NSC LM2792LDX-H, LM2792LD-L, LM2792EVAL Datasheet

LM2792
Current Regulated Switched Capacitor LED Driver with
Analog Brightness Control

General Description

The LM2792 is a CMOS charge-pump voltage doubler and
regulator that provides two regulated current sources. They
are designed to drive two white (or blue) LEDs with matched
currents (within

±

0.3%) to produce balanced light sources
for display backlights. The LM2792 accepts an input voltage
range from 3.0V to 5.8V and maintain a constant current
determined by an external set resistor.

The LM2792 delivers up to 34mA of load current to accom­modate two high forward voltage (typically white) LEDs. The
switching frequency is 900kHz (min.) to keep the conducted
noise spectrum away from sensitive frequencies within por­table RF devices.

The LM2792 offers full off to maximum current control
through the BRGT pin. The output current linearly tracks the
BRGT pin voltage. The LM2792 is available in active high or
low shutdown versions. The shutdown pin reduces the op­erating current to 1µA (max.).

The LM2792 is available in a 10 pin leadless leadframe
(LLP) CSP package.

Features

n Output matching of±0.3% (typ.)
n Drives up to two LED’s
n 3.0V to 5.8V Input Voltage
n Up to 34mA output current
n Soft start limits inrush current
n Analog brightness control
n Separate shutdown input
n Very small solution size and no inductor
n 1.4mA typical operating current
n 1µA (max.) shutdown current
n 900kHz 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

20024201

July 2002

LM2792 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control

© 2002 National Semiconductor Corporation DS200242 www.national.com

Connection Diagram

20024203

Top View

10-Lead LLP

Ordering Information

Order Number Shutdown Polarity NSC Package

Drawing

Package

Marking

Supplied As

LM2792LD-L Active Low LLP-10 SRB 1000 Units, Tape and Reel

LM2792LDX-L Active Low LLP-10 SRB 4500 Units, Tape and Reel

LM2792LD-H Active High LLP-10 SPB 1000 Units, Tape and Reel

LM2792LDX-H Active High LLP-10 SPB 4500 Units, Tape and Reel

LM2792

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Pin Description

Pin Name Function

1 BRGT Variable voltage input controls output current.

2P

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.

8V

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

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LM2792

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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.

V

IN

−0.3 to 6.0V

BRGT, SD −0.3 to (V

IN

+0.2V)

Power Dissipation (Note 2) 400 mW

T

JMAX

(Note 2) 150˚C

θ

JA

(Note 7) 55˚C/W

Storge Temperature −65˚C to +150˚C

Lead Temp. (Soldering, 5 sec.) 260˚C

ESD Rating

Human Body Model 2KV

Machine Model 200V

Operating Conditions

Input Voltage (VIN) 3.0V to 5.8V

BRGT 0 to 3.0V

Ambient Temperature (T

A

) −30˚C to +85˚C

34mAOperating Junction
Temperature

−30˚C to 100˚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, C

1=CIN=CHOLD

= 1 µF, VIN= 3.6V, BRGT pin = 2.75V.

Symbol Parameter Conditions Min Typ Max Units

I

DX

Available Current at Output Dx 3.0V ≤ VIN≤ 5.8V

V

Dx

≤ 3.6V

R

SET

= 1650Ω V

BRGT

=3V

14.5 16.8 mA

I

DX

Line Regulation of Dx Output
Current

3.0V ≤ VIN≤ 4.4V
V

Dx

≤ 3.6V

R

SET

= 1800Ω V

BRGT

= 2.75V

13.5 16 17.8 mA

V

DX

Load Regulation of Dx Output
Current

VIN= 3.6V
V

Dx

= 3.0V

V

Dx

= 4.0V

R

SET

= 1800Ω V

BRGT

= 2.75V

16.1

15.4

mA

I

D-MATCH

Current Matching Between Any
Two Outputs

VD1=VD2= 3.6V,
BRGT = 2.75V, V

IN

= 3.6V,

R

SET

= 1800Ω

0.3 %

I

Q

Quiescent Supply Current 3.0V ≤ VIN≤ 4.4V, Active, No

Load Current

1.4 2.7 mA

I

SD

Shutdown Supply Current 3.0V ≤ VIN≤ 5.5V, Shutdown

At 85˚C

0.1

0.3

1µA

V

IH

SD Input Logic High 3.0V ≤ VIN≤ 5.5V, Note5 0.8* V

IN

V

V

IL

SD Input Logic Low 3.0V ≤ VIN≤ 5.5V, Note5 0.2* V

IN

V

I

LEAK-SD

SD Input Leakage Current 0V ≤ VSD≤ V

IN

0.01 µA

R

BRGT

BRGT Input Resistance 250 kΩ

BRGT Brightness Voltage Range 0 3.0 V

I

SET

I

SET

Pin Output Current IDx/25 mA

f

SW

Switching Frequency (Note 4) 3.0V ≤ VIN≤ 4.4V

I

D1=ID2

≤ 16mA

900 1100 1800 kHz

t

START

Startup Time (Note 6) IDx= 90% steady state 10 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. P

OUT

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, f

OSC

=2fSW.

Note 5: The internal thresholds of the Shutdown bar are set at about 40% of V

IN

Note 6: This electrical specification is quarantee by design

Note 7: For more information regarding the LLP package, please refer to National Semiconductor Application note AN1187

LM2792

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Typical Performance Characteristics Unless otherwise specified, C

1=CIN=CHOLD

= 1uF, VIN=

3.6V, BRGT pin =2.75V

Input Supply Current vs. V

IN(ID1=ID2

=16mA) I

DIODE

vs. V

IN

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Shutdown Threshold vs. V

IN

I

DIODE

vs. Temperature

20024219

20024207

I

Q(SHUTSOWN)

vs. Temperature I

DIODE

vs. R

SET

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Typical Performance Characteristics Unless otherwise specified, C

1=CIN=CHOLD

= 1uF, VIN=

3.6V, BRGT pin =2.75V (Continued)

I

DIODE

vs. BRGT Switch Frequency vs. Temperature

20024216 20024211

V

SET

vs. V

BRGT

I

DIODE

vs. V

DIODE

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LM2792

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Circuit Description

The LM2792 provides two matched current sources for driv­ing high forward voltage drop LEDs from Li-Ion battery
sources. The part 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
34mA 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 section).

Application Information

SOFT START

LM2792 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.

SHUTDOWN MODE

A shutdown pin (SD or SD) is available to disable the
LM2792 and reduce the quiescent current to 1µA maximum.
The LM2792 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 V

IN

, will
enable the device. Pulling SD low or connecting SD 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 V

IN

will disable the device.

CAPACITOR SELECTION

Low equivalent series resistance (ESR) capacitors such as
X5R or X7R are recommended to be used for C

IN

, C1, and

C

HOLD

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

Manufacturer Contact website

TDK [phone] www.component.tdk.com

MuRata [phone] www.murata.com

Taiyo Yuden [phone] www.t-yuden.com

SCHOTTKY DIODE SELECTION

A schottky diode (SD1) must be used between V

IN

and P

OUT

for proper operation. During start-up, the low voltage drop
across this diode is used to charge C

OUT

and start the
oscillator. It is necessary to protect the device from
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

20024204

FIGURE 1.

LM2792

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Application Information (Continued)

be used for most applications or a surface mount diode such
as BAT54-series and MA2J704 can be used to reduce the
circuit size.Table 2 below lists suggested schottky diode
manufactures.

TABLE 2. Diode Manufactures

Manufacturer Contact website

ON

Semiconductor

(800) 344

3860

www.onsemi.com

Phillips

Semiconductors

(800) 234

7381

www.philipssemi-

conductor.com

Panasonic

Semiconductors

(408) 945

5622

www.panasonic.com

LED SELECTION

The LM2792 is designed to drive LEDs with a forward volt­age of about 3.0V to 4.0V or higher. The typical and maxi­mum V

F

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 LM2792. 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 LM2792. For best fit
selection for an application, consult the manufacturer for
detailed information.

TABLE 3. White LED Selection:

Component Manufacture Contact

LWT673/LWT67C Osram www.osram-os.com

NSCW100/NSCW215 Nichia www.nichia.com

I

SET

PIN

An external resistor, R

SET

, sets the mirror current that is
required to provide a constant current through the LEDs. The
current through R

SET

and the LED is set by the internal
current mirror circuitry with a ratio of 25:1 The currents
through each LED are matched within 0.3%. R

SET

should be
chosen not to exceed the maximum current delivery capa­bility of the device. Table 3 shows a list of R

SET

values when
maximun BRGT=3V is applied. For other BRGT voltages,
R

SET

can be calculated using this formula:

R

SET

= ((0.42*BRGT) / I

SET

)*25

TABLE 4. R

SET

Selections ( when BRGT pin = 3V

maximum)

I

LED

per LED R

SET

15mA 2.1KΩ

10mA 3.15KΩ

5mA 6.3KΩ

BRGT PIN

The BRGT pin can be used to smoothly vary the brightness
of the LEDs. In the LM2792, current on BRGT is connected
to an internal resistor divider which gives a factor of 0.42
(see Figure 1) . This voltage is fed to the operational ampli­fier that controls the current through the mirror resistor R

SET

.

The nominal range on BRGT is 0V to 3V.
This means some current must be provided on the BRGT pin

or no current will flow through the LEDs. The LM2792 can
provide an infinite ratio, from fully off (essentially zero cur­rent) to the maximum current set by the R

SET

resistor. Care
must be taken to prevent voltages on BRGT that cause LED
current to exceed a total of 34mA. Although this will not
cause damage to the IC, it will not meet the guaranteed
specifications listed in the Electrical Characteristics.

Calculation of LED Current When Using BRGT :

V

IN

= 3.6V

R

SET

= 1800Ω

I

SET

= ((V

BRGT

*

(0.42) / R

SET

)*25

I

SET

= ((2.75*(0.42)) / 1800 )*25 = 16mA

Note that making V

BRGT

= 0V results in I

SET

∼ = 0mA

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
LM2792, connect BRGT pin to a maximun of 3V to ensure
the widest range. Similarly, the voltage at the BRGT pin can
be set higher than 3V without damage to the IC, it will not
increase the brigthness of the LED significantly. R

SET

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 R

SET

is set to produce a
maximum LED current of 15mA. So the LED current varies
linearly with the duty cycle.

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. R

SET

and

V

BRGT

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, R

SET

and V

BRGT

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 LM2792 and 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.

THERMAL PROTECTION

The LM2792 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

LM2792

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Application Information (Continued)

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:

Efficiency = (V

DIODE*IDIODE

)/(VIN*I

DIODE

* Gain) =

V

DIODE

/2V

IN

In the case of the LM2792, a more accurate efficiency cal­culation can be applied as the given formula below.

Efficiency = ((V

D1*ID1

)+(VD2*ID2))/(I

SUPPLY*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:

P

DMax

=(T

JMax-TA

)/θ

JA

where T

JMax

is the maximum junction temperature, TAis the

ambient temperature, and θ

JA

is the junction-to-ambient

thermal resistance of the specified package.

The actual power dissipation of the device can be calculated
using this equation:

P

Dissipation

= (2VIN-V

DIODE

)*I

LOAD

As an example, if VINin the target application is 4.2V, V

DIODE

= 3.0V and worse case current consumption is 32mA (17mA
for each diode).

P

Dissipation

= ((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 LM2792.

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.

LM2792

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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
the life support device or system, or to affect its
safety or effectiveness.

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LM2792 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control

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.