lm2707 National Semiconductor Corporation, lm2707 Datasheet - Page 13

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

Product Description
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
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
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 122kΩ
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
R
IN
: Internal Resistance Betwen V
OVP
pin to ground.
OVP
pin and prevents the LM2707
(Continued)
IN
and L
X
Pins.
13
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:
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
tor (C
quency (C
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
• TDK VLF4012A Series
• Coilcraft DT1608C Series
• Coilcraft DO1608C Series
• Vishay MBR0520L
• Sanyo SB07-03C
• ON Semiconductor MBR0520L
OUT
), and a capacitor to program the oscillator fre-
X
).
IN
), an output capaci-
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IN
and