lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 15

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

Application Information
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 capabil-
ity and their ability to withstand the heat generated since that
ripple current passes through their ESR. Input rms current is
approximately:
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 (∆I
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. How-
ever, as shown in the efficiency calculations, a smaller in-
ductor 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 charg-
ing and discharging of the gates. Generally the switching
FIGURE 11. Maximum Duty Cycle vs V
T
OUT
J
= 125˚C
). The inductance is chosen by
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frequency 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
current), 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 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-arranging 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 (∆V
during fast load transients.
In this example the output current is 4A and the expected
type of capacitor is an aluminum electrolytic, as with the
input capacitors. Other possibilities include ceramic, tanta-
lum, 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 ca-
pacitors 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 fre-
quency, 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, ∆V
put current ripple, ∆I
In this example, in order to maintain a 2% peak-to-peak
output voltage ripple and a 40% peak-to-peak inductor cur-
rent ripple, the required maximum ESR is 20mΩ. The Sanyo
IN(MAX)
is assumed to be 10% above the steady state input
OUT
OUT
+ 0.5*∆I
, is:
L = 1.6µH
OUT
) and to supply load current
OUT
OUT
) = 4.8A, for a 4A design.
and the designed out-
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