LM2707
Inductive-Boost Series LED Driver with Programmable
Oscillator Frequency
General Description
Features
The LM2707 is a magnetic boost regulator specifically designed for white LED drive applications. Tightly regulated
LED currents, exceptional LED brightness uniformity, and
very high LED-drive power efficiency ( > 80%) can all be
achieved by stacking the LEDs in series between the
LM2707 output and the low-voltage feedback pin (0.515V).
n Excellent LED Drive Capability:
3 LED String: 30mA
4 LED String: 20 mA
6 LEDs (2 strings of 3): 15 mA
n Very High LED Drive Efficiency: > 80%
n Low Feedback Voltage: 515mV
n Low-Resistance NMOS Power Switch: 0.6Ω
n High-Speed PWM Brightness Control Capability
n Over-Voltage Protection (18V min, 19V typ, 20V max)
n Inrush and Inductor Current Limiting
n 2.3V - 7V Input Voltage Range
n Requires Only a Few External Components
n No External Compensation Needed
n Programmable Oscillator Frequency
n ON/OFF Pin
n Small SOT23-8 Package
The LM2707 requires only a few small external components.
A small inductor with a low saturation current rating can
safely be used because the tightly controlled current limit
circuit prevents large inductor current spikes, even at startup. The output capacitor can also be small due to the tightly
controlled output over-voltage protection circuit.
The LM2707 is an excellent choice for display backlighting
and other general-purpose lighting functions in battery powered applications. The 2.3V-to-7V input voltage range of the
device easily accommodates single-cell Lithium-Ion batteries and battery chargers.
The LM2707 features 18V output capability, PFM regulation,
and a high-current switching transistor (400mA peak). These
characteristics allow the part to drive a series string of 2-to-4
LEDs with forward currents between 0 and 40mA. LED
brightness can be adjusted dynamically simply by applying a
PWM signal to the enable (EN) pin. The PWM signal can be
as fast as 50kHz because the LM2707 has a fast turn-on
time.
In addition to LED-drive applications, the LM2707 can also
be used as a general purpose DC-DC voltage regulator in
boost converter applications.
The LM2707 is available in a SOT23-8 surface mount package.
Applications
n
n
n
n
White LED Drive for Display Backlights
LED Flashlights
General Purpose LED Lighting
Step-up DC/DC Voltage Conversion
Typical Application Circuit
20099225
© 2005 National Semiconductor Corporation
DS200992
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LM2707 Inductive-Boost Series LED Driver with Programmable Oscillator Frequency
February 2005
LM2707
Connection Diagram
8-Pin SOT23 Package
National Semiconductor Package Number MF08A
20099226
Pin Descriptions
Pin #
Name
Description
1
VIN
Input Voltage Connection. Input Voltage Range: 2.3V to 7.0V
2
LX
Inductor Input Connection
3
SW
Switching Node
4
VOVP
Output Sense Pin for Over-Voltage Protection Circuit
5
FB
Output Voltage Feedback. Reference Voltage is 0.515V (typ.)
6
GND
Ground
7
CX
Oscillator Frequency Adjustment
8
EN
Active-High Enable Pin
LM2707 is ON when V(EN) is above 1.2V
LM2707 is Shutdown when V(EN) is below 0.3V
Order Information
Order Number
Package Marking
Package
Supplied as:
LM2707MF
S0TB
Tape and Reel, 1000 Units/Reel
LM2707MFX
S0TB
SOT23-8
(MF08A)
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Tape and Reel, 3500 Units/Reel
2
Operating Ratings
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN, FB, and EN pins
-0.3V to 7.5V
SW and VOVP pins
-0.3V to 21V
Continuous Power Dissipation
(TA = 25oC)
800mW
Switch Peak Current
400mA
Input Voltage Range
-65oC to +150o C
Maximum Lead Temperature
(Soldering)
(Note 3)
ESD Rating (Note 4)
Human Body Model:
Machine Model:
Electrical Characteristics
10pF
Junction Temperature (TJ) Range
-30˚C to +125˚C
Ambient Temperature (TA) Range
(Note 5)
-30˚C to +85˚C
Thermal Properties
150 C
Storage Temperature Range
2.3V to 7.0V
Minimum CX Capacitance
o
Junction Temperature (TJ-MAX)
(Notes 1, 2)
125oC/W
Juntion-to-Ambient Thermal
Resistance (θJA) (Note 6)
2kV
200V
(Notes 2, 7)
Unless otherwise specified: VIN = 3.0V, Lx = Open, VOVP = Open, VFB = GND, Cx = 300pF, VEN = VIN, TA = 25˚C.
Symbol
Parameter
Condition
Min
Typ
Max
Units
Oscillator Frequency Programming (Cx pin)
Ichg
Cx Charging Current
VCx = 0.1V, VFB = 1V
16
24
30
µA
Idis
Cx Discharging Current
VCx = 1.0V, VFB = 1V
35
52
65
µA
Idis/Ichg
Charge and Discharge Current
Ratio
VCx, High
Cx Threshold Voltage +
VCx, Low
Cx Threshold Voltage -
VOSC
CX Oscillation Voltage
2.2
(VCx, High) - (VCx, Low)
810
860
910
mV
260
300
340
mV
520
560
600
mV
Current Limiting Comparator (Lx pin)
ILIMIT
Inductor Current Limit
(Note 8)
220
260
300
mA
RIN
Pin 1-2 Total Resistance
Measured between pin 1 and pin 2
380
440
500
mΩ
RSC
Internal Effective Resistance for
Inductor Current Limit Sence
(Notes 9, 10)
300
mΩ
Output Switch Section (SW pin)
Vsw, DS
Output Transistor Drain-to-Source
Voltage
VCx = 0.1V, ISW = 200mA
0.12
0.22
V
RDS-ON
Switch ON Resistance
RDS-ON = Vsw,DS ÷ ISW
VCx = 0.1V, ISW = 200mA
0.60
1.1
Ω
Isw,Off
Output Transistor Off Leak Current VFB = 1V, VSW = 20V
0.01
2.0
µA
0.535
Feedback Comparator section (FB pin)
Vref
Reference Voltage
IFBin
FB Pin Output Current
0.495
0.515
VFB = 0.4V
-0.2
-0.075
VEN, High EN Input Voltage +
ON mode
1.2
VEN, Low
EN Input Voltage -
Shutdown Mode
IENin
EN pin Input Bias Current
VEN = 3.0V
V
µA
Shutdown Section (EN pin)
7.0
25
3
V
0.3
V
40
µA
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LM2707
Absolute Maximum Ratings (Notes 1, 2)
LM2707
Electrical Characteristics (Notes 2, 7)
(Continued)
Unless otherwise specified: VIN = 3.0V, Lx = Open, VOVP = Open, VFB = GND, Cx = 300pF, VEN = VIN, TA = 25˚C.
Symbol
Parameter
Condition
Min
Typ
Max
Units
Protection Activation Threshold
17.5
18.75
20.0
V
Protection Deactivation Threshold
17.0
18.25
19.5
V
100
µA
Open Circuit Protection Section (VOVP pin)
VOVP
IOVP
Output Over-Voltage Protection
(Open Circuit)
VOVP Pin Input Current
Hysteresis
0.5
VOVP = 18.5V, VEN = 3V
50
VOVP = 18.5V, VEN = 0V
0.03
V
µA
Input Voltage Section (VIN pin)
VIN, Low
Undervoltage Lockout (Low
Voltage Stop)
Lockout Deactivation Threshold
1.8
2.0
2.3
V
Lockout Activation Threshold
1.7
1.9
2.2
V
Hysteresis
0.1
0.01
1
µA
0.5
0.8
mA
IIN, Off
Shutdown Supply Current
VEN = 0.3V
IIN, On
Quiescent Supply Current
VFB = 1.0V
0.2
V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: For detailed soldering specifications and information, please consult the National Semiconductor Application Note titled: "Mounting of Surface Mount
Components".
Note 4: The Human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7) The machine model is a 200pF
capacitor discharged directly into each pin. (EAIJ)
Note 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be
derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJMAX-OP = 125oC), the maximum power
dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the
following equation: TA-MAX = TJ-MAX-OP – (θJA x PD-MAX).
Note 6: Junction-to-ambient thermal resistance (θJA) is highly application and board-layout dependent. The 125oC/W figure provided was measured on a 4-layer
test board conforming to JEDEC standards. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues
when designing the board layout.
Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical (Typ) numbers are not guaranteed, but do represent the most likely norm.
Note 8: ILIMIT: The value of current source IL (DC measurement) when the switching operation is stopped by the IS comparator.
Note 9: RSC: Guaranteed by the design equation: ILIMIT = { 0.1V - [(2.3V x VIN) / 300] } / RSC
Note 10: Do not connect the output circuit directly to GND: RSC might be damaged. (Excessive current will pass through RSC , the Schottky Diode, and the coil.)
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4
Oscillator Frequency vs. Temperature
CX = 100pF
Oscillator Frequency vs. Temperature
CX = 10pF
20099221
20099220
Oscillator Period vs. Cx Capacitance
Maximum Duty Cycle vs. Oscillator Frequency
20099222
20099215
Maximum Duty Cycle vs. Temperature
CX = 100pF
Maximum Duty Cycle vs. Temperature
CX = 10pF
20099216
20099217
5
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LM2707
Typical Performance Characteristics Unless otherwise specified: VIN = 3.0V, VEN = 3.0V, L = 22µH
(Coilcraft DT1608C-223), D = MBR0520L (Vishay), CIN = 1.0µF, COUT = 2x1µF, Cx = 68pF, TA = 25oC.
LM2707
Typical Performance Characteristics Unless otherwise specified: VIN = 3.0V, VEN = 3.0V, L = 22µH
(Coilcraft DT1608C-223), D = MBR0520L (Vishay), CIN = 1.0µF, COUT = 2x1µF, Cx = 68pF, TA = 25oC. (Continued)
Feedback Trip Point vs. Supply Voltage
Feedback Trip Point vs. Temperature
20099213
20099212
Switch Resistance (RDS-ON) vs. Switch Current
Inductor Current Limit vs. Supply Voltage
20099203
20099214
Pin 1-2 Resistance vs. Temperature
VOVP Thresholds vs. Temperature
20099210
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20099211
6
Quiescent Supply Current vs. Supply Voltage
VFB = 1V
Shutdown Supply Current vs. Supply Voltage
20099204
20099205
Supply Current vs. EN Input Voltage
Supply Current vs. EN Input Bias Current
20099206
20099207
EN Threshold vs. Supply Voltage
EN Input Bias Current vs. EN Input Voltage
20099201
20099202
7
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LM2707
Typical Performance Characteristics Unless otherwise specified: VIN = 3.0V, VEN = 3.0V, L = 22µH
(Coilcraft DT1608C-223), D = MBR0520L (Vishay), CIN = 1.0µF, COUT = 2x1µF, Cx = 68pF, TA = 25oC. (Continued)
LM2707
Typical Performance Characteristics Unless otherwise specified: VIN = 3.0V, VEN = 3.0V, L = 22µH
(Coilcraft DT1608C-223), D = MBR0520L (Vishay), CIN = 1.0µF, COUT = 2x1µF, Cx = 68pF, TA = 25oC. (Continued)
LED Drive Efficiency vs. Supply Voltage
2 LEDs (Note 11)
LED Drive Efficiency vs. Supply Voltage
3 LEDs (Note 11)
20099218
20099219
LED Drive Efficiency vs. Supply Voltage
4 LEDs (Note 11)
LED Current vs. Duty Cycle
20099253
20099209
* 20mA, 4-LED operation requires increasing the current limit.
A 1Ω resistor was placed between the VIN and LX pins.
Note 11: LED drive efficiency is the ratio of the power consumed by the LEDs
to the power drawn at the LM2707 input (E = PLEDs / PIN). Approximate LED
forward voltage characteristics of the LEDs used for the efficiency curve data: IF
= 5mA: VF = 3.1V; IF = 10mA: VF = 3.3V; IF = 15mA: VF = 3.5V; IF = 20mA: VF
= 3.7V.
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8
LM2707
Block Diagram
20099227
FIGURE 1. LM2707 Block Diagram
9
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LM2707
Simplified Switching Circuit
20099228
FIGURE 2. LM2707 Simplified Switching Circuit
oscillator signal at the R-S latch. A current limit circuit brings
switching to a halt when current through the power switch
becomes excessive. Similar interrupts in switching are triggered by an over-voltage protection circuit on the output and
an under-voltage lockout circuit on the input. An external
shutdown signal can also be applied to place the LM2707 in
a low-power shutdown mode.
Product Description
OVERVIEW
The LM2707 is a magnetic switch-mode boost converter that
has been designed specifically for driving white LEDs. The
device is an asynchronous boost regulator that uses a lowresistance internal NFET power transistor and an external
rectifier diode. An internal high-power gate driver quickly
turns the power switch ON and OFF.
Operation of the LM2707 can be best understood by referring to the block diagram of Figure 1, the simplified switching
circuit in Figure 2, and the switching waveforms in Figure 3.
The part regulates the feedback voltage with pulsefrequency-modulated (PFM) control. The key blocks in this
control circuit are the R-S latch, the oscillator, and the feedback error amplifier. Several sense-and-control circuit
blocks, including the oscillator and the error amplifier, are
inputs to the R-S latch. The output of the R-S latch is the
control signal for the power transistor gate driver. The power
transistor turns ON and OFF at a frequency and duty cycle
that is generated by the oscillator. The oscillator frequency
can be programmed with an external capacitor (CX). The
part switches continuously until one of the LM2707 sense
circuits asserts a reset signal on the R-S latch.
The error amplifier resets the R-S latch when the output
feedback voltage is above the 515mV (typ.) reference voltage. The part will idle in a low-power state until the feedback
voltage falls below the reference voltage. At this point, the
oscillator signal again becomes the output signal of the R-S
latch, and switching resumes.
In addition to the feedback circuit, a few other internal protection and control circuits stop switching by overriding the
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20099229
FIGURE 3. CX Oscillator Waveform and Power Switch
Operation
PROGRAMMABLE OSCILLATOR
The LM2707 contains an oscillator with an internally fixed
duty cycle. The frequency of the oscillator is programmed
externally with capacitor CX. The oscillator frequency is:
10
LM2707
Product Description
(Continued)
An example with CX = 68pF:
FOSC = 26.3MHz / (15 + 68) = 317kHz.
The minimum recommended CX capacitance is 10pF.
The rise time (tr) of the CX signal is 2.2x longer than the fall
time (tf). This gives an oscillator duty cycle (DOSC) of 0.69.
The duty cycle of the switching converter (DSW) is actually
slightly greater than the duty cycle of the oscillator because
there is a delay (tD) of approximately 200ns in turning off the
power transistor.
An example: when VIN = 4.0V, ILIMIT ) 228mA.
When the current limit comparator trips, the comparator
output causes the R-S latch to reset, and the power transistor is turned off. The transistor does not turn off immediately,
though. There is a 200ns (typ.) delay between when the
comparator trips and the power transistor turns off. Because
of this delay, the peak inductor current rises above the
current limit threshold. Peak inductor current is discussed
and calculated in the section to follow: Peak Inductor Current.
The transistor Q1 in Figure 4 opens when the EN signal is
low, blocking the current path from input to ground through
resistors RS, R1, and R2. This keeps the input current very
low during shutdown.
PEAK INDUCTOR CURRENT
When conditions exist such that current limit is not reached
during normal steady-state operation, peak inductor current
is determined by the power switch ON time and can be
predicted with the following equation:
The output of the oscillator connects to the R-S latch. When
the reset signal of the latch is low, the oscillator signal
becomes the ON/OFF signal for the power transistor, as
described in the previous section.
CURRENT LIMIT
The LM2707 current limit circuit senses the current through
the inductor and interrupts switching when the current limit
threshold is exceeded. The current limit circuit is shown in
Figure 4.
VIN: Input voltage (Note 12)
L: Inductance
tON: Switch ON time. (See the Programmable Oscillator
section)
An example -- VIN = 3.0V, L = 22µH, CX = 38pF:
When the current limit is engaged before the switch is turned
off by the oscillator, the peak inductor current of the circuit
(IL-PK-LIMIT) is determined by the current limit value and the
turn-off delay of the power switch:
20099230
FIGURE 4. LM2707 Internal Current Limit Circuit
The current limit circuit operates by comparing the voltage
across sense resistor RS to a 100mV (typ.) reference voltage
VR. Resistors R1 and R2 provide an input-voltage component to the current limit that causes the current limit to be
lower at higher input voltages.
The current limit threshold can be calculated by determining
when the voltages on the current limit comparator inputs are
equal:
ILIMIT: Current Limit -- 330mA - (VIN x 25.5mA/V)
tD: Power transistor turn-off delay (200ns typ.)
An example -- VIN = 3.6V, L = 22µH:
11
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LM2707
Product Description
(Continued)
Figure 5 graphs the relationship between inductor current
and current limit. Figure 6 plots ideal inductor current waveforms to illustrate inductor current behavior. Figure 7 gives
peak inductor current versus input voltage and shows the
two regions where the oscillator and current limit, respectively, determine peak inductor current.
20099252
FIGURE 7. Peak Inductor Current vs. Input Voltage.
L = 22µH, CX = 68pF.
Note 12: VIN is a good approximation of the voltage across the inductor
during the charge phase. Actual voltage across the inductor will be slightly
lower due to the VDS voltage of the power transistor, but this factor is minimal
due to the low RDS-ON of the power transistor.
INCREASING CURRENT LIMIT AND PEAK INDUCTOR
CURRENT
It is possible to increase the current limit by placing an
external resistor between the VIN and LX pins, as shown in
Figure 8. With the addition of the external resistor, only a
fraction of the total inductor current passes through internal
sense resistor. Thus, it takes more inductor current for the
voltage across the internal sense resistor to become large
enough to trip the current limit comparator.
20099231
FIGURE 5. Peak Inductor Current and Current Limit vs.
Input Voltage
20099233
FIGURE 8. Increase Current Limit and Peak Inductor
Current by Adding REXT
20099232
The addition of an external current limit resistor modifies the
associated peak inductor equation to:
FIGURE 6. Ideal Inductor Current Waveforms
REXT: External Current Limit Adjust Resistance
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12
• 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.
(Continued)
RIN: 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:
DIODE SELECTION
For high efficiency and good circuit performance, a fast
schottky rectifier diode with a low forward voltage is recommended 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
OUTPUT OVER-VOLTAGE PROTECTION
The LM2707 contains an over-voltage protection circuit that
limits the voltage at the VOVP 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 VOVP pin to ground.
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 (CIN), an output capacitor (COUT), and a capacitor to program the oscillator frequency (CX).
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 capacitor 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 capacitor (or two 1µF capacitors in parallel) is sufficient for a typical
LM2707 application. To accommodate the over-voltage protection circuit, a voltage rating of at least 25V is recommended for the output capacitor.
Surface-mount multi-layer ceramic capacitors are recommended for both the input and output capacitors. These
capacitors are small, inexpensive and have very low equivalent series resistance (ESR ≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors 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 ceramic 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 CIN and
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 undervoltage 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 hysteresis 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 consumes 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 application. (See the Peak Inductor Current section of the
datasheet.
A few inductors recommended for use with the LM2707 are:
• TDK VLF3010A Series
13
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LM2707
Product Description
LM2707
Components and Connectivity
(Continued)
COUT. 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 tolerance (+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 conditions will result in very high ripple voltages and currents.
Net capacitance of a ceramic capacitor decreases with increased DC bias. This capacitance reduction can give lower
capacitance than expected on the input and/or output, resulting 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 capacitor manufacturers for information on capacitor DC bias characteristics.
A ceramic capacitor can also be used for the CX capacitor. A
small capacitor with a good temperature coefficient (COG,
for example) should be chosen.
Below is a list of some leading ceramic capacitor manufacturers:
• TDK < www.component.tdk.com
• AVX < www.avx.com >
• Murata < www.murata.com >
• Taiyo Yuden < www.t-yuden.com >
20099234
FIGURE 9. Example LM2707 LED Driver Board Layout
(LEDs not shown)
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 consumption 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-VF) will achieve maximum LED drive
efficiency.
LED drive efficiency (E) is commonly measured and calculated by taking the ratio of power consumed by the LEDs to
the power consumed at the input of the LED drive circuit:
• 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 impedance 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 capacitor, output capacitor, CX capacitor, and feedback resistor.
• Place the input capacitor as close to the LM2707 VIN 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 connections of this high-frequency switching node.
See Figure 9 for an example of a recommended board layout
of an LM2707 circuit.
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Efficiency curves for a representative LM2707 LED drive
circuits can be referenced in the Typical Performance Characteristics 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 proportional 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.
14
LM2707
Application Information
(Continued)
Table 1. Time-Averaged LED Current vs. PWM Frequency and Duty Cycle
PWM Frequency
D = 10%
D = 20%
D = 30%
D = 50%
D = 90%
200 Hz
2.3
3.8
5.3
8.2
13.9
1 kHz
3.7
6.0
7.4
10.0
14.4
10 kHz
2.6
5.9
9.1
13.4
14.8
20 kHz
1.0
4.7
8.6
13.6
14.8
40 kHz
OFF
1.8
5.1
12.0
14.8
50 kHz
OFF
OFF
5.7
10.3
14.8
VIN = 3.6V, 4 LEDs, RFB = 34Ω, ILED = 14.9mA when V(EN) is HIGH.
A benefit of PWM brightness adjustment is the characteristic
that LEDs are driven with the same current level (max current) at all brightness levels. Other methods that adjust
brightness by changing the DC forward current of the LEDs
may see a slight change in color at different brightness
levels.
feedback node. In order to keep the feedback voltage regulated, the LM2707 responds by reducing the current through
the LEDs. Conversely, LED current increases when the analog voltage is below the feedback voltage.
A 4-level digital brightness adjustment, shown in Figure 11,
can be implemented with a pair of external resistors and two
digital logic signals. The workings of the circuit are similar to
the analog voltage control: LED currents are set based on
the current that is added to or removed from the FB node
from the digital voltage supplies.
With the addition of an external resistor, an analog voltage
can be used to adjust LED brightness, as shown in Figure
10. When the analog voltage is above the feedback voltage,
0.515V (typ.), the analog voltage source adds current to
20099235
FIGURE 10. LM2707 LED-Drive Circuit with Analog Voltage Brightness Control
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LM2707
Application Information
(Continued)
20099236
FIGURE 11. LM2707 LED-Drive Circuit with 2-Bit Digital Logic Brightness Control
Application Circuits
LM2707 DRIVING 6 LEDs
20099237
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16
LM2707
Application Circuits
(Continued)
LM2707 DRIVING 3 LEDs
20099238
LM2707 DRIVING 2 LEDs
20099239
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LM2707
Application Circuits
(Continued)
LM2707 DC-DC VOLTAGE CONVERTER CIRCUIT
20099240
Curves for VOUT = 12V. RFB1 = 126kΩ, RFB2 = 10kΩ, L = 22µH (Coilcraft DT1608C-223), D = MBR0520L (Vishay), CIN = 1µF,
COUT = 2x1µF, CX = 68pF, TA = 25oC. A 1Ω resistor was placed between the VIN and LX pins to increase the current limit and
accomodate load currents above of 15mA.
Output Voltage vs. Output Current
Output Voltage vs. Input Voltage
20099254
20099255
Power Efficiency vs. Input Voltage
Power Efficiency vs. Output Current
20099256
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20099257
18
inches (millimeters) unless otherwise noted
NS Package Number MF08A: SOT23-8
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
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LM2707 Inductive-Boost Series LED Driver with Programmable Oscillator Frequency
Physical Dimensions