LM2744EVAL National Semiconductor, LM2744EVAL Datasheet - Page 14

• Manufacturer Part Number: LM2744EVAL
• Description: BOARD EVALUATION LM2744
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2744EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 3.5A
• Voltage - Input: 1.8 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2744
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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connected directly to V
to V
DESIGN CONSIDERATIONS
The following is a design procedure for all the components
needed to create the Typical Application Circuit shown on the
front page. This design converts 3.3V (V
a maximum load of 4A with an efficiency of 89% and a switch-
ing frequency of 300kHz. The same procedures can be fol-
lowed to create many other designs with varying input
voltages, output voltages, and load currents.
Input Capacitor
The input capacitors in a Buck converter are subjected to high
stress due to the input current trapezoidal waveform. Input
capacitors are selected for their ripple current capability and
their ability to withstand the heat generated since that ripple
current passes through their ESR. Input rms ripple current is
approximately:
where duty cycle D = V
The power dissipated by each input capacitor is:
where n is the number of capacitors, and ESR is the equiva-
lent series resistance of each capacitor. The equation above
indicates that power loss in each capacitor decreases rapidly
as the number of input capacitors increases. The worst-case
ripple for a Buck converter occurs during full load and when
the duty cycle (D) is 0.5. For this 3.3V to 1.2V design the duty
cycle is 0.364. For a 4A maximum load the ripple current is
1.92A.
Output Inductor
The output inductor forms the first half of the power stage in
a Buck converter. It is responsible for smoothing the square
wave created by the switching action and for controlling the
output current ripple (ΔI
selecting between tradeoffs in efficiency and response time.
The smaller the output inductor, the more quickly the con-
verter can respond to transients in the load current. However,
as shown in the efficiency calculations, a smaller inductor re-
quires a higher switching frequency to maintain the same
level of output current ripple. An increase in frequency can
mean increasing loss in the MOSFETs due to the charging
and discharging of the gates. Generally the switching fre-
quency is chosen so that conduction loss outweighs switching
loss. The equation for output inductor selection is:
Here we have plugged in the values for output current ripple,
input voltage, output voltage, switching frequency, and as-
CC
(see the Electrical Characteristics table).
CC
OUT
OUT
L = 1.6µH
or to another voltage between 1.3V
/V
). The inductance is chosen by
IN
.
IN
) to 1.2V (V
OUT
) at
14
sumed a 40% peak-to-peak output current ripple. This yields
an inductance of 1.6 µH. The output inductor must be rated
to handle the peak current (also equal to the peak switch cur-
rent), which is (I
Coilcraft DO3316P-222P is 2.2 µH, is rated to 7.4A peak, and
has a direct current resistance (DCR) of 12mΩ.
After selecting an output inductor, inductor current ripple
should be re-calculated with the new inductance value, as this
information is needed to select the output capacitor. Re-ar-
ranging the equation used to select inductance yields the
following:
V
voltage, or 3.6V. The actual current ripple will then be 1.2A.
Peak inductor/switch current will be 4.6A.
Output Capacitor
The output capacitor forms the second half of the power stage
of a Buck switching converter. It is used to control the output
voltage ripple (ΔV
load transients.
In this example the output current is 4A and the expected type
of capacitor is an aluminum electrolytic, as with the input ca-
pacitors. Other possibilities include ceramic, tantalum, and
solid electrolyte capacitors, however the ceramic type often
do not have the large capacitance needed to supply current
for load transients, and tantalums tend to be more expensive
than aluminum electrolytic. Aluminum capacitors tend to have
very high capacitance and fairly low ESR, meaning that the
ESR zero, which affects system stability, will be much lower
than the switching frequency. The large capacitance means
that at the switching frequency, the ESR is dominant, hence
the type and number of output capacitors is selected on the
basis of ESR. One simple formula to find the maximum ESR
based on the desired output voltage ripple, ΔV
designed output current ripple, ΔI
In this example, in order to maintain a 2% peak-to-peak output
voltage ripple and a 40% peak-to-peak inductor current ripple,
the required maximum ESR is 20mΩ. The Sanyo 4SP560M
electrolytic capacitor will give an equivalent ESR of 14mΩ.
The capacitance of 560 µF is enough to supply energy even
to meet severe load transient demands.
MOSFETs
Selection of the power MOSFETs is governed by a tradeoff
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. Breaking down the
losses in the high-side and low-side MOSFETs and then cre-
ating spreadsheets is one way to determine relative efficien-
cies between different MOSFETs. Good correlation between
the prediction and the bench result is not guaranteed, how-
ever. Single-channel buck regulators that use a controller IC
and discrete MOSFETs tend to be most efficient for output
currents of 2-10A.
IN(MAX)
is assumed to be 10% above the steady state input
OUT
OUT
+ 0.5*ΔI
) and to supply load current during fast
OUT
) = 4.8A, for a 4A design. The
OUT
, is:
OUT
and the