LM2756
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SNVS504C – JULY 2007 – REVISED MAY 2013
LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in
DSBGA
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FEATURES
APPLICATIONS
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2
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Drives up to 8 LEDs with up to 30mA of Diode
Current Each
32 Exponential Dimming Steps with 800:1
Dimming Ratio for Group A (Up to 6 LEDs)
8 Linear Dimming States for Groups B (Up to 3
LEDs) and D1C (1 LED)
Programmable Auto-Dimming Function
3 Independently Controlled LED Groups Via
I2C Compatible Interface
Up to 90% Efficiency
Total Solution Size < 21mm2
Low Profile 20 Bump DSBGA Package
(1.615mm × 2.015mm × 0.6mm)
0.4% Accurate Current Matching
Internal Soft-Start Limits Inrush Current
True Shutdown Isolation for LED’s
Wide Input Voltage Range (2.7V to 5.5V)
Active High Hardware Enable
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Dual Display LCD Backlighting for Portable
Applications
Large Format LCD Backlighting
Display Backlighting with Indicator Light
DESCRIPTION
The LM2756 is a highly integrated, switchedcapacitor, multi-display LED driver that can drive up
to 8 LEDs in parallel with a total output current of
180mA. Regulated internal current sources deliver
excellent current and brightness matching in all LEDs.
The LED driver current sinks are split into three
independently controlled groups. The primary group
(Group A) can be configured to drive four, five or six
LEDs for use in the main phone display, while the
secondary group (Group B) can be configured to
drive one, two or three LEDs for driving secondary
displays, keypads and/or indicator LEDs. An
additional driver, D1C, is provided for additional
indicator lighting functions.
Typical Application Circuit
GROUP A
GROUP B
D1A D2A D3A D4A
D53
D62
GROUP C
D1B
D1C
VIN
+ -
1 µF
C1+
1 µF
C1C2+
VOUT
LM2756
1 µF
GND
1 µF
C2-
HWEN
SDIO
SCL ISET
2
I C Control
Signals
Capacitors: Murata GNM1M2R61C105ME18D 1 µF dual
capacitors, or 1 µF single capacitor equivalent
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2007–2013, Texas Instruments Incorporated
LM2756
SNVS504C – JULY 2007 – REVISED MAY 2013
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DESCRIPTION (CONTINUED)
The device provides excellent efficiency without the use of an inductor by operating the charge pump in a gain of
3/2 or in Pass-Mode. The proper gain for maintaining current regulation is chosen, based on LED forward
voltage, so that efficiency is maximized over the input voltage range.
The LM2756 is available in TI's tiny 20-bump, 0.4mm pitch, thin DSBGA package.
Figure 1. Minimum Layout
Connection Diagram
4
4
3
3
2
2
1
1
A
B
C
D
E
E
Top View
D
C
B
A
Bottom View
Figure 2. 20 Bump DSBGA Package
Package Number YFQ0020AAA
PIN DESCRIPTIONS
Bump #s
YFQ0020AAA
2
Pin Names
Pin Descriptions
A3
VIN
A2
VOUT
Input voltage. Input range: 2.7V to 5.5V.
Charge Pump Output Voltage
A1, C1, B1, B2
C1+, C1-, C2+, C2-
Flying Capacitor Connections
D3, E3,E4, D4
D1A-D4A
LED Drivers - GroupA
C4, B4
D53, D62
LED Drivers - Configurable Current Sinks. Can be assigned to GroupA or GroupB
B3
D1B
LED Drivers - GroupB
C3
D1C
LED Driver - Indicator LED
D2
ISET
Placing a resistor (RSET) between this pin and GND sets the full-scale LED current for
DxA , DxB, D53, D62 and D1C LEDs.
Full-Scale LED Current = 189 × (1.25V ÷ RSET)
E1
HWEN
C2
SDIO
Serial Data Input/Output Pin
E2
SCL
Serial Clock Pin
A4, D1
GND
Ground
Hardware Enable Pin. High = Normal Operation, Low = RESET
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings
(1) (2) (3)
VIN pin voltage
-0.3V to 6.0V
SCL, SDIO, HWEN pin voltages
-0.3V to (VIN+0.3V)
w/ 6.0V max
IDxx Pin Voltages
-0.3V to (VVOUT+0.3V)
w/ 6.0V max
Continuous Power Dissipation
Internally Limited
(4)
Junction Temperature (TJ-MAX)
150°C
Storage Temperature Range
-65°C to +150° C
(5)
Maximum Lead Temperature (Soldering)
(6)
ESD Rating
Human Body Model
(1)
(2)
(3)
(4)
(5)
(6)
2.0kV
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is ensured. Operating Ratings do not imply specified performance limits. For ensured performance limits
and associated test conditions, see the Electrical Characteristics tables.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
All voltages are with respect to the potential at the GND pins.
Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ = 160°C (typ.) and
disengages at TJ = 155°C (typ.).
For detailed soldering specifications and information, please refer to TI Application Note 1112: Micro SMD Wafer Level Chip Scale
Package (AN-1112) SNVA009.
The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7)
Operating Rating
(1) (2)
Input Voltage Range
2.7V to 5.5V
LED Voltage Range
2.0V to 4.0V
Junction Temperature (TJ) Range
-30°C to +105°C
Ambient Temperature (TA) Range (3)
(1)
(2)
(3)
-30°C to +85°C
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is ensured. Operating Ratings do not imply specified performance limits. For ensured performance limits
and associated test conditions, see the Electrical Characteristics tables.
All voltages are with respect to the potential at the GND pins.
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 (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP =
105°C), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the
part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).
Thermal Properties
Junction-to-Ambient Thermal
Resistance (θJA),
YFQ0020 Package
40°C/W
(1)
(1)
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. For more information, please refer to TI
Application Note 1112: Micro SMD Wafer Level Chip Scale Package (AN-1112) SNVA009.
Electrical Characteristics (1) (2)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = 3.6V; VHWEN = VIN; VDxA = VDxB = VDxC = 0.4V; RSET = 11.8kΩ; GroupA = GroupB = GroupC
= Fullscale Current; ENA, ENB, ENC Bits = “1”; SD53, SD62, 53A, 62A Bits = "0"; C1 = C2 = CIN= COUT= 1.0µF;
(1)
(2)
All voltages are with respect to the potential at the GND pins.
Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the
most likely norm.
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Electrical Characteristics(1)(2) (continued)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = 3.6V; VHWEN = VIN; VDxA = VDxB = VDxC = 0.4V; RSET = 11.8kΩ; GroupA = GroupB = GroupC
= Fullscale Current; ENA, ENB, ENC Bits = “1”; SD53, SD62, 53A, 62A Bits = "0"; C1 = C2 = CIN= COUT= 1.0µF;
Specifications related to output current(s) and current setting pins (IDxx and ISET) apply to GroupA and GroupB. (3)
Specifications related to output current(s) and current setting pins (IDxx and ISET) apply to GroupA and GroupB.
Symbol
Parameter
Min
Typ
Max
Units
2.7V ≤ VIN ≤ 5.5V
ENA = '1', 53A = 62A = '0'', ENB = ENC = '0'
4 LEDs in GroupA
18.65
(-8%)
20.28
21.90
(+8%)
mA
(%)
2.7V ≤ VIN ≤ 5.5V
ENA = '1', 53A = 62A = '1', ENB = ENC = '0'
6 LEDs in GroupA
18.70
(-8.5%)
20.40
22.10
(+8.5%)
mA
(%)
Output Current Regulation
GroupB
2.7V ≤ VIN ≤ 5.5V
ENB = '1', 53A = 62A = '0', ENA = ENC = '0'
3 LEDs in GroupB
18.40
(-8%)
20.00
21.60
(+8%)
mA
(%)
Output Current Regulation
IDC
2.7V ≤ VIN ≤ 5.5V
ENC = '1', ENA = ENB = '0'
18.20
(-7.5%)
19.70
21.20
(+7.5%)
mA
(%)
Maximum Diode Current per Dxx
Output (4)
RSET = 8.33kΩ
Output Current Regulation
GroupA
IDxx
Condition
(3)
30
mA
22.5
DxA
Output Current Regulation
3.2V ≤ VIN ≤ 5.5V
GroupA, GroupB, and GroupC Enabled VLED = 3.6V
(4)
RSET = 10.5kΩ
22.5
DxB
mA
22.5
DxC
IDxx-
LED Current Matching (5)
MATCH
VDxTH
VDxx 1x to 3/2x Gain Transition
Threshold
VHR
Current sink Headroom Voltage
Requirement
ROUT
2.7V ≤ VIN ≤ 5.5V
GroupA (4 LEDs)
0.4
1.8
GroupA (6 LEDs)
1.0
2.7
GroupB (3 LEDs)
0.7
2.5
%
VDxA and/or VDxB Falling
150
mV
IDxx = 95% ×IDxx (nom.)
(IDxx (nom) ≈ 20mA)
65
mV
Open-Loop Charge Pump Output
Resistance
Gain = 3/2
2.4
Gain = 1
0.9
IQ
Quiescent Supply Current
Gain = 1.5x, No Load
2.1
2.5
mA
ISD
Shutdown Supply Current
All ENx bits = "0"
3.7
5.5
µA
VSET
ISET Pin Voltage
2.7V ≤ VIN ≤ 5.5V
1.25
IDxA-B-C /
ISET
Output Current to Current Set Ratio
GroupA, GroupB, GroupC
fSW
Switching Frequency
tSTART
Start-up Time
(6)
VHWEN
HWEN Voltage Thresholds
Ω
V
189
1.0
VOUT = 90% steady state
2.7V ≤ VIN ≤ 5.5V
1.3
1.6
250
Reset
Normal Operation
MHz
µs
0
0.580
1.075
VIN
V
I2C Compatible Interface Voltage Specifications (SCL, SDIO)
(3)
(4)
(5)
(6)
4
CIN, CVOUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
The maximum total output current for the LM2756 should be limited to 180mA. The total output current can be split among any of the
three Groups (IDxA = IDxB = IDxC = 30mA Max.). Under maximum output current conditions, special attention must be given to input
voltage and LED forward voltage to ensure proper current regulation. See the Maximum Output Current section of the datasheet for
more information.
For the two groups of current sinks on a part (GroupA and GroupB), the following are determined: the maximum sink current in the
group (MAX), the minimum sink current in the group (MIN), and the average sink 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.
For each Dxxpin, headroom voltage is the voltage across the internal current sink connected to that pin. For Group A, B, and C current
sinks, VHRx = VOUT -VLED. If headroom voltage requirement is not met, LED current regulation will be compromised.
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Electrical Characteristics(1)(2) (continued)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = 3.6V; VHWEN = VIN; VDxA = VDxB = VDxC = 0.4V; RSET = 11.8kΩ; GroupA = GroupB = GroupC
= Fullscale Current; ENA, ENB, ENC Bits = “1”; SD53, SD62, 53A, 62A Bits = "0"; C1 = C2 = CIN= COUT= 1.0µF;
Specifications related to output current(s) and current setting pins (IDxx and ISET) apply to GroupA and GroupB. (3)
Symbol
Parameter
Condition
Min
Typ
Max
Units
V
VIL
Input Logic Low "0"
2.7V ≤ VIN ≤ 5.5V
0
0.710
VIH
Input Logic High "1"
2.7V ≤ VIN ≤ 5.5V
1.225
VIN
V
VOL
Output Logic Low "0"
ILOAD = 3.5mA
400
mV
I2C Compatible Interface Timing Specifications (SCL, SDIO) (7)
t1
SCL (Clock Period)
t2
Data In Setup Time to SCL High
t3
Data Out stable After SCL Low
t4
SDIO Low Setup Time to SCL Low
(Start)
t5
SDIO High Hold Time After SCL High
(Stop)
(7)
(8)
(8)
294
ns
100
ns
0
ns
100
ns
100
ns
SCL and SDIO should be glitch-free in order for proper brightness control to be realized.
SCL is tested with a 50% duty-cycle clock.
Figure 3.
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BLOCK DIAGRAM
1 PF
C1+
VIN
2.7V to 5.5V
C1-
COUT
1 PF
1 PF
C2+
VOUT D1A D2A D3A D4A D53
C2-
D1B
D1C
3/2X and 1X
Regulated Charge Pump
CIN
1 PF
GAIN
CONTROL
1.3 MHz.
Switch
Frequency
SCL
SDIO
D62
SoftStart
1.25V
Ref.
GroupA Current Sinks
GroupB
Current Sinks
Brightness
Control
Brightness
Control
D1C Current
Sink
Brightness
Control
General Purpose Register
2
I C Interface
Block
HWEN
LM2756
Brightness Control Registers
Group A and Group B
Brightness Control Register
D1C
ISET
GND
RSET
6
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Typical Performance Characteristics
Unless otherwise specified: TA = 25°C; VIN = 3.6V; VHWEN = VIN; VLEDxA = VLEDxB = VLED1C = 3.6V; RSET = 11.8kΩ; C1=C2= CIN =
CVOUT = 1µF; ENA = ENB = ENC = '1'.
LED Drive Efficiency
vs
Input Voltage
100
VLED = 3.0V
LED Drive Efficiency
vs
Input Voltage
100
4 LEDs
BankA = 6 LEDs
90
VLED = 3.3V
90
äLED (%)
äLED (%)
VLED = 3.6V
80
70
80
70
5 LEDs
60
60
VLED = 3.3V
50
2.7
3.1
3.5
3.9
4.3
4.7
6 LEDs
5.1
50
2.7
5.5
3.1
3.5
3.9
VIN (V)
4.3
4.7
5.1
5.5
VIN (V)
Figure 4.
Figure 5.
Input Current
vs
Input Voltage
GroupA Diode Current
vs
Input Voltage
200
21.50
BankA = 6 LEDs
TA = +85°C
21.00
VLED = 3.6V
175
TA = +25°C
150
IDxA (mA)
IIN (mA)
20.50
VLED = 3.3V
VLED = 3.0V
20.00
19.50
TA = -30°C
125
19.00
100
2.7
3.1
3.5
3.9
4.3
4.7
5.1
18.50
2.7
5.5
3.9
4.3
4.7
5.1
Figure 6.
Figure 7.
GroupB Diode Current
vs
Input Voltage
GroupC Diode Current
vs
Input Voltage
5.5
20.50
TA = +25°C
20.50
20.00
TA = +25°C
TA = +85°C
TA = +85°C
ID1C (mA)
20.00
19.50
TA = -30°C
19.50
19.00
TA = -30°C
19.00
18.50
2.7
3.5
VIN (V)
21.00
IDxB (mA)
3.1
VIN (V)
3.1
3.5
3.9
4.3
4.7
5.1
18.50
2.7
5.5
3.1
3.5
3.9
4.3
4.7
VIN (V)
VIN (V)
Figure 8.
Figure 9.
5.1
5.5
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Typical Performance Characteristics (continued)
Unless otherwise specified: TA = 25°C; VIN = 3.6V; VHWEN = VIN; VLEDxA = VLEDxB = VLED1C = 3.6V; RSET = 11.8kΩ; C1=C2= CIN =
CVOUT = 1µF; ENA = ENB = ENC = '1'.
GroupA Current Matching
vs
Input Voltage
6 LEDs
GroupA Current Matching
vs
Input Voltage
4 LEDs
21.6
22.10
21.1
D2A
D62
D3A
D2A
D3A
20.6
D1A
IDxA (mA)
IDx (mA)
21.25
20.40
20.1
D4A
19.6
19.55 D53
D1A
D4A
19.1
18.70
2.7
3.1
3.5
3.9
4.3
4.7
5.1
18.6
5.5
2.7
VIN (V)
3.1
3.5
3.9
4.3
4.7
5.1
5.5
VIN (V)
Figure 10.
Figure 11.
GroupB Current Matching
vs
Input Voltage
3 LEDs
GroupA Diode Current
vs
GroupA Brightness Code
21.6
IDx (mA)
20.8
D62
D1B
20.0
19.2
D53
18.4
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
VIN (V)
8
Figure 12.
Figure 13.
GroupB Diode Current
vs
GroupB Brightness Code
GroupC Diode Current
vs
GroupC Brightness Code
Figure 14.
Figure 15.
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Typical Performance Characteristics (continued)
Unless otherwise specified: TA = 25°C; VIN = 3.6V; VHWEN = VIN; VLEDxA = VLEDxB = VLED1C = 3.6V; RSET = 11.8kΩ; C1=C2= CIN =
CVOUT = 1µF; ENA = ENB = ENC = '1'.
Quiescent Current in Gain 1.5×
vs
Input Voltage
Shutdown Current
vs
Input Voltage
10
3.00
GAIN = 3/2
9
RSET = 11.8 kΩ
2.80
2.60
7
TA = +25°C
2.40
I SD (μA)
IQ (mA)
TA = +85°C
8
TA = +85°C
2.20
TA = -30°C
TA = +25°C
6
5
4
3
2.00
TA = -30°C
2
1.80
1
0
1.60
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
VIN (V)
VIN (V)
Figure 16.
Figure 17.
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CIRCUIT DESCRIPTION
Overview
The LM2756 is a white LED driver system based upon an adaptive 3/2× - 1× CMOS charge pump capable of
supplying up to 180mA of total output current. With three separately controlled Groups of constant current sinks,
the LM2756 is an ideal solution for platforms requiring a single white LED driver for main display, sub display,
and indicator lighting. The tightly matched current sinks ensure uniform brightness from the LEDs across the
entire small-format display.
Each LED is configured in a common anode configuration, with the peak drive current being programmed
through the use of an external RSET resistor. An I2C compatible interface is used to enable the device and vary
the brightness within the individual current sink Groups. For GroupA , 32 exponentially-spaced analog brightness
control levels are available. GroupB and GroupC have 8 linearly-spaced analog brightness levels.
Circuit Components
Charge Pump
The input to the 3/2× - 1× charge pump is connected to the VIN pin, and the regulated output of the charge pump
is connected to the VOUT pin. The recommended input voltage range of the LM2756 is 2.7V to 5.5V. The device’s
regulated charge pump has both open loop and closed loop modes of operation. When the device is in open
loop, the voltage at VOUT is equal to the gain times the voltage at the input. When the device is in closed loop,
the voltage at VOUT is regulated to 4.6V (typ.). The charge pump gain transitions are actively selected to maintain
regulation based on LED forward voltage and load requirements.
LED Forward Voltage Monitoring
The LM2756 has the ability to switch gains (1x or 3/2x) based on the forward voltage of the LED load. This ability
to switch gains maximizes efficiency for a given load. Forward voltage monitoring occurs on all diode pins. At
higher input voltages, the LM2756 will operate in pass mode, allowing the VOUT voltage to track the input voltage.
As the input voltage drops, the voltage on the Dxx pins will also drop (VDXX = VVOUT – VLEDx). Once any of the
active Dxx pins reaches a voltage approximately equal to 150mV, the charge pump will switch to the gain of 3/2.
This switch-over ensures that the current through the LEDs never becomes pinched off due to a lack of
headroom across the current sinks. Once a gain transition occurs, the LM2756 will remain in the gain of 3/2
until an I2C write to the part occurs. At that time, the LM2756 will re-evaluate the LED conditions and
select the appropriate gain.
Only active Dxx pins will be monitored. For example, if only GroupA is enabled, the LEDs in GroupB or GroupC
will not affect the gain transition point. If all 3 Groups are enabled, all diodes will be monitored, and the gain
transition will be based upon the diode with the highest forward voltage.
Configurable Gain Transition Delay
To optimize efficiency, the LM2756 has a user selectable gain transition delay that allows the part to ignore short
duration input voltage drops. By default, the LM2756 will not change gains if the input voltage dip is shorter than
3 to 6 milliseconds. There are four selectable gain transition delay ranges available on the LM2756. All delay
ranges are set within the VF Monitor Delay Register . Please refer to the Internal Registers of LM2756 section of
this datasheet for more information regarding the delay ranges.
HWEN Pin
The LM2756 has a hardware enable/reset pin (HWEN) that allows the device to be disabled by an external
controller without requiring an I2C write command. Under normal operation, the HWEN pin should be held high
(logic '1') to prevent an unwanted reset. When the HWEN is driven low (logic '0'), all internal control registers
reset to the default states and the part becomes disabled. Please see the Electrical Characteristics section of the
datasheet for required voltage thresholds.
10
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I2C Compatible Interface
Data Validity
The data on SDIO line must be stable during the HIGH period of the clock signal (SCL). In other words, state of
the data line can only be changed when SCL is LOW.
SCL
SDIO
data
change
allowed
data
valid
data
change
allowed
data
valid
data
change
allowed
Figure 18. Data Validity Diagram
A pull-up resistor between the controller's VIO line and SDIO must be greater than [(VIO-VOL) / 3.5mA] to meet
the VOL requirement on SDIO. Using a larger pull-up resistor results in lower switching current with slower edges,
while using a smaller pull-up results in higher switching currents with faster edges.
Start and Stop Conditions
START and STOP conditions classify the beginning and the end of the I2C session. A START condition is
defined as SDIO signal transitioning from HIGH to LOW while SCL line is HIGH. A STOP condition is defined as
the SDIO transitioning from LOW to HIGH while SCL is HIGH. The I2C master always generates START and
STOP conditions. The I2C bus is considered to be busy after a START condition and free after a STOP condition.
During data transmission, the I2C master can generate repeated START conditions. First START and repeated
START conditions are equivalent, function-wise.
SDIO
SCL
S
P
START condition
STOP condition
Figure 19. Start and Stop Conditions
Transfering Data
Every byte put on the SDIO line must be eight bits long, with the most significant bit (MSB) transferred first. Each
byte of data has to be followed by an acknowledge bit. The acknowledge related clock pulse is generated by the
master. The master releases the SDIO line (HIGH) during the acknowledge clock pulse. The LM2756 pulls down
the SDIO line during the 9th clock pulse, signifying an acknowledge. The LM2756 generates an acknowledge
after each byte is received.
After the START condition, the I2C master sends a chip address. This address is seven bits long followed by an
eighth bit which is a data direction bit (R/W). The LM2756 address is 36h. For the eighth bit, a “0” indicates a
WRITE and a “1” indicates a READ. The second byte selects the register to which the data will be written. The
third byte contains data to write to the selected register.
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ack from slave
ack from slave
ack from slave
start
msb Chip Address lsb
w
ack
msb Register Add lsb
ack
msb DATA lsb
ack
stop
start
Id = 36h
w
ack
addr = 10h
ack
DGGUHVV K¶06 data
ack
stop
SCL
SDIO
w = write (SDIO = "0")
r = read (SDIO = "1")
ack = acknowledge (SDIO pulled down by either master or slave)
id = chip address, 36h for LM2756
Figure 20. Write Cycle
I2C Compatible Chip Address
The chip address for LM2756 is 0110110, or 36h.
MSB
LSB
ADR6
bit7
ADR5
bit6
ADR4
bit5
ADR3
bit4
ADR2
bit3
ADR1
bit2
ADR0
bit1
0
1
1
0
1
1
0
R/W
bit0
2
I C Slave Address (chip address)
Figure 21. Chip Address
Internal Registers of LM2756
Register
Internal Hex Address
Power On Value
General Purpose Register
10h
0000 0000
Group A Brightness Control Register
A0h
1110 0000
Group B Brightness Control Register
B0h
1111 1000
Group C Brightness Control Register
C0h
1111 1000
Ramp Step Time Register
20h
1111 0000
VF Monitor Delay Ragister
60h
1111 1100
MSB
0
bit7
LSB
62A
bit6
53A
bit5
SD62
bit4
SD53
bit3
ENC
bit2
ENB
bit1
ENA
bit0
Figure 22. General Purpose Register Description
Internal Hex Address: 10h
12
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NOTE
ENA: Enables DxA LED drivers (Main Display)
ENB: Enables DxB LED drivers (Aux Lighting)
ENC: Enables D1C LED driver (Indicator Lighting)
SD53: Shuts down driver D53
SD62: Shuts down driver D62
53A: Configures D53 to GroupA
62A: Configures D62 to GroupA
DxA Brightness Control
Register Address: 0xA0
MSB
1
bit7
1
bit6
1
bit5
1
bit6
1
bit5
DxA2
bit2
DxA1
bit1
1
bit4
1
bit3
1
bit6
1
bit5
1
bit4
1
bit3
DxA0
bit0
LSB
DxB2
bit2
DxB1
bit1
DxC Brightness Control
Register Address: 0xC0
MSB
1
bit7
DxA3
bit3
DxB Brightness Control
Register Address: 0xB0
MSB
1
bit7
DxA4
bit4
LSB
DxB0
bit0
LSB
D1C2
bit2
D1C1
bit1
D1C0
bit0
Figure 23. Brightness Control Register Description
Internal Hex Address: 0xA0 (GroupA), 0xB0 (GroupB), 0xC0 (GroupC)
NOTE
DxA4-DxA0, D53, D62: Sets Brightness for DxA pins (GroupA). 11111=Fullscale
DxB2-DxB0: Sets Brightness for DxB pins (GroupB). 111=Fullscale
DxC2-DxC0: Sets Brightness for D1C pin. 111 = Fullscale
Full-Scale Current set externally by the following equation:
IDxx = 189 × 1.25V / RSET
Table 1. Brightness Level Control Table (GroupA)
Brightness Code (hex)
Perceived Brightness Level (%)
00
0.125
01
0.313
02
0.625
03
1
04
1.125
05
1.313
06
1.688
07
2.063
08
2.438
09
2.813
0A
3.125
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Table 1. Brightness Level Control Table (GroupA) (continued)
Brightness Code (hex)
Perceived Brightness Level (%)
0B
3.75
0C
4.375
0D
5.25
0E
6.25
0F
7.5
10
8.75
11
10
12
12.5
13
15
14
16.875
15
18.75
16
22.5
17
26.25
18
31.25
19
37.5
1A
43.75
1B
52.5
1C
61.25
1D
70
1E
87.5
1F
100
GroupB and GroupC Brightness Levels (% of Full-Scale) = 10%, 20%, 30%, 40%, 50%, 60%, 70%, 100%
Ramp Step Time Register
Register Address: 0x20
MSB
1
bit7
1
bit6
1
bit5
1
bit4
0
bit3
LSB
0
bit2
RS1
bit1
RS0
bit0
Figure 24. Ramp Step Time Register Description
Internal Hex Address: 20h
NOTE
RS1-RS0: Sets Brightness Ramp Step Time. The Brightness ramp settings only affect
GroupA current sinks. ('00' = 100µs, '01' = 25ms, '10' = 50ms, '11' = 100ms).
VF Monitor Delay Register
Register Address: 0x60
MSB
1
bit7
1
bit6
1
bit5
1
bit4
1
bit3
LSB
1
bit2
VF1
bit1
VF0
bit0
Figure 25. VF Monitor Delay Register Description
Internal Hex Address: 60h
NOTE
VF1-VF0: Sets the Gain Transition Delay Time. The VF Monitor Delay can be set to four
different delay times. ('00' (Default) = 3-6msec., '01' = 1.5-3msec., '10' = 0.4-0.8msec., '11'
= 60-90µsec.).
14
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Application Information
Led configurations
The LM2756 has a total of 8 current sinks capable of sinking 180mA of total diode current. These 8 current sinks
are configured to operate in three independently controlled lighting regions. GroupA has four dedicated current
sinks, while GroupB and GroupC each have one. To add greater lighting flexibility, the LM2756 has two
additional drivers (D53 and D62) that can be assigned to either GroupA or GroupB through a setting in the
general purpose register.
At start-up, the default condition is four LEDs in GroupA, three LEDs in GroupB and a single LED in GroupC
(NOTE: GroupC only consists of a single current sink (D1C) under any configuration). Bits 53A and 62A in the
general purpose register control where current sinks D53 and D62 are assigned. By writing a '1' to the 53A or
62A bits, D53 and D62 become assigned to the GroupA lighting region. Writing a '0' to these bits assigns D53
and D62 to the GroupB lighting region. With this added flexibility, the LM2756 is capable of supporting
applications requiring 4, 5, or 6 LEDs for main display lighting, while still providing additional current sinks that
can be used for a wide variety of lighting functions.
Setting LED Current
The current through the LEDs connected to DxA and DxB can be set to a desired level simply by connecting an
appropriately sized resistor (RSET) between the ISET pin of the LM2756 and GND. The DxA, DxB and D1C LED
currents are proportional to the current that flows out of the ISET pin and are a factor of 189 times greater than the
ISET current. The feedback loops of the internal amplifiers set the voltage of the ISET pin to 1.25V (typ.). The
statements above are simplified in the equations below:
IDxA/B/C (A)= 189 × (VISET / RSET)
RSET (Ω)= 189 × (1.25V / IDxA/B/C)
(1)
(2)
Once the desired RSET value has been chosen, the LM2756 has the ability to internally dim the LEDs using
analog current scaling. The analog current level is set through the I2C compatible interface. LEDs connected to
GroupA can be dimmed to 32 different levels. GroupB and GroupC(D1C) have 8 analog current levels.
Please refer to the I2C Compatible Interface section of this datasheet for detailed instructions on how to adjust
the brightness control registers.
LED Current Ramping
The LM2756 provides an internal LED current ramping function that allows the GroupA LEDs to turn on and turn
off gradually over time. The target current level is set in the GroupA Brightness Control Register (0xA0). The total
ramp-up/ramp-down time is determind by the GroupA brightness level (0-31) and the user configurable ramp
step time.
Bits RS1 and RS2 in the Ramp Step Time Register (0x20) set the ramp step time to the following four times: '00'
= 100µsec., '01' = 25msec., '10' = 50msec., '11' = 100msec.
The LM2756 will always ramp-up (upon enable) and ramp-down (upon disable) through the brightness levels
until the target level is reached. At the default setting of '00', the LM2756's current ramping feature looks more
like a current step rather than a current ramp. Table 2 gives the approximate ramp-up/ramp-down times if the
GroupA brightness register is set to full-scale, or brightness code 31.
Table 2. Brightness Ramp-Up/Ramp-Down Times
Ramp Code
RS1-RS0
Ramp Step
Time
Total Ramp
Time
00
100µs
3.2ms
01
25ms
0.8s
10
50ms
1.6s
11
100ms
3.2s
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Maximum Output Current, Maximum LED Voltage, Minimum Input Voltage
The LM2756 can drive 8 LEDs at 22.5mA each (GroupA , GroupB, GroupC) from an input voltage as low as
3.2V, 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 capability of the LM2756. The statement contains the
key application parameters that are required to validate an LED-drive design using the LM2756: LED current
(ILEDx), number of active LEDs (Nx), LED forward voltage (VLED), and minimum input voltage (VIN-MIN).
The equation below can be used to estimate the maximum output current capability of the LM2756:
ILED_MAX = [(1.5 x VIN) - VLED - (IADDITIONAL × ROUT)] / [(Nx x ROUT) + kHRx]
ILED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL × 2.4Ω)] / [(Nx x 2.4Ω) + kHRx]
(3)
(4)
IADDITIONAL is the additional current that could be delivered to the other LED Groups.
ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage
droop at the pump output VOUT. Since the magnitude of the voltage droop is proportional to the total output
current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the LM2756
is typically 2.4Ω (VIN = 3.6V, TA = 25°C). In equation form:
VVOUT = (1.5 × VIN) – [(NA× ILEDA + NB × ILEDB + NC × ILEDC) × ROUT]
(5)
kHR – Headroom constant. This parameter models the minimum voltage required to be present across the current
sinks 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 kHR of the LM2756 is 3.25mV/mA. In equation form:
(VVOUT – VLEDx) > kHRx × ILEDx
Typical Headroom Constant Values kHRA = kHRB = kHRC = 3.25 mV/mA
(6)
(7)
The "ILED-MAX" equation (Equation 3) is obtained from combining the ROUT equation (Equation 5) with the kHRx
equation (Equation 6) and solving for ILEDx. Maximum LED current is highly dependent 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.
Total Output Current Capability
The maximum output current that can be drawn from the LM2756 is 180mA. Each driver Group has a maximum
allotted current per Dxx sink that must not be exceeded.
DRIVER TYPE
MAXIMUM Dxx CURRENT
DxA
30mA per DxA Pin
DxB
30mA per DxB Pin
D1C
30mA
The 180mA load can be distributed in many different configurations. Special care must be taken when running
the LM2756 at the maximum output current to ensure proper functionality.
Parallel Connected and Unused Outputs
Connecting the outputs in parallel does not affect internal operation of the LM2756 and has no impact on the
Electrical Characteristics and limits previously presented. The available diode output current, maximum diode
voltage, and all other specifications provided in the Electrical Characteristics table apply to this parallel output
configuration, just as they do to the standard LED application circuit.
All Dx current sinks utilize LED forward voltage sensing circuitry to optimize the charge-pump gain for maximum
efficiency. Due to the nature of the sensing circuitry, it is not recommended to leave any of the DxA (D1A-D4A,
D53, D62) pins open if diode GroupA is going to be used during normal operation. Leaving DxA pins
unconnected will force the charge-pump into 3/2× mode over the entire VIN range negating any efficiency gain
that could have been achieved by switching to 1× mode at higher input voltages.
If the D1B or D1C drivers are not going to be used, make sure that the ENB and ENC bits in the general purpose
register are set to '0' to ensure optimal efficiency.
The D53 and D62 pins can be completely shutdown through the general purpose register by writing a '1' to the
SD53 or SD62 bits.
16
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Care must be taken when selecting the proper RSET value. The current on any DxX pin must not exceed the
maximum current rating for any given current sink pin.
Power Efficiency
Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (PLED) to the power
drawn at the input of the part (PIN). With a 3/2× - 1× charge pump, the input current is equal to the charge pump
gain times the output current (total LED current). The efficiency of the LM2756 can be predicted as follow:
PLEDTOTAL = (VLEDA × NA × ILEDA) + (VLEDB × NB × ILEDB) + (VLEDC × ILEDC)
PIN = VIN × IIN
PIN = VIN × (GAIN × ILEDTOTAL + IQ)
E = (PLEDTOTAL ÷ PIN)
(8)
(9)
(10)
(11)
The LED voltage is the main contributor to the charge-pump gain selection process. Use of low forward-voltage
LEDs (3.0V- to 3.5V) will allow the LM2756 to stay in the gain of 1× for a higher percentage of the lithium-ion
battery voltage range when compared to the use of higher forward voltage LEDs (3.5V to 4.0V). See the LED
Forward Voltage Monitoring section of this datasheet for a more detailed description of the gain selection and
transition process.
For an advanced analysis, it is recommended that power consumed by the circuit (VIN x IIN) for a given load be
evaluated rather than power efficiency.
Power Dissipation
The power dissipation (PDISS) and junction temperature (TJ) can be approximated with the equations below. PIN is
the power generated by the 3/2× - 1× charge pump, PLED is the power consumed by the LEDs, TA is the ambient
temperature, and θJA is the junction-to-ambient thermal resistance for the DSBGA 20-bump package. VIN is the
input voltage to the LM2756, VLED is the nominal LED forward voltage, N is the number of LEDs and ILED is the
programmed LED current.
PDISS = PIN - PLEDA - PLEDB - PLEDC
PDISS= (GAIN × VIN × IGroupA + GroupB + GroupC ) - (VLEDA × NA × ILEDA) - (VLEDB × NB × ILEDB) - (VLEDC × ILEDC)
TJ = TA + (PDISS x θJA)
(12)
(13)
(14)
The junction temperature rating takes precedence over the ambient temperature rating. The LM2756 may be
operated outside the ambient temperature rating, so long as the junction temperature of the device does not
exceed the maximum operating rating of 105°C. The maximum ambient temperature rating must be derated in
applications where high power dissipation and/or poor thermal resistance causes the junction temperature to
exceed 105°C.
Thermal Protection
Internal thermal protection circuitry disables the LM2756 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 155°C (typ.). It is important that the board layout provide good thermal conduction to keep the junction
temperature within the specified operating ratings.
Capacitor selection
The LM2756 requires 4 external capacitors for proper operation (C1 = C2 = CIN = COUT = 1µF). Surface-mount
multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low
equivalent series resistance (ESR
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