NSC LM2791LDX-L Current regulated switched capacitor led driver with analog brightness control Datasheet

LM2791
Current Regulated Switched Capacitor LED Driver with
Analog Brightness Control
General Description
Features
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 accommodate two high forward voltage (typically white) LEDs. The
switching frequency is 450kHz (min.) to keep the conducted
noise spectrum away from sensitive frequencies within portable 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 shutdown pin reduces the operating current to 1µA (max.).
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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
Output matching of ± 0.3%
Drives up to two LED’s
3V to 5.8V Input Voltage
Up to 36mA output current
Soft start limits inrush current
Analog brightness control
Separate shutdown input
Very small solution size - no inductor
0.7mA typical operating current
1µA (max.) shutdown current
450kHz switching frequency (min.)
Linear regulation generates predictable noise spectrum
LLP-10 package: 3mm X 3mm X 0.8mm
Applications
Basic Application Circuit
20018301
© 2002 National Semiconductor Corporation
DS200183
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LM2791 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control
July 2002
LM2791
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
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2
LM2791
Pin Description
Pin
Name
Function
1
BRGT
Variable voltage input controls output current.
2
POUT
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
VIN
Power supply voltage input.
9
SD/SD
10
ISET
Shutdown input. Device operation is inhibited when pin is asserted.
Current Sense Input. Connect resistor to ground to set constant current through LED.
Block Diagram
20018302
3
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LM2791
Absolute Maximum Ratings
ESD Rating
(Note 1)
Human Body Model
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
−0.3 to 6V
VIN
BRGT, SD
Input Voltage (VIN)
400 mW
TJMAX (Note 2)
3.0V to 5.8V
BRGT
150˚C
0 to 3.0V
55˚C/W
Ambient Temperature (TA)
−30˚C to +85˚C
−65˚C to +100˚C
Junction Temperature (TJ)
−30˚C to +100˚C
θJA (Note 7)
Storge Temperature
200V
Operating Conditions
-0.3 to (VIN +0.2V)
Power Dissipation(Note 2)
2KV
Machine Model
Lead Temp. (Soldering, 5 sec.)
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
IDx
Parameter
Conditions
Min
Diode Current at ID1,2
VIN= 3V, RSET = 270Ω
16.5
Available Current at Output Dx
VIN= 3V
Load Regulation at Output Dx
Typ
Max
Units
mA
18
14.5
VIN= 3.3V
12.8
15.1
17.7
VIN= 3.6V
13.3
15.7
18.4
VIN = 4.4V
16.8
VIN =3.6V
VDX=3.0V
VDX=4.0V
16
15.4
mA
IDX
Line Regulation of Dx Output
Current
3.3V ≤ VIN ≤ 4.4V
VDX = 3.6V
15.7
mA
ID-MATCH
Current Matching Between Any
Two Outputs
3.0V ≤ VIN ≤ 4.4V
VD1, VD2 = 3.6V
0.3
%
IQ
Quiescent Supply Current
3.0V ≤ VIN ≤ 4.4V, Active, No
Load Current
0.7
2
mA
ISD
Shutdown Supply Current
3.0V ≤ VIN ≤ 5.5V, Shutdown at
85˚C
0.1
0.3
1
µA
VIH
SD Input Logic High
3.0V ≤ VIN ≤ 5.5V, (Note 5)
VIL
SD Input Logic Low
3.0V ≤ VIN ≤ 5.5V, (Note 5)
ILEAK-SD
SD Input Leakage Current
0V ≤ VSD ≤ VIN
RBRGT
BRGT Input Resistance
ISET
ISET Pin Output Current
fSW
Switching Frequency (Note 4)
3.0V ≤ VIN ≤ 4.4V
tSTART
Startup Time(Note 6)
IDx = 90% steady state
0.8VIN
V
0.2VIN
µA
250
kΩ
IDx/25
450
V
0.1
650
10
mA
850
kHz
µs
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. POUT may be shorted to GND for 1sec without damage.
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, fOSC = 2fSW.
Note 5: The interanl thresholds of the shutdown bar are set at about 40% of VIN.
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
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Unless otherwise specified, C1 = CIN, CHOLD= 1uF, VIN= 3.6V,
Supply Current at No Load
Supply Current vs Supply Voltage
20018312
20018305
SD-Threshold
IDIODE vs VDIODE
20018308
20018310
Diode Current vs. Temperature
IDIODE vs RSet
20018320
20018311
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LM2791
Typical Performance Characteristics
VDIODE = 3.6V, BRGT pin =0V, RSET = 330Ω
LM2791
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 @ 3.0VIN
Start Up Time @ 3.6VIN
20018314
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20018315
6
Start Up Time @ 4.2 VIN
20018316
7
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LM2791
Typical Performance Characteristics Unless otherwise specified, C1 = CIN, CHOLD= 1uF, VIN= 3.6V,
VDIODE = 3.6V, BRGT pin =0V, RSET = 330Ω (Continued)
LM2791
Circuit Description
The LM2791 provides two matched current sources for driving 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 lowest cost, the LM2791 can be used for constant brightness by
grounding BRGT and enabling the shutdown pin.
20018304
FIGURE 1.
Application Information
CAPACITOR SELECTION
SOFT START
LM2791 includes a soft start function to reduce the inrush
currents and high peak current during power up of the device. 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
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.
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.
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.
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Manufacturer
Contact
website
TDK
(847) 803
6100
www.component.tdk.com
MuRata
(800) 831
9172
www.murata.com
Taiyo Yuden
(800) 348
2496
www.t-yuden.com
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
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 capability 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:
RSET = (((BRGT * 0.42) + VOFFSET))/(ILED)* 25
(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 manufactures.
TABLE 4. RSETSelections ( when BRGT pin = 0V)
ILED per LED
*RSET
15mA
330Ω
Schottky Diodes
10mA
500Ω
5mA
1K
TABLE 2. Diode Manufactures
Manufacturer
Contact
ON
Semiconductor
(800) 344
3860
www.onsemi.con
Phillips
Semiconductors
(800) 234
7381
www.philipssemiconduc
tor.com
Panasonic
Semiconductor
(408) 945
5622
www.panasonic.com
* Use 1% resitor for Rset
Calculation of LED Current When Grounding BRGT:
VIN = 3.6V
VOFFSET = 200mV(Reference Voltage)
RSET = 330Ω
ILED = ( VOFFSET/RSET) *25
ILED = (200mV/330) *25 = 15mA
LED SELECTION
The LM2791 is designed to drive LEDs with a forward voltage of about 3.0V to 4.0V or higher. The typical and maximum VF depends highly on the manufacturer and their technology. 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 configuration 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 damage 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
VOFFSET = 200mV(Reference Voltage)
ILED = (((BRGT * 0.42) + VOFFSET)/ RSET )* 25
ILED = (((2.5*0.42) + 0.20)/2000 )*25 =15.6mA
TABLE 3. White LED Selection:
Component
Manufacture Contact
LWT673/LWT67C
Osram
NSCW100/ NSCW215 Nichia
www.osram-os.com
www.nichia.com
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)
1000Ω
1500Ω
2000Ω
2500Ω
BRGT (V)
ILED (mA)
ILED (mA)
ILED (mA)
ILED (mA)
0.5
10.25
6.84
5.10
4.1
1.0
15.5
10.3
7.75
6.2
1.5
20.75
13.8
10.37
8.3
2.0
26
17.3
13.00
10.4
2.5
31.25
20.80
15.6
12.5
3.0
36.5
24.3
18.3
14.6
9
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LM2791
Application Information
LM2791
Application Information
Efficiency = (VDIODE* IDIODE) / ( VIN * IDIODE* Gain) =
VDIODE / 2VIN
In the case of the LM2791, a more accurate efficiency calculation can be applied as the given formula below.
Efficiency = ((VD1* ID1) + (VD2* ID2)) / (ISUPPLY* VIN)
(Continued)
BRIGHTNESS CONTROL USING PWM
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.
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:
PDissipation = (2VIN -VDIODE)*ILOAD
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.
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT
DRIVE
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 Characteristics and limits previously presented. The available Dx output current, maximum Dx voltage, and all other specifications provided in the Electrical Characteristics table apply to
this parallel output configuration, just as they do to the
standard 2-LED application circuit.
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 package 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 Application Note AN-1187.
THERMAL PROTECTION
The LM2791 has internal thermal protection circuitry to disable 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
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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LM2791 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control
Physical Dimensions
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