition, it may be possible to observe
a large hysteresis in the current limit. This can affect the out-
put voltage of the device until the load current is reduced
sufficiently to allow the current limit protection circuit to reset
itself.
Under current limiting conditions, the LM267x is designed to
respond in the following manner:
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 vari-
ations.
1. At the moment when the inductor current reaches the
current limit threshold, the ON-pulse is immediately
terminated. This happens for any application condition.
2. However, the current limit block is also designed to
momentarily reduce the duty cycle to below 50% to avoid
subharmonic oscillations, which could cause the inductor
to saturate.
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 capac-
itance. 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 nor-
mal operation and should be considered when selecting an
input capacitor.
3. Thereafter, once the inductor current falls below the
current limit threshold, there is a small relaxation time
during which the duty cycle progressively rises back
above 50% to the value required to achieve regulation.
If the output capacitance is sufficiently ‘large’, it may be pos-
sible that as the output tries to recover, the output capacitor
charging current is large enough to repeatedly re-trigger the
current limit circuit before the output has fully settled. This
condition is exacerbated with higher output voltage settings
because the energy requirement of the output capacitor
varies as the square of the output voltage (½CV2), thus re-
quiring an increased charging current.
The input capacitor should be placed very close to the input
pin of the LM2678. Due to relative high current operation with
fast transient changes, the series inductance of input con-
necting 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 de-
signs to add a small valued (0.1μF to 0.47μF) ceramic type
capacitor in parallel with the input capacitor to prevent or min-
imize any ringing.
A simple test to determine if this condition might exist for a
suspect application is to apply a short circuit across the output
of the converter, and then remove the shorted output condi-
tion. In an application with properly selected external compo-
nents, the output will recover smoothly.
Practical values of external components that have been ex-
perimentally found to work well under these specific operating
conditions are COUT = 47µF, L = 22µH. It should be noted that
even with these components, for a device’s current limit of
ICLIM, the maximum load current under which the possibility of
the large current limit hysteresis can be minimized is ICLIM/2.
For example, if the input is 24V and the set output voltage is
18V, then for a desired maximum current of 1.5A, the current
limit of the chosen switcher must be confirmed to be at least
3A.
CATCH DIODE
When the power switch in the LM2678 turns OFF, the current
through the inductor continues to flow. The path for this cur-
rent is through the diode connected between the switch output
and ground. This forward biased diode clamps the switch out-
put 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 recommended. Total effi-
ciency of the entire power supply is significantly 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.
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