LM5642MH NSC [National Semiconductor], LM5642MH Datasheet - Page 18

• Manufacturer Part Number: LM5642MH
• Description: High Voltage, Dual Synchronous Buck Converter with Oscillator Synchronization
• Manufacturer: NSC [National Semiconductor]

www.national.com
Output Capacitor Selection
In applications that exhibit large, fast load current swings, the
slew rate of such a load current transient will likely be beyond
the response speed of the regulator. Therefore, to meet volt-
age transient requirements during worst-case load transients,
special consideration should be given to output capacitor se-
lection. 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 regulation 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
(ΔVc_s) is:
Where ±δ% is the output voltage regulation window and ±ε%
is the output voltage initial accuracy.
Example: Vnom = 5V, δ% = 7%, ε% = 3.4%, Vrip = 40mV peak
to peak.
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 com-
bined ESR (Re) is too high, the load transient requirement will
not be met, no matter how large the capacitance.
The maximum allowed total combined ESR is:
Since the ripple voltage is included in the calculation of
ΔVc_s, the inductor ripple current should not be included in
the worst-case load current excursion. Simply use the worst-
case load current excursion for ΔIc_s.
Example: ΔVc_s = 160mV, ΔIc_s = 3A. Then Re_max =
53.3mΩ.
FIGURE 10. Output Voltage Range
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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 tran-
sient is an unloading transient that happens when the input
voltage is the highest and when the current switching cycle
has just finished. The corresponding minimum capacitance 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 cal-
culated before C
below). Example: Re = 20mΩ, Vnom = 5V, ΔVc_s = 160mV,
ΔIc_s = 3A, L = 8µH
Generally speaking, C
ΔIc_s, and L, but with increasing Vnom and ΔVc_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 frequency
is very close to Re. In the case of ceramic capacitors, replace
Re with the true impedance at the switching frequency.
Example: Vin = 36V, Vo = 3.3V, V
F = 200kHz.
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
transient requirements should be considered. If an induc-
tance value larger than Lmin is selected, make sure that the
Cmin requirement is not violated.
Priority should be given to parameters that are not flexible or
more costly. For example, if there are very few types of ca-
pacitors to choose from, it may be a good idea to adjust the
inductance value so that a requirement of 3.2 capacitors can
be reduced to 3 capacitors.
min
and after Re (see Inductor Selection
min
decreases with decreasing Re,
RIP
= 60mV, Re = 20mΩ,
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