LM2796
Dual-Display White LED Driver with 3/2x Switched
Capacitor Boost
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 pulse-
width 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 high-
efficiency 3/2x-gain charge pump.
The LM2796 is available in National's chip-scale 18-bump
micro SMD package.
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
Typical Application Circuit
20093801
February 2004
LM2796
Dual-Display
White
LED
Driver
with
3/2x
Switched
Capacitor
Boost
2004 National Semiconductor Corporation
DS200938
www.national.com
Connection Diagram
18-Bump Thin Micro SMD Package, Large Bump
NS Package Number TLA18
20093802
Pin Description
Pin #s
Pin Names
Pin Descriptions
C1
V
IN
Input voltage. Input range: 2.7V to 5.5V.
D2
GND
Ground
A3
P
OUT
Charge pump output. Approximately 1.5xV
IN
A1, B2, A5, E1
C1+, C1-, C2+,
C2-
Flying capacitor connections.
A7
EN
Enable pin. Logic input. High = normal operation, Low = shutdown (charge
pump and all current sources OFF).
D6, E5, D4, E3
D1A, D2A, D3A,
D4A
LED Outputs - Group A
C5, B4, C3
D1B, D2B, D3B
LED Outputs - Group B
B6
EN-A
Enable for Group-A LEDs (current outputs). Logic input.
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.
E7
EN-B
Enable for Group-B LEDs (current outputs). Logic input.
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.
C7
I
SET
Placing a resistor (R
SET
) between this pin and GND sets the LED current for
all LEDs. LED Current = 100 x (1.25V R
SET
).
Ordering Information
Order Information
Package
Supplied As
LM2796TL
TLA18 Micro SMD
250 Units, Tape & Reel
LM2796TLX
3000 Units, Tape & Reel
LM2796
www.national.com
2
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.
V
IN
pin voltage
-0.3V to 7.1V
EN, ENA, ENB pin voltages
-0.3V to (V
IN
+0.3V)
w/ 5.6V max
Continuous Power Dissipation
(Note 3)
Internally Limited
Junction Temperature (T
J-MAX
)
150
o
C
Storage Temperature Range
-65
o
C to +150
o
C
Maximum Lead Temperature
(Soldering, 10 sec.)
265
o
C
ESD Rating (Note 4)Human Body
Model:
Machine Model
2.0kV
200V
Operating Rating
(Notes 1, 2)
Input Voltage Range
2.7V to 5.5V
Junction Temperature (T
J
) Range
-30C to +125C
Ambient Temperature (T
A
) Range
(Note 5)
-30C to +85C
Thermal Properties
Juntion-to-Ambient Thermal
Resistance (
JA
), (Note 6)
100C/W
Electrical Characteristics
(Notes 2, 7)
Limits in standard typeface and typical values apply for T
J
= 25
o
C. Limits in boldface type apply over the full operating junction
temperature range (-30C
T
J
+85C) . Unless otherwise specified: V
IN
= 3.6V; V
Dxx
= 3.6V; V(EN) = 2.0V; Group A and
Group B LEDs not ON simultaneously (ENA = V
IN
and ENB = GND, or ENA = GND and ENB = V
IN
); R
SET
= 8.35k
; C
IN
, C
1
,
C
2
, and C
POUT
= 1F. (Note 8)
Symbol
Parameter
Condition
Min
Typ
Max
Units
I
Dxx
Output Current Regulation
3.0V
V
IN
4.2V, and V
IN
= 5.5V
2.5V
V
Dxx
3.8V;
R
SET
= 8.35k
13.8
(-8%)
15
16.2
(+8%)
mA
(%)
3.0V
V
IN
5.5V;
2.5V
V
Dxx
3.6V;
R
SET
= 6.25k
20
mA
3.0V
V
IN
5.5V;
2.5V
V
Dxx
3.9V;
R
SET
= 12.5k
10
mA
2.7V
V
IN
3.0V;
2.5V
V
Dxx
3.3V;
R
SET
= 8.35k
15
mA
ENA and ENB ON (all 7 I
DX
outputs active),
V
IN
= 3.0V, C
IN
= C
OUT
= 2.2F
15
mA
I
Dxx-MATCH
Current Matching Between Any
Two Group A Outputs or Group
B Outputs
V
IN
= 3.0V (Note 10)
1
%
I
Q
Quiescent Supply Current
2.7V
V
IN
4.2V;
No Load Current,
EN = ON, ENA = ENB = OFF
3.5
6
mA
I
SD
Shutdown Supply Current
2.7V
V
IN
5.5V, EN = OFF
3
4.5
A
V
SET
I
SET
Pin Voltage
2.7V
V
IN
5.5V
1.25
V
I
Dxx
/I
SET
Output Current to Current Set
Ratio
100
R
OUT
Charge Pump Output Resistance
(Note 11)
V
IN
= 3.0V
2.7
V
HR
Current Source Headroom
Voltage Requirement (Note 12)
I
Dxx
= 95% X I
Dxx
(nom)
R
SET
= 8.35k
(I
Dxx
(nom)
15mA)
320
mV
I
Dxx
= 95%X I
Dxx
(nom)
R
SET
= 12.5k
(I
Dxx
(nom)
10mA)
220
LM2796
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3
Electrical Characteristics
(Notes 2, 7) (Continued)
Limits in standard typeface and typical values apply for T
J
= 25
o
C. Limits in boldface type apply over the full operating junction
temperature range (-30C
T
J
+85C) . Unless otherwise specified: V
IN
= 3.6V; V
Dxx
= 3.6V; V(EN) = 2.0V; Group A and
Group B LEDs not ON simultaneously (ENA = V
IN
and ENB = GND, or ENA = GND and ENB = V
IN
); R
SET
= 8.35k
; C
IN
, C
1
,
C
2
, and C
POUT
= 1F. (Note 8)
Symbol
Parameter
Condition
Min
Typ
Max
Units
f
SW
Switching Frequency
3.0V
V
IN
4.2V
325
500
675
kHz
t
START
Start-up Time
I
Dx
= 90% steady state
100
s
1.5x/1x
Charge pump gain cross-over:
Gain = 1.5 when V
IN
is below
threshold. Gain = 1 when V
IN
is
above threshold.
1.5x to 1x Threshold
4.75
V
1x to 1.5x Threshold
4.55
V
Logic Pin Specifications: EN, ENA, ENB
V
IL
Input Logic Low
2.7V
V
IN
5.5V
0
0.5
V
V
IH
Input Logic High
2.7V
V
IN
5.5V
1.1
V
IN
V
I
LEAK
Input Leakage Current
V
ENx
= 0V
0.1
A
V
ENx
= 3V (Note 13)
10
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
J
= 160C (typ.) and disengages at T
J
=
120C (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
A-MAX
) is dependent on the maximum operating junction temperature (T
J-MAX-OP
= 125C), the maximum power
dissipation of the device in the application (P
D-MAX
), and the junction-to ambient thermal resistance of the part/package in the application (
JA
), as given by the
following equation: T
A-MAX
= T
J-MAX-OP
(
JA
x P
D-MAX
).
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
IN
, C
OUT
, C
1
, and C
2
: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
Note 9: If both LED groups are to be ON simultaneously, the maximum V
Dxx
voltage may need to be derated, depending on minimum input voltage conditions. Refer
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
OUT
) models all voltage losses in the charge pump. R
OUT
can be used to estimate the voltage at the charge pump output (P
OUT
):
V
Pout
= (1.5 x V
IN
) (R
OUT
x I
OUT
). In the equation, I
OUT
is the total output current: the sum of all active Dxx output currents and all current drawn from P
OUT
. The
equation applies when the charge pump is operating with a gain of 3/2 (V
IN
4.75V typ.).
Note 12: Headroom voltage: V
HR
= V
Pout
V
Dxx
. If headroom voltage requirement is not met, LED current regulation will be compromised.
Note 13: There is a 300k
(typ.) pull-down resistor connected internally between each enable pin (EN, ENA, ENB) and GND.
LM2796
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