LM3743_08 Datasheet, PDF (18/28 Pages) National Semiconductor (TI) – High-Performance Synchronous Buck Controller with Comprehensive Fault Protection Features

chosen by selecting between trade-offs in efficiency, size, and
response time. The recommended percentage of AC compo-
nent to DC current is 30% to 40%, this will provide the best
trade-off between energy requirements and size, (read
AN-1197 for theoretical analysis). Another criteria is the ability
to respond to large load transient responses; the smaller the
output inductor, the more quickly the converter can respond.
The equation for output inductor selection is:
Here we have plugged in the values for input voltage, output
voltage, switching frequency, and 30% of the maximum load
current. This yields an inductance of 1.34 µH. The output in-
ductor must be rated to handle the peak current (also equal
to the peak switch current), which is (I_OUT + (0.5 x ΔI_OUT)) =
11.5A, for a 10A design and a AC current of 3A.
The Coiltronics DR125–1R5 is 1.5 µH, is rated to 13.8A RMS
current, and has a direct current resistance (DCR) of 3 mΩ.
After selecting the Coiltronics DR125–1R5 for the output in-
ductor, actual inductor current ripple must be re-calculated
with the selected inductance value. This information is need-
ed to determine the RMS current through the input and output
capacitors. Re-arranging the equation used to select induc-
tance yields the following:
number and type of capacitors used depends mainly on their
size and cost. One exception to this is multi-layer ceramic ca-
pacitors. MLCCs have very low ESR, but also low capaci-
tance in comparison with other types. This makes them
attractive for lower power designs. For higher power or for fast
load transients the number of MLCCs needed often increases
the size and cost to unacceptable levels. Because the load
could transition quickly from 0 to 10A, more bulk capacitance
is needed than the MLCCs can provide. One compromise is
a solid electrolytic POSCAP from Sanyo or SP-caps from
Panasonic. POSCAP and SPcaps often have large capaci-
tances needed to supply currents for load transients, and low
ESRs. The 6TPD470M by Sanyo has 470 µF, and a maximum
ESR of 10 mΩ. Solid electrolytics have stable ESR relative to
temperature, and capacitance change is relatively immune to
bias voltage. Tantalums (Ta), niobium (Nb), and Al-E are
good solutions for ambient operating temperatures above 0°
C, however their ESR tends to increase quickly below 0°C
ambient operating temperature, so these capacitor types are
not recommended for this area of operation.
Input Capacitor
The input capacitors in a buck converter are subjected to high
RMS current stress. Input capacitors are selected for their
ability to withstand the heat generated by the RMS current
and the ESR as specified by the manufacturer. Input RMS
ripple current is approximately:
Where duty cycle D = V_OUT/V_IN. The worst-case ripple for a
buck converter occurs during full load and when the duty cycle
(D) is 0.5.
When multiple capacitors of the same type and value are par-
alleled, the power dissipated by each input capacitor is:
V_IN(MAX) is assumed to be 10% above the steady state input
voltage, or 5.5V at V_IN = 5.0V. The re-calculated current ripple
will then be 2.69A. This gives a peak inductor/switch current
will be 11.35A.
Output Capacitor
The output capacitor in a switching regulator is selected on
the basis of capacitance, equivalent series resistance (ESR),
size, and cost. In this example the output current is 10A and
the expected type of capacitor is an aluminum electrolytic, as
with the input capacitors. An important specification in switch-
ing converters is the output voltage ripple ΔV_OUT. At 300 kHz
the impedance of most capacitors is very small compared to
ESR, hence ESR becomes the main selection criteria. In this
design the load requires a 2% ripple , which results in a
ΔV_OUT of 36 mV_P-P. Thus the maximum ESR is then:
ESR_MAX is 13 mΩ. Aluminum electrolytic (Al-E), tantalum
(Ta), solid aluminum, organic, and niobium (Nb) capacitors
are all popular in switching converters. In general, by the time
enough capacitors have been paralleled to obtain the desired
ESR, the bulk capacitance is more than enough to supply the
load current during a transient from no-load to full load. The
where n is the number of paralleled capacitors, and ESR is
the equivalent series resistance of each capacitor. The equa-
tion above indicates that power loss in each capacitor de-
creases rapidly as the number of input capacitors increases.
For this 5V to 1.8V design the duty cycle is 0.36. For a 10A
maximum load the RMS current is 4.8A.
Connect one or two 22 µF MLCC as close as possible across
the drain of the high-side MOSFET and the source of the low-
side MOSFET, this will provide high frequency decoupling
and satisfy the RMS stress. A bulk capacitor is recommended
in parallel with the MLCC in order to prevent switching fre-
quency noise from reflecting back into the input line, this
capacitor should be no more than 1inch away from the MLCC
capacitors.
MOSFETs
Selection of the power MOSFETs is governed by a trade-off
between cost, size, and efficiency. One method is to deter-
mine the maximum cost that can be endured, and then select
the most efficient device that fits that price. Using a spread-
sheet to estimate the losses in the high-side and low-side
MOSFETs is one way to determine relative efficiencies be-
tween different MOSFETs. Good correlation between the
prediction and the bench result is not guaranteed.
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