LM2707
Inductive-Boost Series LED Driver with Programmable
Oscillator Frequency
General Description
The LM2707 is a magnetic boost regulator specifically de-
signed 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).
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.
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.
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 pack-
age.
Features
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: 0.6
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
Applications
n
White LED Drive for Display Backlights
n
LED Flashlights
n
General Purpose LED Lighting
n
Step-up DC/DC Voltage Conversion
Typical Application Circuit
20099225
February 2005
LM2707
Inductive-Boost
Series
LED
Driver
with
Programmable
Oscillator
Frequency
2005 National Semiconductor Corporation
DS200992
www.national.com
Connection Diagram
8-Pin SOT23 Package
National Semiconductor Package Number MF08A
20099226
Pin Descriptions
Pin #
Name
Description
1
V
IN
Input Voltage Connection. Input Voltage Range: 2.3V to 7.0V
2
L
X
Inductor Input Connection
3
SW
Switching Node
4
V
OVP
Output Sense Pin for Over-Voltage Protection Circuit
5
FB
Output Voltage Feedback. Reference Voltage is 0.515V (typ.)
6
GND
Ground
7
C
X
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
SOT23-8
(MF08A)
Tape and Reel, 1000 Units/Reel
LM2707MFX
S0TB
Tape and Reel, 3500 Units/Reel
LM2707
www.national.com
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.
V
IN
, FB, and EN pins
-0.3V to 7.5V
SW and V
OVP
pins
-0.3V to 21V
Continuous Power Dissipation
(T
A
= 25
o
C)
800mW
Switch Peak Current
400mA
Junction Temperature (T
J-MAX
)
150
o
C
Storage Temperature Range
-65
o
C to +150
o
C
Maximum Lead Temperature
(Soldering)
(Note 3)
ESD Rating (Note 4)
Human Body Model:
Machine Model:
2kV
200V
Operating Ratings
(Notes 1, 2)
Input Voltage Range
2.3V to 7.0V
Minimum C
X
Capacitance
10pF
Junction Temperature (T
J
) Range
-30C to +125C
Ambient Temperature (T
A
) Range
(Note 5)
-30C to +85C
Thermal Properties
Juntion-to-Ambient Thermal
Resistance (
JA
) (Note 6)
125
o
C/W
Electrical Characteristics
(Notes 2, 7)
Unless otherwise specified: V
IN
= 3.0V, L
x
= Open, V
OVP
= Open, V
FB
= GND, C
x
= 300pF, V
EN
= V
IN
, T
A
= 25C.
Symbol
Parameter
Condition
Min
Typ
Max
Units
Oscillator Frequency Programming (C
x
pin)
I
chg
C
x
Charging Current
V
Cx
= 0.1V, V
FB
= 1V
16
24
30
A
I
dis
C
x
Discharging Current
V
Cx
= 1.0V, V
FB
= 1V
35
52
65
A
I
dis
/I
chg
Charge and Discharge Current
Ratio
2.2
V
Cx, High
C
x
Threshold Voltage +
810
860
910
mV
V
Cx, Low
C
x
Threshold Voltage -
260
300
340
mV
V
OSC
C
X
Oscillation Voltage
(V
Cx, High
) - (V
Cx, Low
)
520
560
600
mV
Current Limiting Comparator (L
x
pin)
I
LIMIT
Inductor Current Limit
(Note 8)
220
260
300
mA
R
IN
Pin 1-2 Total Resistance
Measured between pin 1 and pin 2
380
440
500
m
R
SC
Internal Effective Resistance for
Inductor Current Limit Sence
(Notes 9, 10)
300
m
Output Switch Section (SW pin)
V
sw, DS
Output Transistor Drain-to-Source
Voltage
V
Cx
= 0.1V, I
SW
= 200mA
0.12
0.22
V
R
DS-ON
Switch ON Resistance
R
DS-ON
= V
sw,DS
I
SW
V
Cx
= 0.1V, I
SW
= 200mA
0.60
1.1
I
sw,Off
Output Transistor Off Leak Current
V
FB
= 1V, V
SW
= 20V
0.01
2.0
A
Feedback Comparator section (FB pin)
V
ref
Reference Voltage
0.495
0.515
0.535
V
I
FBin
FB Pin Output Current
V
FB
= 0.4V
-0.2
-0.075
A
Shutdown Section (EN pin)
V
EN, High
EN Input Voltage +
ON mode
1.2
7.0
V
V
EN, Low
EN Input Voltage -
Shutdown Mode
0.3
V
I
ENin
EN pin Input Bias Current
V
EN
= 3.0V
25
40
A
LM2707
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3
Electrical Characteristics
(Notes 2, 7) (Continued)
Unless otherwise specified: V
IN
= 3.0V, L
x
= Open, V
OVP
= Open, V
FB
= GND, C
x
= 300pF, V
EN
= V
IN
, T
A
= 25C.
Symbol
Parameter
Condition
Min
Typ
Max
Units
Open Circuit Protection Section (V
OVP
pin)
V
OVP
Output Over-Voltage Protection
(Open Circuit)
Protection Activation Threshold
17.5
18.75
20.0
V
Protection Deactivation Threshold
17.0
18.25
19.5
V
Hysteresis
0.5
V
I
OVP
V
OVP
Pin Input Current
V
OVP
= 18.5V, V
EN
= 3V
50
100
A
V
OVP
= 18.5V, V
EN
= 0V
0.03
A
Input Voltage Section (V
IN
pin)
V
IN, 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
V
I
IN, Off
Shutdown Supply Current
V
EN
= 0.3V
0.01
1
A
I
IN, On
Quiescent Supply Current
V
FB
= 1.0V
0.2
0.5
0.8
mA
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 1.5k
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
A-MAX
) is dependent on the maximum operating junction temperature (T
JMAX-OP
= 125
o
C), the maximum power
dissipation of the device in the application (P
D-MAX
), and the junction-to ambient thermal resistance of the part/package in the application (
JA
), as given by the
following equation: T
A-MAX
= T
J-MAX-OP
(
JA
x P
D-MAX
).
Note 6: Junction-to-ambient thermal resistance (
JA
) is highly application and board-layout dependent. The 125
o
C/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: I
LIMIT
: The value of current source I
L
(DC measurement) when the switching operation is stopped by the I
S
comparator.
Note 9: R
SC
: Guaranteed by the design equation: I
LIMIT
= { 0.1V - [(2.3V x V
IN
) / 300] } / R
SC
Note 10: Do not connect the output circuit directly to GND: R
SC
might be damaged. (Excessive current will pass through R
SC
, the Schottky Diode, and the coil.)
LM2707
www.national.com
4
Typical Performance Characteristics
Unless otherwise specified: V
IN
= 3.0V, V
EN
= 3.0V, L = 22H
(Coilcraft DT1608C-223), D = MBR0520L (Vishay), C
IN
= 1.0F, C
OUT
= 2x1F, C
x
= 68pF, T
A
= 25
o
C.
Oscillator Frequency vs. Temperature
C
X
= 10pF
Oscillator Frequency vs. Temperature
C
X
= 100pF
20099221
20099220
Oscillator Period vs. Cx Capacitance
Maximum Duty Cycle vs. Oscillator Frequency
20099215
20099222
Maximum Duty Cycle vs. Temperature
C
X
= 10pF
Maximum Duty Cycle vs. Temperature
C
X
= 100pF
20099216
20099217
LM2707
www.national.com
5
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