Datasheet

V
IN(MAX)
- V
O
'I
OUT
=
f
SW
x L
ACTUAL
x D
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)
LM2745, LM2748
SNOSAL2E –APRIL 2005–REVISED APRIL 2013 www.ti.com
DESIGN CONSIDERATIONS
The following is a design procedure for all the components needed to create the Typical Application Circuit. This
design converts 3.3V (V
IN
) to 1.2V (V
OUT
) at a maximum load of 4A with an efficiency of 89% and a switching
frequency of 300 kHz. The same procedures can be followed 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
OUT
/V
IN
.
The power dissipated by each input capacitor is:
where n is the number of paralleled 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 = 1.6 µH
Here we have plugged in the values for output current ripple, input voltage, output voltage, switching frequency,
and assumed 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 current), which is (I
OUT
+ (0.5 x ΔI
OUT
)) =
4.8A, for a 4A design.
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 the Coilcraft DO3316P-222P for the output inductor, actual inductor current ripple should be
re-calculated with the selected inductance value, as this information is needed to select the output capacitor. Re-
arranging the equation used to select inductance yields the following:
V
IN(MAX)
is assumed to be 10% above the steady state input voltage, or 3.6V at V
IN
= 3.3V. The re-calculated
current ripple will then be 1.2A. This gives a peak inductor/switch current will be 4.6A.
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