LM2743-19AEVAL National Semiconductor, LM2743-19AEVAL Datasheet - Page 15

• Manufacturer Part Number: LM2743-19AEVAL
• Description: BOARD EVALUATION LM2743-19A
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2743-19AEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2 ~ 3.3V
• Current - Output: 19A
• Voltage - Input: 8 ~ 14V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2743
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

followed to create many other designs with varying input volt-
ages, output voltages, and load currents.
Duty Cycle Calculation
The complete duty cycle for a buck converter is defined with
the following equation:
where V
drops that develop across the low side and high side MOS-
FETs. Assuming the inductor ripple current is 20% to 30% of
the output current, therefore:
To calculate the maximum duty cycle use the estimated 'hot'
R
and maximum load. As shown in
maximum duty cycles of the LM2743 occurs at 125°C junction
temperature vs V
the operating duty cycle is below the curve in
condition is not satisfied, the system will be unable to develop
the required duty cycle to derive the necessary system power
and so the output voltage will fall out of regulation.
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:
The power dissipated by each input capacitor is:
DS(on)
value of the MOSFETs, the minimum input voltage,
FIGURE 12. Maximum Duty Cycle vs V
SWL
and V
V
CC
V
SWH
SWL
(High-Side MOSFET)
SWH
(Low-Side MOSFET)
(IC control section voltage). Ensure that
= I
= I
are the respective forward voltage
T
OUT
OUT
J
= 125°C
x R
x R
DS(ON)HIGH
DS(ON)LOW
Figure
12, the worst case
Figure
20095291
CC
12, if this
15
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-
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
The Coilcraft DO3316P-222P is 2.2 µH, is rated to 7.4A peak,
and has a direct current resistance (DCR) of 12 mΩ.
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
IN(MAX)
is assumed to be 10% above the steady state input
OUT
+ (0.5 x ΔI
OUT
L = 1.6µH
). The inductance is chosen by
OUT
)) = 4.8A, for a 4A design.
www.national.com