Datasheet LM2796 Datasheet (National Semiconductor)

LM2796
Dual-Display White LED Driver with 3/2x Switched
Capacitor Boost

LM2796 Dual-Display White LED Driver with 3/2x Switched Capacitor Boost

February 2004

General Description

The LM2796 is a charge-pump based white-LED driver that
is ideal for mobile phone display backlighting. It can drive up
to 7 LEDs in parallel with up to 20mA through each LED.
Regulated internal current sources deliver excellent current
and brightness matching in all LEDs. The LED-driver current
sources are split into two independently controlled groups.
The primary group (4 LEDs) can be used to backlight the
main phone display. The second group (3 LEDs) can be
used to backlight a secondary display or to provide other
lighting features (keypad LEDs, for example). Brightness of
the two groups can be adjusted independently with pulse­width modulated (PWM) digital signals.

The LM2796 works off an extended Li-Ion input voltage
range (2.7V to 5.5V). Voltage boost is achieved with a high­efficiency 3/2x-gain charge pump.

The LM2796 is available in National’s chip-scale 18-bump
micro SMD package.

Typical Application Circuit

Features

n Drives up to 7 LEDs with up to 20mA each
n LEDs controlled in 2 Distinct Groups, for Backlighting 2

Displays (main LCD and sub-LCD)

n Excellent Current and Brightness Matching
n High-Efficiency 3/2x Charge Pump
n Extended Li-Ion Input: 2.7V to 5.5V
n PWM Brightness Control: 100Hz - 1kHz
n 18-bump Thin Micro SMD Package:

(2.1mm x 2.4mm x 0.6mm)

Applications

n Mobile Phone Display Lighting
n Mobile Phone Keypad Lighting
n PDAs
n General LED Lighting

20093801

© 2004 National Semiconductor Corporation DS200938 www.national.com

Connection Diagram

LM2796

Pin Description

Pin #s Pin Names Pin Descriptions

C1 V

D2 GND Ground

A3 P

A1, B2, A5, E1 C1+, C1-, C2+,

A7 EN Enable pin. Logic input. High = normal operation, Low = shutdown (charge

D6, E5, D4, E3 D1A, D2A, D3A,

C5, B4, C3 D1B, D2B, D3B LED Outputs - Group B

B6 EN-A Enable for Group-A LEDs (current outputs). Logic input.

E7 EN-B Enable for Group-B LEDs (current outputs). Logic input.

C7 I

18-Bump Thin Micro SMD Package, Large Bump

NS Package Number TLA18

IN

OUT

Input voltage. Input range: 2.7V to 5.5V.

Charge pump output. Approximately 1.5xV

Flying capacitor connections.

C2-

pump and all current sources OFF).

LED Outputs - Group A

D4A

High = Group-A LEDs ON. Low = Group A LEDs OFF.
Pulsing this pin with a PWM signal (100Hz-1kHz) can be used to dim LEDs.

High = Group-B LEDs ON. Low = Group B LEDs OFF.
Pulsing this pin with a PWM signal (100Hz-1kHz) can be used to dim LEDs.

SET

Placing a resistor (R

) between this pin and GND sets the LED current for

SET

all LEDs. LED Current = 100 x (1.25V ÷ R

IN

SET

20093802

).

Ordering Information

Order Information Package Supplied As

LM2796TL TLA18 Micro SMD 250 Units, Tape & Reel

LM2796TLX 3000 Units, Tape & Reel

www.national.com 2

LM2796

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.

pin voltage -0.3V to 7.1V

V

IN

EN, ENA, ENB pin voltages -0.3V to (V

+0.3V)

IN

Operating Rating (Notes 1, 2)

Input Voltage Range 2.7V to 5.5V

Junction Temperature (T

Ambient Temperature (T
(Note 5)

) Range -30˚C to +125˚C

J

) Range

A

-30˚C to +85˚C

w/ 5.6V max

Continuous Power Dissipation

Internally Limited

(Note 3)

Junction Temperature (T

) 150oC

J-MAX

Storage Temperature Range -65

Maximum Lead Temperature

o

C to +150oC

265

o

C

Thermal Properties

Juntion-to-Ambient Thermal
Resistance (θ

), (Note 6)

JA

100˚C/W

(Soldering, 10 sec.)

ESD Rating (Note 4)Human Body
Model:

2.0kV
200V

Machine Model

Electrical Characteristics (Notes 2, 7)

Limits in standard typeface and typical values apply for TJ=25oC. Limits in boldface type apply over the full operating junction
temperature range (-30˚C ≤ T
Group B LEDs not ON simultaneously (ENA = V

, and C

C

2

= 1µF. (Note 8)

POUT

≤ +85˚C) . Unless otherwise specified: VIN= 3.6V; V

J

and ENB = GND, or ENA = GND and ENB = VIN); R

IN

Symbol Parameter Condition Min Typ Max Units

≤ 4.2V, and VIN= 5.5V

IN

≤ 3.8V;

Dxx

= 8.35kΩ

≤ 5.5V;

IN

≤ 3.6V;

Dxx

= 6.25kΩ

≤ 5.5V;

IN

≤ 3.9V;

Dxx

= 12.5kΩ

≤ 3.0V;

IN

≤ 3.3V;

Dxx

= 8.35kΩ

outputs active),

DX

= 3.0V, CIN=C

OUT

= 2.2µF

= 3.0V (Note 10) 1 %

I

Dxx

I

Dxx-MATCH

Output Current Regulation

Current Matching Between Any

3.0V ≤ V

2.5V ≤ V
R

SET

3.0V ≤ V

2.5V ≤ V
R

SET

3.0V ≤ V

2.5V ≤ V
R

SET

2.7V ≤ V

2.5V ≤ V
R

SET

ENA and ENB ON (all 7 I
V

IN

V

IN

Two Group A Outputs or Group
B Outputs

I

Q

Quiescent Supply Current 2.7V ≤ VIN≤ 4.2V;

No Load Current,
EN = ON, ENA = ENB = OFF

I

SD

V

SET

I

Dxx/ISET

Shutdown Supply Current 2.7V ≤ VIN≤ 5.5V, EN = OFF 3 4.5 µA

I

Pin Voltage 2.7V ≤ VIN≤ 5.5V 1.25 V

SET

Output Current to Current Set
Ratio

R

OUT

Charge Pump Output Resistance

VIN= 3.0V 2.7 Ω

(Note 11)

V

HR

Current Source Headroom
Voltage Requirement (Note 12)

I

=95%XI

Dxx

= 8.35kΩ

R

SET

(nom) ≈ 15mA)

(I

Dxx

= 95%X I

I

Dxx

= 12.5kΩ

R

SET

(nom) ≈ 10mA)

(I

Dxx

Dxx

Dxx

(nom)

(nom)

= 3.6V; V(EN) = 2.0V; Group A and

Dxx

13.8

(-8%)

= 8.35kΩ;CIN,C1,

SET

15

16.2

(+8%)mA(%)

20

10

15

15

3.5 6 mA

100

320 mV

220

mA

mA

mA

mA

www.national.com3

Electrical Characteristics (Notes 2, 7) (Continued)

Limits in standard typeface and typical values apply for TJ=25oC. Limits in boldface type apply over the full operating junction

LM2796

temperature range (-30˚C ≤ T
Group B LEDs not ON simultaneously (ENA = V

, and C

C

2

Symbol Parameter Condition Min Typ Max Units

f

SW

t

START

1.5x/1x Charge pump gain cross-over:

Logic Pin Specifications: EN, ENA, ENB

V

IL

V

IH

I

LEAK

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: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at T

120˚C (typ.). The thermal shutdown function is guaranteed by design.

Note 4: The Human body model is a 100pF capacitor discharged through a 1.5k resistor into each pin. The machine model is a 200pF capacitor discharged directly
into each pin. MIL-STD-883 3015.7

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
dissipation of the device in the application (P
following equation: T

Note 6: Junction-to-ambient thermal resistance is highly dependent on application and board layout. In applications where high maximum power dissipation exists,
special care must be paid to thermal dissipation issues in board design.

Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.

Note 8: C

Note 9: If both LED groups are to be ON simultaneously, the maximum V

to the "MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE" section.

Note 10: For the two groups of outputs on a part (Group A and Group B), the following are determined: the maximum output current in the group (MAX), the
minimum output current in the group (MIN), and the average output current of the group (AVG). For each group, two matching numbers are calculated:
(MAX-AVG)/AVG and (AVG-MIN)/AVG. The largest number of the two (worst case) is considered the matching figure for the group. The matching figure for a given
part is considered to be the highest matching figure of the two groups. The typical specification provided is the most likely norm of the matching figure for all parts.

Note 11: Output resistance (R
V

=(1.5xVIN)–(R

Pout

equation applies when the charge pump is operating with a gain of 3/2 (V

Note 12: Headroom voltage: V

Note 13: There is a 300kΩ(typ.) pull-down resistor connected internally between each enable pin (EN, ENA, ENB) and GND.

= 1µF. (Note 8)

POUT

Switching Frequency 3.0V ≤ VIN≤ 4.2V 325 500 675 kHz

Start-up Time IDx= 90% steady state 100 µs

Gain = 1.5 when V
threshold. Gain = 1 when V
above threshold.

Input Logic Low 2.7V ≤ VIN≤ 5.5V 0 0.5 V

Input Logic High 2.7V ≤ VIN≤ 5.5V 1.1 V

Input Leakage Current V

A-MAX=TJ-MAX-OP

IN,COUT,C1

, and C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics

OUTxIOUT

≤ +85˚C) . Unless otherwise specified: VIN= 3.6V; V

J

and ENB = GND, or ENA = GND and ENB = VIN); R

IN

1.5x to 1x Threshold 4.75 V

is below

IN

is

1x to 1.5x Threshold 4.55 V

IN

= 0V 0.1 µA

ENx

V

= 3V (Note 13) 10

ENx

) is dependent on the maximum operating junction temperature (T

A-MAX

), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the

D-MAX

–(θJAxP

) models all voltage losses in the charge pump. R

OUT

). In the equation, I

HR=VPout–VDxx

).

D-MAX

voltage may need to be derated, depending on minimum input voltage conditions. Refer

Dxx

is the total output current: the sum of all active Dxx output currents and all current drawn from P

OUT

. If headroom voltage requirement is not met, LED current regulation will be compromised.

IN

OUT

≤ 4.75V typ.).

= 3.6V; V(EN) = 2.0V; Group A and

Dxx

= 160˚C (typ.) and disengages at TJ=

J

J-MAX-OP

can be used to estimate the voltage at the charge pump output (P

= 8.35kΩ;CIN,C1,

SET

IN

= 125˚C), the maximum power

OUT

V

OUT

. The

):

www.national.com 4

LM2796

Typical Performance Characteristics Unless otherwise specified: V

2.0V; V(ENA) = 2.0V; V(ENB) = 0V; R

LED Current vs. Input Voltage LED Current vs. R

LED Current vs. PWM Duty Cycle,

PWM Applied to ENA and/or ENB

= 8.3 kΩ;CIN,C1,C2, and C

SET

20093803

= 1 µF.

POUT

Charge Pump Output Resistance

vs. Ambient Temperature

= 3.6V; V

IN

Resistance

SET

= 3.6V; V(EN) =

DXX

20093804

20093805

20093806

www.national.com5

Circuit Description

LM2796

OVERVIEW

The LM2796 is primarily intended for Lithium-Ion battery
driven white-LED drive applications, and is well suited to
drive white LEDs that are used for backlighting small-format
displays. The part has seven matched constant-current out­puts, each capable of driving up to 20mA (or more) through
white LEDs. The well-matched current sources ensure the
current through all the LEDs is virtually identical. This keeps
brightness of all LEDs matched to near perfection so that
they can provide a consistent backlight over the entire dis­play.

The core of the LM2796 is a 1.5x/1x dual-mode charge
pump. The input of the charge pump is connected to the V
pin. The recommended input voltage range of the LM2796 is

2.7V to 5.5V. The output of the charge pump is the P
( “Pump OUTput”). The output voltage of the charge pump is
unregulated and varies with input voltage and load current.

The charge pump operates in the 1.5x mode when the input
voltage is below 4.75V (typ.). In this mode, the input-to­output voltage gain of the charge pump is 1.5, and the
voltage at the output of the charge pump will be approxi­mately 1.5x the input voltage (V(P
the 1.5x mode, the charge pump provides the voltage boost
that is required to drive white LEDs from a Li-Ion battery.
(White LEDs typically have a forward voltage in the range of

3.3V to 4.0V. A Li-Ion battery typically is not considered to be
fully discharged until the battery voltage falls to 3.0V (ap­prox.) )

The charge pump operates in the 1x mode when the input
voltage is above 4.75V (typ.). In these conditions, voltage
boost is not required to drive the LEDs, so the charge pump
merely passes the input voltage to P
This reduces the input current and the power dissipation of
the LM2796 when the input voltage is high.

The matched current outputs are generated with a precision
current mirror that is biased off the charge pump output.
Matched currents are ensured with the use of tightly
matched internal devices and internal mismatch cancellation
circuitry. Top-side current drive allows LEDs to be connected
between each current output and GND, simplifying PWB
routing and connectivity.

There are seven regulated current outputs. These seven
outputs are split into two groups, a group of 4 outputs and a
group of 3 outputs. There is an ON/OFF control pin for each
group.

The DC current through the LEDs is programmed with an
external resistor. Changing currents on-the-fly can be
achieved with the use of digital pulse (PWM) signals.

ENABLE PINS: EN, ENA, ENB

The LM2796 has 3 enable pins. All three are active-high
logic (HIGH = ON). There are internal pull-down resistors
(300kΩ typ.) that are connected internally between each of
the enable pins and GND.

The EN pin is the master enable pin for the part. When
voltage on this pin is low (

<

mode. All internal circuitry is OFF and the part consumes
very little supply current when the LM2796 is shutdown.
When the voltage on the EN pin is high (
active. The charge pump is ON, and it is possible to turn on
the output currents to drive the LEDs.

) ≈ 1.5*VIN). When in

OUT

(V(P

OUT

0.5V), the part is in shutdown

>

1.1V), the part is

OUT

pin

OUT

) ≈ VIN).

ENA and ENB are used to turn the output currents ON and
OFF. ENA activates/deactivates the four group-A outputs
(D1A-D4A). ENB activates/deactivates the three group-B
outputs (D1B-D3B).

SETTING LED CURRENTS

The output currents of the LM2796 can be set to a desired
value simply by connecting an appropriately sized resistor
(R

) between the I

SET

pin of the LM2796 and GND. The

SET

output currents (LED currents) are proportional to the current
that flows out of the I
of 100 greater than the I
internal amplifier sets the voltage of the I
(typ.). Placing a resistor between I

IN

above are simplified in the equations below:

the I

current, and thus the LED currents. The statements

SET

R

pin. The output currents are a factor

SET

current. The feedback loop of an

SET

and GND programs

SET

= 100 x(V

I

Dxx

= 100 x (1.25V / I

SET

SET/RSET

)

Dxx

pin to 1.25V

SET

)

Maximum Output Current, Maximum LED Voltage, Minimum Input Voltage

The LM2796 can drive 7 LEDs at 15mA each from an input
voltage as low as 3.0V, so long as the LEDs have a forward
voltage of 3.6V or less (room temperature).

The statement above is a simple example of the LED drive
capabilities of the LM2796. The statement contains the key
application parameters that are required to validate an LED­drive design using the LM2796: LED current (I
of active LEDs (N), LED forward voltage (V
mum input voltage (V

IN-MIN

).

), number

LED

), and mini-

LED

The equation below can be used to estimate the total output
current capability of the LM2796:

I

LED_MAX

I

LED_MAX

R

OUT

= ((1.5 x VIN)-V

= ((1.5 x VIN)-V

– Output resistance. This parameter models the inter-

)/((NxR

LED

) / ((N x 2.7Ω) + 22mV/mA)

LED

)+kHR) (eq. 1)

OUT

nal losses of the charge pump that result in voltage droop at
the pump output P

. Since the magnitude of the voltage

OUT

droop is proportional to the total output current of the charge
pump, the loss parameter is modeled as a resistance. The

(eq. 2)

= 3.0V,

IN

output resistance of the LM2796 is typically 2.7Ω (V

= 25˚C). In equation form:

T

A

= 1.5xVIN– NxI

V

POUT

– Headroom constant. This parameter models the mini-

k

HR

LEDxROUT

mum voltage required to be present across the current
sources for them to regulate properly. This minimum voltage
is proportional to the programmed LED current, so the con­stant has units of mV/mA. The typical k

of the LM2796 is

HR

22mV/mA. In equation form:

–V

The "I

LED-MAX

the R

OUT

solving for I

(V

POUT

)>kHRxI

LED

" equation (eq. 1) is obtained from combining

equation (eq. 2) with the kHRequation (eq. 3) and

. Maximum LED current is highly dependent

LED

LED

(eq. 3)

on minimum input voltage and LED forward voltage. Output
current capability can be increased by raising the minimum
input voltage of the application, or by selecting an LED with
a lower forward voltage. Excessive power dissipation may
also limit output current capability of an application.

Soft Start

The LM2796 contains internal soft-start circuitry to limit input
inrush currents when the part is enabled. Soft start is imple­mented internally with a controlled turn-on of the internal
voltage reference. During soft start, the current through the

www.national.com 6

Circuit Description (Continued)

LED outputs rise at the rate of the reference voltage ramp.
Due to the soft-start circuitry, turn-on time of the LM2796 is
approximately 100µs (typ.).

Thermal Protection

Internal thermal protection circuitry disables the LM2796
when the junction temperature exceeds 160˚C (typ.). This
feature protects the device from being damaged by high die
temperatures that might otherwise result from excessive
power dissipation. The device will recover and operate nor­mally when the junction temperature falls below 120˚C (typ.).
It is important that the board layout provides good thermal
conduction. This will help to keep the junction temperature
within specified operating ratings.

Applications Information

ADJUSTING LED BRIGHTNESS (PWM control)

Perceived LED brightness can be adjusted using a PWM
control signal to turn the LM2796 current sources ON and
OFF at a rate faster than perceptible by the eye. When this
is done, the total brightness perceived is proportional to the
duty cycle (D) of the PWM signal (D = the percentage of time
that the LED is on in every PWM cycle). A simple example: if
the LEDs are driven at 15mA each with a PWM signal that
has a 50% duty cycle, perceived LED brightness will be
about half as bright as compared to when the LEDs are
driven continuously with 15mA. A PWM signal thus provides
brightness (dimming) control for the solution.

The minimum recommended PWM frequency is 100Hz. Fre­quencies below this may be visibly noticeable as flicker or
blinking. The maximum recommended PWM frequency is
1kHz. Frequencies above this may cause interference with
internal current driver circuitry.

The preferred method for applying a PWM signal to adjust
brightness is to keep the master EN voltage ON continuously
and to apply the PWM signal(s) to the current source enable
pin(s): ENA and/or ENB. The benefit of this type of connec­tion can be best understood with a contrary example. When
a PWM signal is connected to the master enable (EN) pin,
the charge pump repeatedly turns on and off. Every time the
charge pump turns on, there is an inrush of current as
capacitances, both internal and external, are recharged. This
inrush current results in a current and voltage spike at the
input of the part. By only applying the PWM signal to ENA/
ENB, the charge pump stays on continuously and much
lower input noise results.

In cases where a PWM signal must be connected to the EN
pin, measures can be taken to reduce the magnitude of the
charge-pump turn-on voltage spikes. More input capaci­tance, series resistors and/or ferrite beads may provide ben­efits.

If the current and voltage spikes can be tolerated, connect­ing the PWM signal to the EN pin does provide a benefit:
lower supply current when the PWM signal is active. When
the PWM signal is low, the LM2796 will be shutdown and
input current will only be a few micro-amps. This results in a
lower time-averaged input current than the prior suggestion,
where EN is kept on continuously.

CAPACITOR SELECTION

The LM2796 requires 4 external capacitors for proper opera­tion. Surface-mount multi-layer ceramic capacitors are rec­ommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR
typ.). Tantalum capacitors, OS-CON capacitors, and alumi­num electrolytic capacitors are not recommended for use
with the LM2796 due to their high ESR, as compared to
ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2796. These capacitors have tight capacitance tolerance
(as good as
(X7R:
85˚C).

Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2796. Ca­pacitors with these temperature characteristics typically
have wide capacitance tolerance (+80%, -20%) and vary
significantly over temperature (Y5V: +22%, -82% over -30˚C
to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C
range). Under some conditions, a nominal 1µF Y5V or Z5U
capacitor could have a capacitance of only 0.1µF. Such
detrimental deviation is likely to cause Y5V and Z5U capaci­tors to fail to meet the minimum capacitance requirements of
the LM2796.

The minimum recommended voltage rating for these capaci­tors is 10V.

MICRO SMD MOUNTING

The LM2796 is an 18-bump micro SMD with a bump size of
approximately 300 micron diameter. The micro SMD pack­age requires specific mounting techniques detailed in Na­tional Semiconductor Application Note 1112 (AN-1112).

±

±

10%) and hold their value over temperature

15% over -55˚C to 125˚C; X5R:±15% over -55˚C to

<

20mW

LM2796

www.national.com7

Physical Dimensions inches (millimeters) unless otherwise noted

TLA18EHA: 18-Bump Thin Micro SMD, Large Bump

±

X1 = 2.098
X2 = 2.403mm

X3 = 0.600mm

0.030mm

±

0.030

±

0.075mm

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant

LM2796 Dual-Display White LED Driver with 3/2x Switched Capacitor Boost

into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the

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.

labeling, can be reasonably expected to result in a
significant injury to the user.

BANNED SUBSTANCE COMPLIANCE

National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products
Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification
(CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2.

National Semiconductor
Americas Customer
Support Center

Email: [e-mail]
Tel: [phone]

www.national.com

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.

National Semiconductor
Europe Customer Support Center

Fax: [phone]

Email: [e-mail]
Deutsch Tel: [phone]
English Tel: [phone]
Français Tel: [phone]

National Semiconductor
Asia Pacific Customer
Support Center

Email: [e-mail]

National Semiconductor
Japan Customer Support Center

Fax: [phone]
Email: [e-mail]
Tel: [phone]