lm2707 National Semiconductor Corporation, lm2707 Datasheet - Page 14

• Manufacturer Part Number: lm2707
• Description: Inductive-boost Series Led Driver With Programmable Oscillator Frequency
• Manufacturer: National Semiconductor Corporation

www.national.com
Components and Connectivity
C
good as +/-10%) and hold their value over temperature
(X7R: +/-15% over –55˚C to 125˚C; X5R: +/-15% over
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
small capacitor with a good temperature coefficient (COG,
for example) should be chosen.
Below is a list of some leading ceramic capacitor manufac-
turers:
BOARD LAYOUT RECOMMENDATIONS
For optimal LM2707 circuit performance, the following board
layout suggestions are recommended:
See Figure 9 for an example of a recommended board layout
of an LM2707 circuit.
–55˚C to 85˚C).
• TDK
• AVX
• Murata
• Taiyo Yuden
• Vishay
• Use short, wide traces and/or fills to connect the LM2707
• Utilize low impedance traces and an internal ground
• Place the input capacitor as close to the LM2707 V
• Place the inductor and rectifier diode as close as possible
(Continued)
OUT
and the external components. This results in low imped-
ance connections that minimize parasitic losses and
noise emissions.
plane to connect the LM2707 GND pin to the input ca-
pacitor, output capacitor, C
sistor.
as possible to minimize input noise.
to the SW pin and minimize the lengths of the connec-
tions of this high-frequency switching node.
. These capacitors have tight capacitance tolerance (as
<
<
www.avx.com
www.component.tdk.com
<
<
www.vishay.com
www.murata.com
<
www.t-yuden.com
>
>
>
X
capacitor, and feedback re-
>
X
capacitor. A
IN
pin
14
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
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.
FIGURE 9. Example LM2707 LED Driver Board Layout
(LEDs not shown)
F
) will achieve maximum LED drive
20099234