LM2707MF [NSC]
Inductive-Boost Series LED Driver with Programmable Oscillator Frequency; 电感式升压系列LED驱动器，可编程振荡器频率
| 型号： | LM2707MF |
| 厂家： | 
National Semiconductor |
| 描述： | Inductive-Boost Series LED Driver with Programmable Oscillator Frequency |
| 文件： | 总19页 (文件大小：1035K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
February 2005
LM2707
Inductive-Boost Series LED Driver with Programmable
Oscillator Frequency
General Description
Features
n Excellent LED Drive Capability:
3 LED String: 30mA
The LM2707 is a magnetic boost regulator specifically de-
signed for white LED drive applications. Tightly regulated
LED currents, exceptional LED brightness uniformity, and
4 LED String: 20 mA
6 LEDs (2 strings of 3): 15 mA
>
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 Very High LED Drive Efficiency: 80%
n Low Feedback Voltage: 515mV
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 start-
up. The output capacitor can also be small due to the tightly
controlled output over-voltage protection circuit.
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
The LM2707 is an excellent choice for display backlighting
and other general-purpose lighting functions in battery pow-
ered applications. The 2.3V-to-7V input voltage range of the
device easily accommodates single-cell Lithium-Ion batter-
ies and battery chargers.
n Small SOT23-8 Package
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.
Applications
n White LED Drive for Display Backlights
n LED Flashlights
n General Purpose LED Lighting
n Step-up DC/DC Voltage Conversion
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 pack-
age.
Typical Application Circuit
20099225
© 2005 National Semiconductor Corporation
DS200992
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Connection Diagram
8-Pin SOT23 Package
National Semiconductor Package Number MF08A
20099226
Pin Descriptions
Pin #
Name
VIN
Description
1
2
3
4
5
6
7
8
Input Voltage Connection. Input Voltage Range: 2.3V to 7.0V
Inductor Input Connection
LX
SW
VOVP
FB
Switching Node
Output Sense Pin for Over-Voltage Protection Circuit
Output Voltage Feedback. Reference Voltage is 0.515V (typ.)
Ground
GND
CX
Oscillator Frequency Adjustment
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
LM2707MF
Package Marking
Package
SOT23-8
(MF08A)
Supplied as:
S0TB
S0TB
Tape and Reel, 1000 Units/Reel
Tape and Reel, 3500 Units/Reel
LM2707MFX
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2
Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Ratings (Notes 1, 2)
Input Voltage Range
2.3V to 7.0V
10pF
Minimum CX Capacitance
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range
(Note 5)
-30˚C to +125˚C
-30˚C to +85˚C
VIN, FB, and EN pins
SW and VOVP pins
Continuous Power Dissipation
(TA = 25oC)
-0.3V to 7.5V
-0.3V to 21V
800mW
Thermal Properties
Juntion-to-Ambient Thermal
Resistance (θJA) (Note 6)
Switch Peak Current
400mA
150oC
-65oC to +150o C
125oC/W
Junction Temperature (TJ-MAX
Storage Temperature Range
Maximum Lead Temperature
(Soldering)
)
(Note 3)
ESD Rating (Note 4)
Human Body Model:
Machine Model:
2kV
200V
Electrical Characteristics (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
Idis
dis/Ichg
Cx Charging Current
Cx Discharging Current
Charge and Discharge Current
Ratio
VCx = 0.1V, VFB = 1V
VCx = 1.0V, VFB = 1V
16
35
24
52
30
65
µA
µA
I
2.2
VCx, High Cx Threshold Voltage +
VCx, Low Cx Threshold Voltage -
810
260
520
860
300
560
910
340
600
mV
mV
mV
VOSC CX Oscillation Voltage
Current Limiting Comparator (Lx pin)
(VCx, High) - (VCx, Low
(Note 8)
)
ILIMIT
RIN
Inductor Current Limit
220
380
260
440
300
300
500
mA
mΩ
mΩ
Pin 1-2 Total Resistance
Internal Effective Resistance for
Inductor Current Limit Sence
Measured between pin 1 and pin 2
(Notes 9, 10)
RSC
Output Switch Section (SW pin)
Vsw, DS
RDS-ON
Isw,Off
Output Transistor Drain-to-Source VCx = 0.1V, ISW = 200mA
0.12
0.60
0.01
0.22
1.1
V
Ω
Voltage
Switch ON Resistance
RDS-ON = Vsw,DS ÷ ISW
VCx = 0.1V, ISW = 200mA
Output Transistor Off Leak Current VFB = 1V, VSW = 20V
2.0
µA
Feedback Comparator section (FB pin)
Vref
Reference Voltage
0.495 0.515
0.535
V
IFBin
FB Pin Output Current
VFB = 0.4V
-0.2
-0.075
µA
Shutdown Section (EN pin)
VEN, High EN Input Voltage +
VEN, Low EN Input Voltage -
ON mode
1.2
7.0
0.3
40
V
V
Shutdown Mode
VEN = 3.0V
IENin
EN pin Input Bias Current
25
µA
3
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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
Open Circuit Protection Section (VOVP pin)
VOVP
Output Over-Voltage Protection
(Open Circuit)
Protection Activation Threshold
Protection Deactivation Threshold
Hysteresis
17.5
17.0
18.75
18.25
0.5
20.0
19.5
V
V
V
IOVP
VOVP Pin Input Current
VOVP = 18.5V, VEN = 3V
VOVP = 18.5V, VEN = 0V
50
100
µA
µA
0.03
Input Voltage Section (VIN pin)
VIN, Low Undervoltage Lockout (Low
Voltage Stop)
Lockout Deactivation Threshold
Lockout Activation Threshold
Hysteresis
1.8
1.7
2.0
1.9
2.3
2.2
V
V
0.1
V
IIN, Off
IIN, On
Shutdown Supply Current
Quiescent Supply Current
VEN = 0.3V
0.01
0.5
1
µA
mA
VFB = 1.0V
0.2
0.8
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 (T
) is dependent on the maximum operating junction temperature (T
= 125oC), the maximum power
A-MAX
JMAX-OP
dissipation of the device in the application (P
), and the junction-to ambient thermal resistance of the part/package in the application (θ ), as given by the
D-MAX
JA
following equation: T
= T
– (θ x P
).
A-MAX
J-MAX-OP
JA
D-MAX
Note 6: Junction-to-ambient thermal resistance (θ ) is highly application and board-layout dependent. The 125oC/W figure provided was measured on a 4-layer
JA
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: I
: The value of current source I (DC measurement) when the switching operation is stopped by the I comparator.
L S
LIMIT
Note 9: R : Guaranteed by the design equation: I
= { 0.1V - [(2.3V x V ) / 300] } / R
IN SC
SC
LIMIT
Note 10: Do not connect the output circuit directly to GND: R might be damaged. (Excessive current will pass through R , the Schottky Diode, and the coil.)
SC
SC
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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.
Oscillator Frequency vs. Temperature
CX = 10pF
Oscillator Frequency vs. Temperature
CX = 100pF
20099221
20099220
Oscillator Period vs. Cx Capacitance
Maximum Duty Cycle vs. Oscillator Frequency
20099222
20099215
Maximum Duty Cycle vs. Temperature
CX = 10pF
Maximum Duty Cycle vs. Temperature
CX = 100pF
20099216
20099217
5
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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
20099211
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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)
Quiescent Supply Current vs. Supply Voltage
Shutdown Supply Current vs. Supply Voltage
VFB = 1V
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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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
LED Drive Efficiency vs. Supply Voltage
2 LEDs (Note 11)
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 V and L pins.
IN
X
Note 11: LED drive efficiency is the ratio of the power consumed by the LEDs
to the power drawn at the LM2707 input (E = P / P ). Approximate LED
LEDs
IN
forward voltage characteristics of the LEDs used for the efficiency curve data: I
F
F
= 5mA: V = 3.1V; I = 10mA: V = 3.3V; I = 15mA: V = 3.5V; I = 20mA: V
F
F
F
F
F
F
= 3.7V.
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Block Diagram
20099227
FIGURE 1. LM2707 Block Diagram
9
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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 trig-
gered 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 low-
resistance 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 refer-
ring 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 pulse-
frequency-modulated (PFM) control. The key blocks in this
control circuit are the R-S latch, the oscillator, and the feed-
back 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.
20099229
FIGURE 3. CX Oscillator Waveform and Power Switch
Operation
The error amplifier resets the R-S latch when the output
feedback voltage is above the 515mV (typ.) reference volt-
age. 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.
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:
In addition to the feedback circuit, a few other internal pro-
tection and control circuits stop switching by overriding the
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Product Description (Continued)
An example with CX = 68pF:
FOSC = 26.3MHz / (15 + 68) = 317kHz.
The minimum recommended CX capacitance is 10pF.
An example: when VIN = 4.0V, ILIMIT ) 228mA.
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.
When the current limit comparator trips, the comparator
output causes the R-S latch to reset, and the power transis-
tor 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 Cur-
rent.
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
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.
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:
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 compo-
nent to the current limit that causes the current limit to be
lower at higher input voltages.
ILIMIT: Current Limit -- 330mA - (VIN x 25.5mA/V)
tD: Power transistor turn-off delay (200ns typ.)
An example -- VIN = 3.6V, L = 22µH:
The current limit threshold can be calculated by determining
when the voltages on the current limit comparator inputs are
equal:
11
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Product Description (Continued)
Figure 5 graphs the relationship between inductor current
and current limit. Figure 6 plots ideal inductor current wave-
forms 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, respec-
tively, determine peak inductor current.
20099252
FIGURE 7. Peak Inductor Current vs. Input Voltage.
L = 22µH, CX = 68pF.
Note 12: V is a good approximation of the voltage across the inductor
IN
during the charge phase. Actual voltage across the inductor will be slightly
lower due to the V voltage of the power transistor, but this factor is minimal
DS
due to the low R
of the power transistor.
DS-ON
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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•
•
•
TDK VLF4012A Series
Coilcraft DT1608C Series
Coilcraft DO1608C Series
Product Description (Continued)
R
IN: Internal Resistance Betwen VIN and LX Pins.
Many other inductors that are not on this list will also function
well with the LM2707.
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 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
OUTPUT OVER-VOLTAGE PROTECTION
Sanyo SB07-03C
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.
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 (CIN), an output capaci-
tor (COUT), and a capacitor to program the oscillator fre-
quency (CX).
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 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 output over-voltage protection feature can be disabled
by connecting the VOVP 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.
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.
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.
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.
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:
For most applications, ceramic capacitors with an X7R or
X5R temperature characteristic should be used for CIN and
•
TDK VLF3010A Series
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Components and Connectivity
(Continued)
C
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.
20099234
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.
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 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-VF) will achieve maximum LED drive
efficiency.
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 manufac-
turers:
<
TDK www.component.tdk.com
•
•
•
•
•
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:
<
>
AVX www.avx.com
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>
Murata www.murata.com
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>
Taiyo Yuden www.t-yuden.com
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>
Vishay www.vishay.com
BOARD LAYOUT RECOMMENDATIONS
For optimal LM2707 circuit performance, the following board
layout suggestions are recommended:
Efficiency curves for a representative LM2707 LED drive
circuits can be referenced in the Typical Performance Char-
acteristics graphs.
•
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.
LED BRIGHTNESS ADJUSTMENT
There are several methods and application circuits that can
be used to dynamically adjust LED brightness.
•
Utilize low impedance traces and an internal ground
plane to connect the LM2707 GND pin to the input ca-
pacitor, output capacitor, CX capacitor, and feedback re-
sistor.
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.
•
•
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 connec-
tions of this high-frequency switching node.
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.
See Figure 9 for an example of a recommended board layout
of an LM2707 circuit.
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14
Application Information (Continued)
Table 1. Time-Averaged LED Current vs. PWM Frequency and Duty Cycle
PWM Frequency
200 Hz
D = 10%
2.3
D = 20%
3.8
D = 30%
5.3
D = 50%
8.2
D = 90%
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
OFF
1.8
5.1
12.0
14.8
50 kHz
OFF
5.7
10.3
14.8
V
= 3.6V, 4 LEDs, R = 34Ω, I
= 14.9mA when V(EN) is HIGH.
LED
IN
FB
A benefit of PWM brightness adjustment is the characteristic
that LEDs are driven with the same current level (max cur-
rent) 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 regu-
lated, the LM2707 responds by reducing the current through
the LEDs. Conversely, LED current increases when the ana-
log 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
15
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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
Application Circuits (Continued)
LM2707 DRIVING 3 LEDs
20099238
LM2707 DRIVING 2 LEDs
20099239
17
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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. Input Voltage
Output Voltage vs. Output Current
20099254
20099255
Power Efficiency vs. Input Voltage
Power Efficiency vs. Output Current
20099256
20099257
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18
Physical Dimensions 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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