LM2679 datasheet(11/24 Pages) NSC

11 / 24 page

Application Hints (Continued)

In some cases multiple capacitors are required either to re-

duce the ESR of the output capacitor, to minimize output

ripple (a ripple voltage of 1% of Vout or less is the assumed

performance condition), or to increase the output capaci-

tance to reduce the closed loop unity gain bandwidth (to less

than 40KHz). When parallel combinations of capacitors are

required it has been assumed that each capacitor is the ex-

act same part type.

The RMS current and working voltage (WV) ratings of the

output capacitor are also important considerations. In a typi-

cal step-down switching regulator, the inductor ripple current

(set to be no more than 30% of the maximum load current by

the inductor selection) is the current that flows through the

output capacitor. The capacitor RMS current rating must be

greater than this ripple current. The voltage rating of the out-

put capacitor should be greater than 1.3 times the maximum

output voltage of the power supply. If operation of the system

at elevated temperatures is required, the capacitor voltage

rating may be de-rated to less than the nominal room tem-

perature rating. Careful inspection of the manufacturer’s

specification for de-rating of working voltage with tempera-

ture is important.

INPUT CAPACITOR

Fast changing currents in high current switching regulators

place a significant dynamic load on the unregulated power

source. An input capacitor helps to provide additional current

to the power supply as well as smooth out input voltage

variations.

Like the output capacitor, the key specifications for the input

capacitor are RMS current rating and working voltage. The

RMS current flowing through the input capacitor is equal to

one-half of the maximum dc load current so the capacitor

should be rated to handle this. Paralleling multiple capacitors

proportionally increases the current rating of the total capaci-

tance. The voltage rating should also be selected to be 1.3

times the maximum input voltage. Depending on the unregu-

lated input power source, under light load conditions the

maximum input voltage could be significantly higher than

normal operation and should be considered when selecting

an input capacitor.

The input capacitor should be placed very close to the input

pin of the LM2679. Due to relative high current operation

with fast transient changes, the series inductance of input

connecting wires or PCB traces can create ringing signals at

the input terminal which could possibly propagate to the out-

put or other parts of the circuitry. It may be necessary in

some designs to add a small valued (0.1µF to 0.47µF) ce-

ramic type capacitor in parallel with the input capacitor to

prevent or minimize any ringing.

CATCH DIODE

When the power switch in the LM2679 turns OFF, the current

through the inductor continues to flow. The path for this cur-

rent is through the diode connected between the switch out-

put and ground. This forward biased diode clamps the switch

output to a voltage less than ground. This negative voltage

must be greater than −1V so a low voltage drop (particularly

at high current levels) Schottky diode is recommended. Total

efficiency of the entire power supply is significantly impacted

by the power lost in the output catch diode. The average cur-

rent through the catch diode is dependent on the switch duty

cycle (D) and is equal to the load current times (1-D). Use of

a diode rated for much higher current than is required by the

actual application helps to minimize the voltage drop and

power loss in the diode.

During the switch ON time the diode will be reversed biased

by the input voltage. The reverse voltage rating of the diode

should be at least 1.3 times greater than the maximum input

voltage.

BOOST CAPACITOR

The boost capacitor creates a voltage used to overdrive the

gate of the internal power MOSFET. This improves efficiency

by minimizing the on resistance of the switch and associated

power loss. For all applications it is recommended to use a

0.01µF/50V ceramic capacitor.

ADJ, ADJUSTABLE CURRENT LIMIT

A key feature of the LM2679 is the ability to control the peak

switch current. Without this feature the peak switch current

would be internally set to 7A or higher to accommodate 5A

load current designs. This requires that both the inductor

(which could saturate with excessively high currents) and the

catch diode be able to safely handle up to 7A which would be

conducted under load fault conditions.

If an application only requires a load current of 3A or 4A the

peak switch current can be set to a limit just over the maxi-

mum load current with the addition of a single programming

resistor. This allows the use of less powerful and more cost

effective inductors and diodes.

The peak switch current is equal to a factor of 37,125 divided

by R

ADJ. A resistance of 5.6KΩ sets the current limit to typi-

cally 6.3A and an R

ADJ of 8.25KΩ reduces the maximum cur-

rent to approximately 4.4A. For predictable control of the cur-

rent limit it is recommended to keep the peak switch current

greater than 3A. For lower current applications a 3A switch-

ing regulator with adjustable current limit, the LM2673, is

available.

When the power switch reaches the current limit threshold it

is immediately turned OFF and the internal switching fre-

quency is reduced. This extends the OFF time of the switch

to prevent a steady state high current condition. As the

switch current falls below the current limit threshold, the

switch will turn back ON. If a load fault continues, the switch

will again exceed the threshold and switch back OFF. This

will result in a low duty cycle pulsing of the power switch to

minimize the overall fault condition power dissipation.

Css SOFTSTART CAPACITOR

This optional capacitor controls the rate at which the LM2679

starts up at power on. The capacitor is charged linearly by an

internal current source. This voltage ramp gradually in-

creases the duty cycle of the power switch until it reaches

the normal operating duty cycle defined primarily by the ratio

of the output voltage to the input voltage. The softstart

turn-on time is programmable by the selection of Css.

The formula for selecting a softstart capacitor is:

Where:

SST = Softstart Current, 3.7µA typical

SS = Softstart time, from design requirements

SST = Softstart Threshold Voltage, 0.63V typical

OUT = Output Voltage, from design requirements

SCHOTTKY = Schottky Diode Voltage Drop, typically 0.5V

IN = Maximum Input Voltage, from design requirements

If this feature is not desired, leave the Softstart pin (pin 7)

open circuited

SIMPLE DESIGN PROCEDURE

LM2679 датащи(PDF) 11 Page - National Semiconductor (TI)

LM2679 датащи(HTML) 11 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

датащи скачать

ссылки URL