LM2678
Application Hints
(Continued)
In some cases multiple capacitors are required either to re-
duce 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 ex-
act 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
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 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 LM2678. 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 out-
put or other parts of the circuitry. It may be necessary in
some designs to add a small valued (0.1µF to 0.47µF) ce-
ramic type capacitor in parallel with the input capacitor to
prevent or minimize any ringing.
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 out-
put 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 recommended. Total
efficiency of the entire power supply is significantly impacted
by the power lost in the output catch diode. The average cur-
rent 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.
11
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.
SIMPLE DESIGN PROCEDURE
Using the nomographs and tables in this data sheet (or use
the available design software at http://www.national.com) a
complete step-down regulator can be designed in a few
simple steps.
Step 1:
Define the power supply operating conditions:
Required output voltage
Maximum DC input voltage
Maximum output load current
Step 2:
Set the output voltage by selecting a fixed output
LM2678 (3.3V, 5V or 12V applications) or determine the re-
quired feedback resistors for use with the adjustable
LM2678−ADJ
Step 3:
Determine the inductor required by using one of the
four nomographs,
Figure 3
through
Figure 6.
Table 1 pro-
vides a specific manufacturer and part number for the induc-
tor.
Step 4:
Using Table 3 (fixed output voltage) or Table 6 (ad-
justable output voltage), determine the output capacitance
required for stable operation. Table 2 provides the specific
capacitor type from the manufacturer of choice.
Step 5:
Determine an input capacitor from Table 4 for fixed
output voltage applications. Use Table 2 to find the specific
capacitor type. For adjustable output circuits select a capaci-
tor from Table 2 with a sufficient working voltage (WV) rating
greater than Vin max, and an rms current rating greater than
one-half the maximum load current (2 or more capacitors in
parallel may be required).
Step 6:
Select a diode from Table 5. The current rating of the
diode must be greater than I load max and the Reverse Volt-
age rating must be greater than Vin max.
Step 7:
Include a 0.01µF/50V capacitor for Cboost in the de-
sign.
FIXED OUTPUT VOLTAGE DESIGN EXAMPLE
A system logic power supply bus of 3.3V is to be generated
from a wall adapter which provides an unregulated DC volt-
age of 13V to 16V. The maximum load current is 4A.
Through-hole components are preferred.
Step 1:
Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 4A
Step 2:
Select an LM2678T-3.3. The output voltage will have
a tolerance of
±
2% at room temperature and
±
3% over the full operating
temperature range.
Step 3:
Use the nomograph for the 3.3V device ,Figure
3.
The intersection of the 16V horizontal line (V
in
max) and the
4A vertical line (I
load
max) indicates that L46, a 15µH induc-
tor, is required.
From Table 1, L46 in a through-hole component is available
from Renco with part number RL-1283-15-43.
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