Datasheet

L =
3.3V - 1.2V
0.4 x 4A x 300 kHz
x
1.2V
3.3V
L =
V
IN
- V
OUT
'I
OUT
x f
SW
x D
P
CAP
=
(I
RMS_RIP
)
2
x ESR
n
2
I
RMS_RIP
= I
OUT
x
D(1 - D)
LM2743
www.ti.com SNVS276G –APRIL 2004–REVISED MARCH 2013
To calculate the maximum duty cycle use the estimated 'hot' R
DS(on)
value of the MOSFETs, the minimum input
voltage, and maximum load. As shown in Figure 34, the worst case maximum duty cycles of the LM2743 occurs
at 125°C junction temperature vs V
CC
(IC control section voltage). Ensure that the operating duty cycle is below
the curve in Figure 34, if this 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.
Figure 34. Maximum Duty Cycle vs V
CC
T
J
= 125°C
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:
where n is the number of capacitors, and ESR is the equivalent 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
OUT
). The inductance
is chosen by selecting between tradeoffs in efficiency and response time. The smaller the output inductor, the
more quickly the converter can respond to transients in the load current. However, as shown in the efficiency
calculations, a smaller inductor requires 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 frequency is chosen so that conduction loss outweighs switching loss. The
equation for output inductor selection is:
L = 1.6µH
Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LM2743