LM27402 datasheet(14/32 Pages) NSC

14 / 32 page

Design Guide

The Design Guide assists the designer with the steps neces-

sary to select the external components to build a fully func-

tional power supply. As with any DC-DC converter numerous

tradeoffs are possible to optimize the design for efficiency,

size, or performance. These tradeoffs will be taken into ac-

count and highlighted throughout the discussion. To facilitate

component selection, the circuit shown in Figure 4 below may

be used as a reference. Unless otherwise indicated, all for-

mulae assume units of Amps (A) for current, Farads (F) for

capacitance, Henries (H) for inductance and Volts (V) for volt-

age.

30092601

FIGURE 4. Typical Application Circuit

The above schematic shows R

F and CF acting as an RC filter

to the input of the LM27402. The filter is used to attenuate

voltage ripple that may exist on the input rail particularly dur-

ing high output currents. The recommended values of R

F and

F are 2.2Ω and 1 µF, respectively. There is a practical limit

to the size of R

F as it can cause a large voltage drop if large

operating bias currents are present. The VIN pin of the

LM27402 should not exceed 150 mV difference from the input

voltage rail (V

IN).

The first equation to calculate for any buck converter is duty

ratio:

Due to the resistive powertrain losses, the duty ratio will in-

crease based on the overall efficiency,

η. Calculation of η can

be found in the POWER/EFFICIENCY CALCULATIONS sec-

tion of this datasheet.

INDUCTOR SELECTION (L)

The inductor value is determined based on the operating fre-

quency, load current, ripple current, and duty ratio. The se-

lected inductor should have a saturation current rating greater

than the peak current limit of the LM27402. To optimize the

performance, the inductance is typically selected such that

the ripple current,

ΔI

L, is between 20% and 40% of the rated

output current. Figure 5 illustrates the switch voltage and in-

ductor ripple current waveforms. Once the nominal input volt-

age, output voltage, operating frequency, and desired ripple

current are known, the minimum inductance value can be

calculated by:

30092670

FIGURE 5. Switch Voltage and Inductor Current

Waveforms

The peak inductor current at maximum load, I

OUT + ΔIL/2,

should be kept adequately below the peak current limit set-

point of the device.

OUTPUT CAPACITOR SELECTION (C

OUT)

The output capacitor, C

OUT, filters the inductor ripple current

and provides a source of charge for transient load events. A

wide range of output capacitors may be used with the

LM27402 that provide excellent performance. The best per-

formance is typically obtained using ceramic, tantalum, or

electrolytic type chemistries. Typically, ceramic capacitors

provide extremely low ESR to reduce the output ripple voltage

and noise spikes, while tantalum and electrolytic capacitors

provide a large bulk capacitance in a small size for transient

loading events. When selecting the output capacitance value,

the two performance characteristics to consider are the output

voltage ripple and transient response. The output voltage rip-

ple can be approximated by:

where

ΔV

OUT (V) is the amount of peak to peak voltage ripple

at the power supply output, R

ESR (Ω) is the series resistance

of the output capacitor, f

SW (Hz) is the switching frequency,

and C

OUT (F) is the output capacitance used in the design.

The amount of output ripple that can be tolerated is applica-

tion specific; however a general recommendation is to keep

the output ripple less than 1% of the rated output voltage. Note

that ceramic capacitors are sometimes preferred because

they have very low ESR; however, depending on package

and voltage rating of the capacitor, the value of capacitance

can drop significantly with applied voltage and operating tem-

perature.

The output capacitor will affect the output voltage droop dur-

ing a load transient. The peak output voltage deviation is

dependent on many factors such as output capacitance, out-

put capacitor ESR, inductor size, control loop bandwidth,

powertrain parasitics, etc. Given sufficient control loop band-

LM27402 Datasheet(PDF) 14 Page - National Semiconductor (TI)

LM27402 Datasheet(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL