LM2707MF datasheet(14/19 Pages) NSC

14 / 19 page

Components and Connectivity

(Continued)

OUT. These capacitors have tight capacitance tolerance (as

good as +/-10%) and hold their value over temperature

(X7R: +/-15% over –55˚C to 125˚C; X5R: +/-15% over

–55˚C to 85˚C).

Capacitors with a Y5V or Z5U temperature characteristic are

generally not recommended for use with the LM2707. These

types of capacitors typically have wide capacitance toler-

ance (+80%, -20%) and vary significantly over temperature

(Y5V: +22%, -82% over –30˚C to +85˚C; Z5U: +22%, -56%

over +10˚C to +85˚C). Under some conditions, a 1uF-rated

Y5V or Z5U capacitor could have a capacitance as low as

0.1uF. The greatly reduced capacitance under some condi-

tions will result in very high ripple voltages and currents.

Net capacitance of a ceramic capacitor decreases with in-

creased DC bias. This capacitance reduction can give lower

capacitance than expected on the input and/or output, result-

ing in higher ripple voltages and currents. Using capacitors

at DC bias voltages significantly below the capacitor voltage

rating will usually minimize DC bias effects. Consult capaci-

tor manufacturers for information on capacitor DC bias char-

acteristics.

A ceramic capacitor can also be used for the C

X capacitor. A

small capacitor with a good temperature coefficient (COG,

for example) should be chosen.

Below is a list of some leading ceramic capacitor manufac-

turers:

•

TDK <www.component.tdk.com

•

AVX <www.avx.com>

•

Murata <www.murata.com>

•

Taiyo Yuden <www.t-yuden.com>

•

Vishay <www.vishay.com>

BOARD LAYOUT RECOMMENDATIONS

For optimal LM2707 circuit performance, the following board

layout suggestions are recommended:

•

Use short, wide traces and/or fills to connect the LM2707

and the external components. This results in low imped-

ance connections that minimize parasitic losses and

noise emissions.

•

Utilize low impedance traces and an internal ground

plane to connect the LM2707 GND pin to the input ca-

pacitor, output capacitor, C

X capacitor, and feedback re-

sistor.

•

Place the input capacitor as close to the LM2707 V

IN pin

as possible to minimize input noise.

•

Place the inductor and rectifier diode as close as possible

to the SW pin and minimize the lengths of the connec-

tions of this high-frequency switching node.

See Figure 9 for an example of a recommended board layout

of an LM2707 circuit.

Application Information

LED DRIVE EFFICIENCY

The LM2707 can be used to build a high efficiency LED drive

circuit. The low ON resistance of the NFET power device and

the sub-bandgap feedback voltage minimize the power con-

sumption of the LED-drive circuit. A circuit that uses an

inductor with a low series resistance and a diode with a low

forward voltage (low-V

F) will achieve maximum LED drive

efficiency.

LED drive efficiency (E) is commonly measured and calcu-

lated by taking the ratio of power consumed by the LEDs to

the power consumed at the input of the LED drive circuit:

Efficiency curves for a representative LM2707 LED drive

circuits can be referenced in the Typical Performance Char-

acteristics graphs.

LED BRIGHTNESS ADJUSTMENT

There are several methods and application circuits that can

be used to dynamically adjust LED brightness.

A pulse-width modulated signal applied to the enable (EN)

pin can be used to strobe the LEDs and adjust the perceived

LED brightness (see the schematic on page 1 of the

datasheet). With this approach, the LEDs are ON and driven

at the current programmed by the feedback resistor when

the pulse voltage is high. The LM2707 and the LEDs are

OFF when the pulse voltage is low. Brightness is propor-

tional to the duty cycle of the pulse signal.

The LM2707 can accommodate a very wide range of PWM

signal frequencies: signals between 100Hz and 50kHz are

acceptable. Signals below 100Hz are not recommended

because these lower frequencies are distinguishable by the

human eye. The brightness vs. duty cycle characteristic of

the circuit may vary slightly with different PWM frequencies.

This is especially noticable at the higher PWM frequencies.

See Table 1 for an example.

20099234

FIGURE 9. Example LM2707 LED Driver Board Layout

(LEDs not shown)

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