LM2707 datasheet(13/19 Pages) NSC

13 / 19 page

Product Description (Continued)

IN: Internal Resistance Betwen VIN and LX Pins.

Rearranging the equation above allows for easy calculation

of an external resistance to obtain a desired peak inductor

current:

OUTPUT OVER-VOLTAGE PROTECTION

The LM2707 contains an over-voltage protection circuit that

limits the voltage at the V

OVP pin and prevents the LM2707

from boosting to voltages that might damage the LM2707 or

external components (LEDs, capacitors, etc.). This circuit is

especially important in LED-drive applications where there is

the possibility that the feedback path might be broken if the

LED string becomes disconnected or if an LED burns out.

The over-voltage protection circuit protects internal circuits

and the NFET power transistor. The over-voltage threshold

is typically centered at 18.75V, and contains approximately

500mV of hysteresis.

The output over-voltage protection feature can be disabled

by connecting the V

OVP pin to ground.

INPUT VOLTAGE RANGE AND UNDER-VOLTAGE

LOCKOUT

The LM2707 input voltage operating range is 2.3V to 7.0V.

When the input voltage becomes excessively low, the under-

voltage lockout circuit interrupts switching cycles to prevent

device malfunction. Lockout typically occurs when the input

voltage falls to 1.9V. There is approximately 100mV of hys-

teresis in the under-voltage lockout threshold.

ENABLE AND SHUTDOWN

The Enable pin (EN) is a logic input that puts the part in

active mode when the voltage on the pin is high. It places the

part in a low-power shutdown mode when the voltage on the

pin is low. When shutdown, the LM2707 input typically con-

sumes only a few nanoamps of current. There is a 122k

Ω

pull-down resistor connected internally between the EN and

GND pins. This resistor pulls the LM2707 into shutdown

mode when the EN pin is left floating.

Components and Connectivity

INDUCTOR SELECTION

Inductor selection is a vital part of LM2707 circuit design.

The inductance value affects input and output ripple voltages

and currents. An inductor with low series resistance will

provide optimal power conversion efficiency. The saturation

current rating of the inductor should be chosen so that it is

above the steady-state peak inductor current of the applica-

tion. (See the Peak Inductor Current section of the

datasheet.

A few inductors recommended for use with the LM2707 are:

•

TDK VLF3010A Series

•

TDK VLF4012A Series

•

Coilcraft DT1608C Series

•

Coilcraft DO1608C Series

Many other inductors that are not on this list will also function

well with the LM2707.

DIODE SELECTION

For high efficiency and good circuit performance, a fast

schottky rectifier diode with a low forward voltage is recom-

mended for use with the LM2707. The average current rating

of the diode should be higher than the peak inductor current

of the application. The reverse breakdown voltage of the

diode should be greater than the maximum output voltage of

the circuit.

Some schottky diodes recommended for use with the

LM2707 are:

•

Vishay MBR0520L

•

Sanyo SB07-03C

•

ON Semiconductor MBR0520L

Many other diodes that are not on this list will also function

well with the LM2707.

CAPACITOR SELECTION

The LM2707 circuit requires three external capacitors for

proper operation: an input capacitor (C

IN), an output capaci-

tor (C

OUT), and a capacitor to program the oscillator fre-

quency (C

X).

The input capacitor keeps input voltage ripple, input current

ripple, and input noise levels low. The ripple magnitudes will

typically be inversely proportional to input capacitance: the

larger the capacitance, the smaller the ripple. A 4.7µF ca-

pacitor is recommended for a typical LM2707 circuit. This

value can be increased or decreased as desired, with the

only impact being a change in input ripple and noise. The

input capacitor should have a voltage rating that is at least as

large as the maximum input voltage of the application.

The capacitor on the output performs a similar function:

keeping ripple voltage, ripple current, and noise levels low.

Like the input, the output ripple magnitudes are inversely

proportional to the capacitance on the output. Due to the

inherently stable ON/OFF control scheme of the LM2707,

the output capacitance does not affect stability of the circuit.

But an undersized capacitor may result in excessive ripple

that could cause the circuit to behave erratically or even

prematurely trip the over-voltage protection. A 2.2µF capaci-

tor (or two 1µF capacitors in parallel) is sufficient for a typical

LM2707 application. To accommodate the over-voltage pro-

tection circuit, a voltage rating of at least 25V is recom-

mended for the output capacitor.

Surface-mount multi-layer ceramic capacitors are recom-

mended for both the input and output capacitors. These

capacitors are small, inexpensive and have very low equiva-

lent series resistance (ESR

≤ 15mΩ typ.). Tantalum capaci-

tors, OS-CON capacitors, and aluminum electrolytic capaci-

tors generally are not recommended for use with the

LM2707 due to their high ESR, as compared to ceramic

capacitors. If one of these types of capacitor is used, it is

recommended that small ceramic capacitors (0.1µF to 1µF)

also be placed in parallel with each of the larger bypass

capacitors to filter high frequency noise. These small ce-

ramic capacitors should be placed as close to the LM2707

as possible for optimal filtering.

For most applications, ceramic capacitors with an X7R or

X5R temperature characteristic should be used for C

IN and

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