LM26001B datasheet(13/18 Pages) NSC

13 / 18 page

For example, at a maximum load of 1.5A and a ripple content

of 33%, peak inductor current is equal to 1.75A which is safely

below the minimum current limit of 1.80A. 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 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%.

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:

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

MAX.

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 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

LM26001B. 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 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.

BOOTSTRAP

The drive voltage for the internal switch is supplied via the

BOOT pin. This pin must be connected to a ceramic capacitor,

Cboot, from the switch node, shown as C4 in the typical ap-

plication. The LM26001B provides the VDD voltage internally,

so no external diode is needed. A minimum value of 0.1 uF is

recommended for Cboot. Smaller values may result in insuf-

ficient hold up time for the drive voltage and increased power

dissipation.

During low Vin operation, when the on-time is extended, the

bootstrap capacitor is at risk of discharging. If the Cboot ca-

pacitor is discharged below approximately 2.5V, the

LM26001B enters a high frequency re-charge mode. The

Cboot cap is re-charged via the LG synchronous FET shown

in the block diagram. Switching returns to normal when the

Cboot cap has been recharged.

CATCH DIODE

When the internal switch is off, output current flows through

the catch diode. Alternately, when the switch is on, the diode

sees a reverse voltage equal to Vin. Therefore, the important

parameters for selecting the catch diode are peak current and

peak inverse voltage. The average current through the diode

is given by:

AVE = Iload x (1-D)

Where D is the duty cycle, defined as Vout/Vin. The catch

diode conducts the largest currents during the lowest duty

cycle. Therefore ID

AVE should be calculated assuming maxi-

mum input voltage. The diode should be rated to handle this

current continuously. For over-current or short circuit condi-

tions, the catch diode should be rated to handle peak currents

equal to the peak current limit.

The peak inverse voltage rating of the diode must be greater

than maximum input voltage.

A Schottky diode must be used. It's low forward voltage max-

imizes efficiency and BOOT voltage, while also protecting the

SW pin against large negative voltage spikes

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