Datasheet

LM22680
www.ti.com SNVS595K –SEPTEMBER 2008–REVISED APRIL 2013
INDUCTOR
The inductor value is determined based on the load current, ripple current, and the minimum and maximum input
voltages. To keep the application in continuous conduction mode (CCM), the maximum ripple current, I
RIPPLE
,
should be less than twice the minimum load current. The general rule of keeping the inductor current peak-to-
peak ripple around 30% of the nominal output current is a good compromise between excessive output voltage
ripple and excessive component size and cost. Using this value of ripple current, the value of inductor, L, is
calculated using the following formula:
where
• F
sw
is the switching frequency and V
in
should be taken at its maximum value, for the given application. (11)
The above formula provides a guide to select the value of the inductor L; the nearest standard value will then be
used in the circuit.
Once the inductor is selected, the actual ripple current can be found from the equation shown below:
(12)
Increasing the inductance will generally slow down the transient response but reduce the output voltage ripple.
Reducing the inductance will generally improve the transient response but increase the output voltage ripple.
The inductor must be rated for the peak current, I
PK
, in a given application, to prevent saturation. During normal
loading conditions, the peak current is equal to the load current plus 1/2 of the inductor ripple current.
During an overload condition, as well as during certain load transients, the controller may trip current limit. In this
case the peak inductor current is given by I
CL
, found in the Electrical Characteristics table. Good design practice
requires that the inductor rating be adequate for this overload condition. If the inductor is not rated for the
maximum expected current, it can saturate resulting in damage to the LM22680 and/or the power diode.
INPUT CAPACITOR
The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the regulator current
during switch on-time. Low ESR ceramic capacitors are preferred. Larger values of input capacitance are
desirable to reduce voltage ripple and noise on the input supply. This noise may find its way into other circuitry,
sharing the same input supply, unless adequate bypassing is provided. A very approximate formula for
determining the input voltage ripple is shown below:
where
• V
ri
is the peak-to-peak ripple voltage at the switching frequency. (13)
Another concern is the RMS current passing through this capacitor. The following equation gives an
approximation to this current:
(14)
The capacitor must be rated for at least this level of RMS current at the switching frequency.
All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature
coefficients. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum
capacitance up to the desired value. This may also help with RMS current constraints by sharing the current
among several capacitors. Many times it is desirable to use an electrolytic capacitor on the input, in parallel with
the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input supply caused by
long power leads. This method can also help to reduce voltage spikes that may exceed the maximum input
voltage rating of the LM22680.
Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM22680