LM2648 National Semiconductor, LM2648 Datasheet - Page 18

• Manufacturer Part Number: LM2648
• Description: Synchronous Step-Down 3-Channel Switching Regulator Controller
• Manufacturer: National Semiconductor

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Component Selection
ture as well as the maximum input voltage. Input RMS ripple
current can be approximately calculated with the equation
below:
Where D1 is the duty cycle of channel 1 or 3a, whichever is
larger, and D2 is the duty cycle of channel 2 or 3b, whichever
is larger. I1 and I2 are the sum of the load currents of
channels 1 nd 3a and of channels 2 and 3b respectively.
Additionally, the input capacitor should always be placed as
close as possible to the current sense resistor.
When using ceramic input capacitors, care must be taken to
limit transient voltage spikes during turn on. These spikes
can be greater than twice the nominal input voltage and can
easily exceed the maximum rating of the device.
Using an additional tantalum or other high ESR capacitor at
the input is usually effective in dampening input spikes and
protecting the device during turn on.
OUTPUT CAPACITOR SELECTION
In applications that exhibit large and fast load current
swings, the slew rate of such a load current transient may be
beyond the response speed of the regulator. Therefore, to
meet voltage transient requirements during worst-case load
transients, special consideration should be given to output
capacitor selection. The total combined ESR of the output
capacitors must be lower than a certain value, while the total
capacitance must be greater than a certain value. Also, in
applications where the specification of output voltage regu-
lation is tight and ripple voltage must be low, starting from the
required output voltage ripple will often result in fewer design
iterations.
ALLOWED TRANSIENT VOLTAGE EXCURSION
The allowed output voltage excursion during a load transient
(∆Vc_s) is:
Where
Example: Vnom = 5V, δ% = 7%, e% = 3.4%, Vrip = 40mV
peak to peak.
Since the ripple voltage is included in the calculation of
∆Vc_s, the inductor ripple current should not be included in
the worst-case load current excursion. That is, the worst-
case load current excursion should be simply maximum load
current change specification, ∆Ic_s.
MAXIMUM ESR CALCULATION
Unless the rise and fall times of a load transient are slower
than the response speed of the control loop, if the total
combined ESR (Re) is too high, the load transient require-
ment will not be met, no matter how large the capacitance.
±
e% is the output voltage initial accuracy.
±
δ% is the output voltage regulation window and
(Continued)
18
The maximum allowed total combined ESR is:
Example: ∆Vc_s = 160mV, ∆Ic_s = 3A. Then Re_max =
53.3mΩ.
Maximum ESR criterion can be used when the associated
capacitance is high enough, otherwise more capacitors than
the number determined by this criterion should be used in
parallel.
MINIMUM CAPACITANCE CALCULATION
In a switch mode power supply, the minimum output capaci-
tance is typically dictated by the load transient requirement.
If there is not enough capacitance, the output voltage excur-
sion will exceed the maximum allowed value even if the
maximum ESR requirement is met. The worst-case load
transient is an unloading transient that happens when the
input voltage is the highest and when the present switching
cycle has just finished. The corresponding minimum capaci-
tance is calculated as follows:
Notice it is already assumed the total ESR, Re, is no greater
than Re_max, otherwise the term under the square root will
be a negative value. Also, it is assumed that L has already
been selected, therefore the minimum L value should be
calculated before Cmin and after Re (see Inductor Selection
below). Example: Re = 20mΩ, Vnom = 5V, ∆Vc_s = 160mV,
∆Ic_s = 3A, L = 8µH
Generally speaking, Cmin decreases with decreasing Re,
∆Ic_s, and L, but with increasing Vnom and ∆Vc_s.
Inductor Selection
The size of the output inductor can be determined from the
desired output ripple voltage, Vrip, and the impedance of the
output capacitors at the switching frequency. The equation to
determine the minimum inductance value is as follows:
In the above equation, Re is used in place of the impedance
of the output capacitors. This is because in most cases, the
impedance of the output capacitors at the switching fre-
quency is very close to Re. In the case of ceramic capaci-
tors, replace Re with the true impedance.
The actual selection process usually involves several itera-
tions of all of the above steps, from ripple voltage selection,
to capacitor selection, to inductance calculations. Both the
highest and the lowest input and output voltages and load