Datasheet

Irms =
Vin
Vout x (Vin ± Vout)
Iload x
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
=
LM26001, LM26001Q
SNVS430G –MAY 2006–REVISED MAY 2006 www.ti.com
For example, at a maximum load of 1.5A and a ripple content of 40%, peak inductor current is equal to 1.8A
which is safely below the minimum current limit of 1.85A. 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%.
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 100 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 LM26001. Capacitors must be
selected that can handle both the maximum ripple RMS current at highest ambient temperature as well as the
maximum input voltage. The equation for calculating the RMS input ripple current is shown below:
(15)
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 VIN pin.
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 threshold if there is not enough input capacitance. Both
tantalum and electrolytic type capacitors are suitable for the bulk capacitor. However, large tantalums may not be
available for high input voltages and their working voltage must be derated by at least 2X.
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