LM26003MH NSC [National Semiconductor], LM26003MH Datasheet - Page 12

• Manufacturer Part Number: LM26003MH
• Description: 3A Switching Regulator with High Efficiency Sleep Mode
• Manufacturer: NSC [National Semiconductor]

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INDUCTOR
The output inductor should be selected based on inductor
ripple current. The amount of inductor ripple current com-
pared to load current, or ripple content, is defined as Iripple/
Iload. Ripple content should be less than 40%. Inductor ripple
current, Iripple, can be calculated as shown below:
Larger ripple content increases losses in the inductor and re-
duces the effective current limit.
Larger inductance values result in lower output ripple voltage
and higher efficiency, but a slightly degraded transient re-
sponse. Lower inductance values allow for smaller case size,
but the increased ripple lowers the effective current limit
threshold.
Remember that inductor value also affects the sleep mode
threshold as shown in Figure 3.
When choosing the inductor, the saturation current rating
must be higher than the maximum peak inductor current and
the RMS current rating should be higher than the maximum
load current. Peak inductor current, Ipeak, is calculated as:
For example, at a maximum load of 3A and a ripple content
of 10%, peak inductor current is equal to 3.15A which is safely
at the minimum current limit of 3.15A. By increasing the in-
ductor size, ripple content and peak inductor current are
lowered, which increases the current limit margin.
The size of the output inductor can also be determined using
the desired output ripple voltage, Vrip. The equation to deter-
mine the minimum inductance value based on Vrip is as
follows:
Where Re is the ESR of the output capacitors, and Vrip is a
peak-to-peak value. This equation assumes that the output
capacitors have some amount of ESR. It does not apply to
ceramic output capacitors.
If this method is used, ripple content should still be verified to
be less than 40% and that the peak currents do not exceed
the minimum current threshold.
OUTPUT CAPACITOR
The primary criterion for selecting an output capacitor is
equivalent series resistance, or ESR.
ESR (Re) can be selected based on the requirements for out-
put ripple voltage and transient response. Once an inductor
value has been selected, ripple voltage can be calculated for
a given Re using the equation above for LMIN. Lower ESR
values result in lower output ripple.
Re can also be calculated from the following equation:
12
Where ΔVt is the allowed voltage excursion during a load
transient, and ΔIt is the maximum expected load transient.
If the total ESR is too high, the load transient requirement
cannot be met, no matter how large the output capacitance.
If the ESR criteria for ripple voltage and transient excursion
cannot be met, more capacitors should be used in parallel.
For non-ceramic capacitors, the minimum output capacitance
is of secondary importance, and is determined only 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 minimum capaci-
tance is calculated as follows:
It is assumed the total ESR, Re, is no greater than Re
Also, it is assumed that L has already been selected.
Generally speaking, the output capacitance requirement de-
creases with Re, ΔIt, and L. A typical value greater than 120
µF works well for most applications.
INPUT CAPACITOR
In a switching converter, very fast switching pulse currents
are drawn from the input rail. Therefore, input capacitors are
required to reduce noise, EMI, and ripple at the input to the
LM26003. Capacitors must be selected that can handle both
the maximum ripple RMS current at highest ambient temper-
ature as well as the maximum input voltage. The equation for
calculating the RMS input ripple current is shown below:
For noise suppression, a ceramic capacitor in the range of 1.0
µF to 10 µF should be placed as close as possible to the PVIN
pin. For the AVIN pin also some decoupling is necessary. It
is very important that the pin is decoupled with such a capac-
itor close to the AGND pin and the GND pin of the IC to avoid
switching noise to couple into the IC. Also some RC input fil-
tering can be implemented using a small resistor between
PVIN and AVIN. In figure 7 the resistor value of R7 is selected
to be 0Ω but can be increased to filter with different time con-
stants depending on the capacitor value used. When using a
R7 resistor, keep in mind that the resistance will increase the
minimum input voltage threshold due to the voltage drop
across the resistor.
The PVIN decoupling should be implemented in a way to
minimize the trace length between the Cin capacitor gnd and
the Schottky diode gnd. A larger, high ESR input capacitor
should also be used. This capacitor is recommended for
damping input voltage spikes during power on and for holding
up the input voltage during transients. In low input voltage
applications, line transients may fall below the UVLO thresh-
old if there is not enough input capacitance. Both tantalum
and electrolytic type capacitors are suitable for the bulk ca-
pacitor. However, large tantalums may not be available for
high input voltages and their working voltage must be derated
by at least 2X.
MAX
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