Datasheet

C
MIN
=
('Vt)
2
- ('It x R
e
)
2
'Vt -
L x
©
§
©
§
V
out
x R
e
2
'Vt
'It
Re
MAX
=
(Vin ± Vout) x Vout x Re
Vin x fsw x Vrip
L
MIN
=
Iripple
2
Ipeak = Iload +
LM26003
www.ti.com SNVS576D –AUGUST 2008–REVISED MARCH 2013
Larger inductance values result in lower output ripple voltage and higher efficiency, but a slightly degraded
transient response. 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 15.
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:
(11)
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 inductor 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 determine the minimum inductance value based on Vrip is as follows:
(12)
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 output 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:
(13)
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 excursion will exceed the maximum allowed value even if
the maximum ESR requirement is met. The minimum capacitance is calculated as follows:
(14)
It is assumed the total ESR, Re, is no greater than Re
MAX
. Also, it is assumed that L has already been selected.
Generally speaking, the output capacitance requirement decreases with Re, ΔIt, and L. A typical value greater
than 120 µF works well for most applications.
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