LM2791
Current Regulated Switched Capacitor LED Driver with
Analog Brightness Control
General Description
The LM2791 is a CMOS charge-pump voltage doubler and
regulator that provides two regulated current sources. The
LM2791 is designed to drive two white (or blue) LEDs with
matched currents (within
0.3%) to produce balanced light
sources for display backlights. They accept an input voltage
range from 3V to 5.8V and maintain a constant current
determined by an external set resistor.
The LM2791 delivers up to 36mA of load current to accom-
modate two high forward voltage (typically white) LEDs. The
switching frequency is 450kHz (min.) to keep the conducted
noise spectrum away from sensitive frequencies within por-
table RF devices.
In the LM2791, brightness is controlled by applying a voltage
between GND and 3.0V to the BRGT pin. The LM2791 is
available in active high or low shutdown versions. The shut-
down pin reduces the operating current to 1A (max.).
The LM2791 is available in a 10 pin leadless leadframe
(LLP) CSP package.
Features
n
Output matching of
0.3%
n
Drives up to two LED's
n
3V to 5.8V Input Voltage
n
Up to 36mA output current
n
Soft start limits inrush current
n
Analog brightness control
n
Separate shutdown input
n
Very small solution size - no inductor
n
0.7mA typical operating current
n
1A (max.) shutdown current
n
450kHz switching frequency (min.)
n
Linear regulation generates predictable noise spectrum
n
LLP-10 package: 3mm X 3mm X 0.8mm
Applications
n
White LED Display Backlights
n
White LED Keypad Backlights
n
1-Cell Li-Ion battery-operated equipment including
PDAs, hand-held PCs, cellular phones
n
Flat Panel Dispalys
Basic Application Circuit
20018301
July 2002
LM2791
Current
Regulated
Switched
Capacitor
LED
Driver
with
Analog
Brightness
Control
2002 National Semiconductor Corporation
DS200183
www.national.com
Connection Diagram
20018303
Top View
10-Lead LLP
Ordering Information
Order Number
Shutdown Polarity
NSC Package
Drawing
Package
Marking
Supplied As
LM2791LD-L
Active Low
LLP-10
SNB
1000 Units, Tape and Reel
LM2791LDX-L
Active Low
LLP-10
SNB
4500 Units, Tape and Reel
LM2791LD-H
Active High
LLP-10
SLB
1000 Units, Tape and Reel
LM2791LDX-H
Active High
LLP-10
SLB
4500 Units, Tape and Reel
LM2791
www.national.com
2
Pin Description
Pin
Name
Function
1
BRGT
Variable voltage input controls output current.
2
P
OUT
Charge pump output.
3
C1-
Connect this pin to the negative terminal of C1.
4
C1+
Connect this pin to the positive terminal of C1.
5
D2
Current source outputs. Connect directly to LED.
6
D1
Current source outputs. Connect directly to LED.
7
GND
Power supply ground input.
8
V
IN
Power supply voltage input.
9
SD/SD
Shutdown input. Device operation is inhibited when pin is asserted.
10
I
SET
Current Sense Input. Connect resistor to ground to set constant current through LED.
Block Diagram
20018302
LM2791
www.national.com
3
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
-0.3 to 6V
BRGT, SD
-0.3 to (V
IN
+0.2V)
Power Dissipation(Note 2)
400 mW
T
JMAX
(Note 2)
150C
JA
(Note 7)
55C/W
Storge Temperature
-65C to +100C
Lead Temp. (Soldering, 5 sec.)
260C
ESD Rating
Human Body Model
2KV
Machine Model
200V
Operating Conditions
Input Voltage (V
IN
)
3.0V to 5.8V
BRGT
0 to 3.0V
Ambient Temperature (T
A
)
-30C to +85C
Junction Temperature (T
J
)
-30C to +100C
Electrical Characteristics
Limits in standard typeface are for T
J
= 25C and limits in boldface type apply over the full Operating Temperature Range.
Unless otherwise specified, C
1
= C
IN
= C
HOLD
= 1 F, V
IN
= 3.6V, V
DIODE
= 3.6V, R
SET
= 332
, BRGT pin = 0V.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
I
DX
Diode Current at ID1,2
V
IN
= 3V, R
SET
= 270
16.5
18
mA
Available Current at Output Dx
V
IN
= 3V
14.5
V
IN
= 3.3V
12.8
15.1
17.7
V
IN
= 3.6V
13.3
15.7
18.4
V
IN
= 4.4V
16.8
I
Dx
Load Regulation at Output Dx
V
IN
=3.6V
V
DX
=3.0V
V
DX
=4.0V
16
15.4
mA
I
DX
Line Regulation of Dx Output
Current
3.3V
V
IN
4.4V
V
DX
= 3.6V
15.7
mA
I
D-MATCH
Current Matching Between Any
Two Outputs
3.0V
V
IN
4.4V
V
D1
, V
D2
= 3.6V
0.3
%
I
Q
Quiescent Supply Current
3.0V
V
IN
4.4V, Active, No
Load Current
0.7
2
mA
I
SD
Shutdown Supply Current
3.0V
V
IN
5.5V, Shutdown at
85C
0.1
0.3
1
A
V
IH
SD Input Logic High
3.0V
V
IN
5.5V, (Note 5)
0.8V
IN
V
V
IL
SD Input Logic Low
3.0V
V
IN
5.5V, (Note 5)
0.2V
IN
V
I
LEAK-SD
SD Input Leakage Current
0V
V
SD
V
IN
0.1
A
R
BRGT
BRGT Input Resistance
250
k
I
SET
I
SET
Pin Output Current
I
Dx
/25
mA
f
SW
Switching Frequency (Note 4)
3.0V
V
IN
4.4V
450
650
850
kHz
t
START
Startup Time(Note 6)
I
Dx
= 90% steady state
10
s
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device
beyond its rated operating conditions.
Note 2: D1 and D2 may be shorted to GND without damage. P
OUT
may be shorted to GND for 1sec without damage.
Note 3: In the test circuit, all capacitors are 1.0F, 0.3
maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output
voltage and efficiency.
Note 4: The output switches operate at one half of the oscillator frequency, f
OSC
= 2f
SW
.
Note 5: The interanl thresholds of the shutdown bar are set at about 40% of V
IN
.
Note 6: This electrical specification is guaranteed by design.
Note 7: For more inforamtion regarding the LLP package, please refer to National Semiconductor Application note AN1187
LM2791
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4
Typical Performance Characteristics
Unless otherwise specified, C1 = C
IN
, C
HOLD
= 1uF, V
IN
= 3.6V,
V
DIODE
= 3.6V, BRGT pin =0V, R
SET
= 330
Supply Current at No Load
Supply Current vs Supply Voltage
20018305
20018312
SD-Threshold
I
DIODE
vs V
DIODE
20018308
20018310
Diode Current vs. Temperature
I
DIODE
vs R
Set
20018320
20018311
LM2791
www.national.com
5
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