LM2791 [NSC]
Current Regulated Switched Capacitor LED Driver with Analog Brightness Control; 电流调节开关电容LED驱动器，提供模拟亮度控制
| 型号： | LM2791 |
| 厂家： | 
National Semiconductor |
| 描述： | Current Regulated Switched Capacitor LED Driver with Analog Brightness Control |
| 文件： | 总11页 (文件大小：225K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
July 2002
LM2791
Current Regulated Switched Capacitor LED Driver with
Analog Brightness Control
General Description
Features
n Output matching of 0.3%
n Drives up to two LED’s
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.
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 1µA (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
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 1µA (max.).
Applications
The LM2791 is available in a 10 pin leadless leadframe
(LLP) CSP package.
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
© 2002 National Semiconductor Corporation
DS200183
www.national.com
Connection Diagram
20018303
Top View
10-Lead LLP
Ordering Information
Order Number
Shutdown Polarity
NSC Package
Package
Marking
SNB
Supplied As
Drawing
LLP-10
LLP-10
LLP-10
LLP-10
LM2791LD-L
LM2791LDX-L
LM2791LD-H
LM2791LDX-H
Active Low
Active Low
Active High
Active High
1000 Units, Tape and Reel
4500 Units, Tape and Reel
1000 Units, Tape and Reel
4500 Units, Tape and Reel
SNB
SLB
SLB
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2
Pin Description
Pin
1
Name
Function
Variable voltage input controls output current.
BRGT
POUT
C1−
C1+
D2
2
Charge pump output.
3
Connect this pin to the negative terminal of C1.
Connect this pin to the positive terminal of C1.
Current source outputs. Connect directly to LED.
Current source outputs. Connect directly to LED.
Power supply ground input.
4
5
6
D1
7
GND
VIN
8
Power supply voltage input.
9
SD/SD
ISET
Shutdown input. Device operation is inhibited when pin is asserted.
Current Sense Input. Connect resistor to ground to set constant current through LED.
10
Block Diagram
20018302
3
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Rating
Human Body Model
Machine Model
2KV
200V
VIN
−0.3 to 6V
-0.3 to (VIN +0.2V)
400 mW
Operating Conditions
BRGT, SD
Input Voltage (VIN
)
3.0V to 5.8V
0 to 3.0V
Power Dissipation(Note 2)
TJMAX (Note 2)
BRGT
150˚C
Ambient Temperature (TA)
Junction Temperature (TJ)
−30˚C to +85˚C
−30˚C to +100˚C
θJA (Note 7)
55˚C/W
Storge Temperature
Lead Temp. (Soldering, 5 sec.)
−65˚C to +100˚C
260˚C
Electrical Characteristics
Limits in standard typeface are for TJ = 25˚C and limits in boldface type apply over the full Operating Temperature Range.
Unless otherwise specified, C1 = CIN = CHOLD = 1 µF, VIN = 3.6V, VDIODE = 3.6V, RSET = 332Ω, BRGT pin = 0V.
Symbol
IDX
Parameter
Conditions
VIN= 3V, RSET = 270Ω
VIN= 3V
Min
Typ
18
Max
Units
Diode Current at ID1,2
16.5
Available Current at Output Dx
14.5
15.1
15.7
16.8
V
IN= 3.3V
IN= 3.6V
12.8
13.3
17.7
18.4
mA
V
VIN = 4.4V
IDx
Load Regulation at Output Dx
VIN =3.6V
16
VDX=3.0V
mA
mA
%
15.4
VDX=4.0V
IDX
Line Regulation of Dx Output
Current
3.3V ≤ VIN ≤ 4.4V
VDX = 3.6V
15.7
0.3
ID-MATCH
IQ
Current Matching Between Any
Two Outputs
3.0V ≤ VIN ≤ 4.4V
VD1, VD2 = 3.6V
3.0V ≤ VIN ≤ 4.4V, Active, No
Load Current
Quiescent Supply Current
0.7
2
mA
µA
ISD
Shutdown Supply Current
3.0V ≤ VIN ≤ 5.5V, Shutdown at
85˚C
0.1
0.3
1
VIH
SD Input Logic High
3.0V ≤ VIN ≤ 5.5V, (Note 5)
3.0V ≤ VIN ≤ 5.5V, (Note 5)
0V ≤ VSD ≤ VIN
0.8VIN
V
V
VIL
SD Input Logic Low
0.2VIN
ILEAK-SD
RBRGT
ISET
SD Input Leakage Current
BRGT Input Resistance
ISET Pin Output Current
Switching Frequency (Note 4)
Startup Time(Note 6)
0.1
250
IDx/25
650
10
µA
kΩ
mA
kHz
µs
fSW
3.0V ≤ VIN ≤ 4.4V
450
850
tSTART
IDx = 90% steady state
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
may be shorted to GND for 1sec without damage.
OUT
Note 3: In the test circuit, all capacitors are 1.0µF, 0.3Ω 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
= 2f
.
OSC
SW
Note 5: The interanl thresholds of the shutdown bar are set at about 40% of V
Note 6: This electrical specification is guaranteed by design.
.
IN
Note 7: For more inforamtion regarding the LLP package, please refer to National Semiconductor Application note AN1187
www.national.com
4
Typical Performance Characteristics Unless otherwise specified, C1 = CIN, CHOLD= 1uF, VIN= 3.6V,
VDIODE = 3.6V, BRGT pin =0V, RSET = 330Ω
Supply Current at No Load
Supply Current vs Supply Voltage
20018305
20018312
SD-Threshold
IDIODE vs VDIODE
20018308
20018310
Diode Current vs. Temperature
IDIODE vs RSet
20018320
20018311
5
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Typical Performance Characteristics Unless otherwise specified, C1 = CIN, CHOLD= 1uF, VIN= 3.6V,
VDIODE = 3.6V, BRGT pin =0V, RSET = 330Ω (Continued)
IDIODE vs BRGT
VSET vs BRGT
20018313
20018309
Switching Frequency vs Supply Voltage
Switching Frequency vs. Temperature
20018306
20018321
@
@
Start Up Time 3.6VIN
Start Up Time 3.0VIN
20018314
20018315
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Typical Performance Characteristics Unless otherwise specified, C1 = CIN, CHOLD= 1uF, VIN= 3.6V,
VDIODE = 3.6V, BRGT pin =0V, RSET = 330Ω (Continued)
@
Start Up Time 4.2 VIN
20018316
7
www.national.com
Circuit Description
The LM2791 provides two matched current sources for driv-
ing high forward voltage drop LEDs from Li-Ion battery
sources. The device has on-chip current regulators which
are composed of current mirrors with a 25 to 1 ratio. The
mirrors control the LED current without using current limiting
resistors in the LED current path. The device can drive up to
a total of 36mA through the LEDs.
The LED brightness can be controlled by both analog and or
digital methods. The digital technique uses a PWM (Pulse
Width Modulation) signal applied to the shutdown input. The
analog technique applies an analog voltage to the brightness
(BRGT) pin (see Application Information sections). For low-
est cost, the LM2791 can be used for constant brightness by
grounding BRGT and enabling the shutdown pin.
20018304
FIGURE 1.
Application Information
SOFT START
CAPACITOR SELECTION
LM2791 includes a soft start function to reduce the inrush
currents and high peak current during power up of the de-
vice. Soft start is implemented internally by ramping the
bandgap more slowly than the applied voltage. This is done
by holding the bandgap in shutdown for a short time. During
soft start, the switch resistances limit the inrush current used
to charge the flying and hold capacitors.
Low equivalent series resistance (ESR) capacitors such as
X5R or X7R are recommended to be used for CIN, C1, C2,
and CHOLD for best performance. Ceramic capacitors with
less than or equal to 0.3 ohms ESR value are recommended
for this application. Table 1 below lists suggested capacitor
suppliers for the typical application circuit.
TABLE 1. Low ESR Capacitor Manufactures
SHUTDOWN MODE
Manufacturer
Contact
(847) 803
6100
website
A shutdown pin (SD or /SD) is available to disable the
LM2791 and reduce the quiescent current to 1µA maximum.
The LM2791 is available with both senses of shutdown
polarity.
TDK
www.component.tdk.com
MuRata
(800) 831
9172
www.murata.com
www.t-yuden.com
During normal operation mode of the ’-L’ options, an active
high logic signal to the SD pin or tying the SD pin to VIN, will
enable the device. Pulling SD low or connectingSD to
ground will disable the device.
Taiyo Yuden
(800) 348
2496
During normal operation mode of the ’-H’ options, an active
low logic signal to the SD pin or tying the SD pin to GND, will
enable the device. Pulling SD high or connecting SD to VIN
will disable the device.
SCHOTTKY DIODE SELECTION
A schottky diode (D1) must be used between VIN and POUT
for proper operation. During start-up, the low voltage drop
across this diode is used to charge COUT and start the
oscillator. It is necessary to protect the device from
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8
current mirror circuitry with a ratio of 25:1 The currents
through each LED are matched within 0.3%. RSET should be
chosen not to exceed the maximum current delivery capa-
bility of the device. Table 4 shows a list of RSET values when
maximun BRGT = 0V is applied. For other BRGT voltages,
RSET can be calculated using this formula:
Application Information (Continued)
turning-on its own parasitic diode and potentially latching-up.
As a result, it is important to select a schottky diode that will
carry at least 200mA or higher current to charge the output
capacitor during start-up. A schottky diode like 1N5817 can
be used for most applications or a surface mount diode such
as BAT54-series and MA2J704 used to reduce the circuit
size. Table 2 below lists suggested schottky diode manufac-
tures.
*
*
25
RSET = (((BRGT 0.42) + VOFFSET))/(ILED
)
TABLE 4. RSETSelections ( when BRGT pin = 0V)
ILED per LED
15mA
*RSET
330Ω
500Ω
1K
TABLE 2. Diode Manufactures
10mA
Manufacturer
ON
Contact
(800) 344
3860
Schottky Diodes
5mA
www.onsemi.con
Semiconductor
Phillips
* Use 1% resitor for Rset
(800) 234
7381
www.philipssemiconduc
tor.com
Calculation of LED Current When Grounding BRGT:
VIN = 3.6V
Semiconductors
Panasonic
(408) 945
5622
www.panasonic.com
VOFFSET = 200mV(Reference Voltage)
RSET = 330Ω
Semiconductor
*
) 25
ILED = ( VOFFSET/RSET
LED SELECTION
*
ILED = (200mV/330) 25 = 15mA
The LM2791 is designed to drive LEDs with a forward volt-
age of about 3.0V to 4.0V or higher. The typical and maxi-
mum VF depends highly on the manufacturer and their tech-
nology. Table 3 lists two suggested manufactures and
example part numbers. Each supplier makes many LEDs
that work well with the LM2791. The LEDs suggested below
are in a surface mount package and TOPLED or SIDEVIEW
configuration with a maximum forward current of 20mA.
These diodes also come in SIDELED or SIDEVIEW configu-
ration and various chromaticity groups. For applications that
demand color and brigthness matching, care must be taken
to select LEDs from the same chromaticity group. Forward
current matching is assured over the LED process variations
due to the constant current output of the LM2791. For best fit
selection for an application, consult the manufacturer for
detailed information.
BRGT PIN
The BRGT pin can be used to smoothly vary the brightness
of the White LEDs. In the LM2791, current on BRGT is
connected to an internal resistor divider which gives a factor
0.42 and summed with an offset voltage from the internal
bandgap (200mV). This voltage is fed to the operational
amplifier that controls the current through the mirror resistor
RSET. The nominal range on BRGT is 0V to 3V. Care must
be taken to prevent voltages on BRGT that cause LED
current to exceed 36mA. Although this will not cause dam-
age to the IC, it will not meet the guaranteed specifications
listed in the Electrical Characteristics.
Table 5 shows the current through each LED for the LM2791
with various BRGT and RSET values.
>
Calculation of LED Current When BRGT Pin 0:
RSET = 2000Ω
BRGT = 2.5V
TABLE 3. White LED Selection:
Component
Manufacture Contact
Osram www.osram-os.com
www.nichia.com
VOFFSET = 200mV(Reference Voltage)
LWT673/LWT67C
*
*
ILED = (((BRGT 0.42) + VOFFSET)/ RSET
)
25
NSCW100/ NSCW215 Nichia
*
*
ILED = (((2.5 0.42) + 0.20)/2000 ) 25 =15.6mA
ISET PIN
An external resistor, RSET, sets the mirror current that is
required to provide a constant current through the LEDs. The
current through RSET and the LED is set by the internal
TABLE 5. LED Current When Using BRGT Input (Values Highlighted in Boldface exceeded maximum current range of
the device if both LEDs are in use)
RSET (Ω)
BRGT (V)
0.5
1000Ω
ILED (mA)
10.25
15.5
1500Ω
ILED (mA)
6.84
2000Ω
ILED (mA)
5.10
2500Ω
ILED (mA)
4.1
1.0
10.3
7.75
6.2
1.5
20.75
26
13.8
10.37
13.00
15.6
8.3
2.0
17.3
10.4
2.5
31.25
36.5
20.80
24.3
12.5
3.0
18.3
14.6
9
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Efficiency = (VDIODE* IDIODE) / ( VIN * IDIODE* Gain) =
VDIODE / 2VIN
Application Information (Continued)
BRIGHTNESS CONTROL USING PWM
In the case of the LM2791, a more accurate efficiency cal-
culation can be applied as the given formula below.
Brigthness control can be implemented by pulsing a signal at
the SD pin. The recommended signal should be between
100Hz to 1kHz. If the operating PWM frequency is much less
than 100Hz, flicker may be seen in the LEDs. Likewise, if
frequency is much higher, brightness in the LEDs will not be
linear. When a PWM signal is used to drive the SD pin of the
LM2791, connect BRGT pin to a maximun of GND. RSET
value is selected using the above I SET equation as if BRGT
pin is used. The brigthness is controlled by increasing and
decreasing the duty cycle of the PWM. Zero duty cycle will
turn off the brigthness and a 50% duty cycle waveform
produces an average current of 7.5mA if RSET is set to
produce a maximum LED current of 15mA. So the LED
current varies linearly with the duty cycle.
Efficiency = ((VD1* ID1) + (VD2* ID2)) / (ISUPPLY* VIN
)
It is clear that the efficiency will depend on the supply voltage
in the above equation. As such, the lower the supply voltage,
the higher the efficiency.
POWER DISSIPATION
The maximum allowable power dissipation that this package
is capable of handling can be determined as follows:
PDMax = (TJMax - TA) / θJA
where TJMax is the maximum junction temperature, TA is the
ambient temperature, and θJA is the junction-to-ambient
thermal resistance of the specified package.
The actual power dissipation of the device can be calculated
using this equation:
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT
DRIVE
*
PDissipation = (2VIN -VDIODE) ILOAD
Outputs D1 and D2 may be connected together to drive a
single LED. In such a configuration, two parallel current
sources of equal value drive the single LED. RSET and
VBRGT should be chosen so that the current through each of
the outputs is programmed to 50% of the total desired LED
current. For example, if 30mA is the desired drive current for
the single LED, RSET and VBRGT should be selected so that
the current through each of the outputs is 15mA. Connecting
the outputs in parallel does not affect internal operation of
the LM2791and has no impact on the Electrical Character-
istics and limits previously presented. The available Dx out-
put current, maximum Dx voltage, and all other specifica-
tions provided in the Electrical Characteristics table apply to
this parallel output configuration, just as they do to the
standard 2-LED application circuit.
As an example, if VIN in the target application is 4.2V, VDIODE
= 3.0V and worse case current consumption is 32mA (16mA
for each diode).
*
*
PDissipation = ((2 4.2) -3.0) 0.032 = 173mW
Power dissipation must be less than that allowed by the
package. Please refer to the Absolute Maximum Rating of
the LM2791.
PCB LAYOUT CONSIDERATIONS
The LLP is a leadframe based Chip Scale Package (CSP)
with very good thermal properties. This package has an
exposed DAP (die attach pad) at the center of the package
measuring 2.0mm x 1.2mm. The main advantage of this
exposed DAP is to offer lower thermal resistance when it is
soldered to the thermal land on the PCB. For PCB layout,
National highly recommends a 1:1 ratio between the pack-
age and the PCB thermal land. To further enhance thermal
conductivity, the PCB thermal land may include vias to a
ground plane. For more detailed instructions on mounting
LLP packages, please refer to National Semiconductor Ap-
plication Note AN-1187.
THERMAL PROTECTION
The LM2791 has internal thermal protection circuitry to dis-
able the charge pump if the junction temperature exceeds
150˚C. This feature will protect the device from damage due
to excessive power dissipation. The device will recover and
operate normally when the junction temperature falls below
the maximum operating junction temperature of 100˚C. It is
important to have good thermal conduction with a proper
layout to reduce thermal resistance.
POWER EFFICIENCY
An ideal power efficiency for a voltage doubler switched
capacitor converter is given as the output voltage of the
doubler over twice the input voltage as follows:
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10
Physical Dimensions inches (millimeters) unless otherwise noted
LLP-10 Pin Package (LDA)
For Ordering, Refer to Ordering Information Table
NS Package Number LDA10A
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whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
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safety or effectiveness.
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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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