NSC LM2755

LM2755
Charge Pump LED Controller with I2C Compatible Interface
in Micro SMD
General Description
Features
The LM2755 is a charge-pump-based, constant current LED
driver capable of driving 3 LEDs with a total output current up
to 90mA. The diode current waveforms of each LED can be
trapezoidal with timing and level parameters (rise time, fall
time, high level, low level, delay, high time, low time) programmed via an I2C compatible interface. The 32 brightness
levels found on the LM2755 are exponentially spaced (as opposed to linearly spaced) to better match the response of the
human eye to changing brightness levels.
The device requires only four small and low-cost ceramic capacitors. The LM2755 provides excellent efficiency without
the use of an inductor by operating the charge pump in a gain
of 3/2 or in a gain of 1. Maximum efficiency is achieved over
the input voltage range by actively selecting the proper gain
based on the LED forward voltage requirements.
The pre-regulation scheme used by the LM2755 is optimized
to ensure low conducted noise on the input. An internal softstart circuitry eliminates high inrush current at start-up. The
LM2755 consumes 3µA (typ.) of supply current in shut-down.
The LM2755 is available in National’s tiny 18-bump thin micro
SMD package.
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90% Peak Efficiency
Total solution size < 13mm2
No Inductor Required: Only 4 Inexpensive Ceramic Caps
3 Independently Controlled Constant Current Outputs
Programmable Trapezoidal Dimming Waveform on Each
Output
Programmable Timing Control Via Internal Registers and
External Clock Synchronization Input
32 Exponential Dimming Steps with 800:1 Dimming Ratio
Programmable brightness control via I2C compatible
interface
Hardware Enable Pin
Wide input voltage range: 2.7V to 5.5V
Tiny 18-bump thin micro SMD: 1.8mm x 1.6mm x 0.6mm
Applications
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Indicator LEDs
Keypad LED Backlight
Display LED Backlight
Fun-light LEDs
Typical Application Circuit
20180904
Minimum Solution Size
20180901
© 2007 National Semiconductor Corporation
201809
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LM2755 Charge Pump LED Controller with I2C Compatible Interface
October 2007
LM2755
Connection Diagram
18-Bump Thin Micro SMD Package
1.615mm × 1.807mm × 0.6mm
NS Package Number TMD18AAA
20180902
Pin Descriptions
Pin #s
Pin Names
A1
ID1
LED Driver 1
Pin Descriptions
A3
ID2
LED Driver 2
A5
ID3
LED Driver 3
A7
SYNC
External clock synchronization input
B2
ISET
LED Driver Current Set Pin
B4
HWEN
B6
SDIO
C1
VIN
Input Voltage Connection
C3
GND
Ground Connection.
C5
VIO
Serial Bus Voltage Level Input
C7
SCL
Serial Clock Pin
D2
POUT
Charge Pump Output
D4
C2-
Flying Capacitor Connect
D6
GND
Ground connection
E1
C1+
Flying Capacitor Connect
E3
C2+
Flying Capacitor Connect
E5
C1-
Flying Capacitor Connect
E7
ADDR
Hardware EN Pin. Low '0' = RESET, High '1' = Normal Operation
Serial data Input/Output pin
Chip Address Select Input. VIN = 0x67. Ground = 0x18.
Ordering Information
Order Information
Package
Supplied As
LM2755TM
TMD18AAA,
µSMD-18
250 Units, Tape & Reel
LM2755TMX
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3000 Units, Tape & Reel
2
Operating Rating
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
(Notes 1, 2)
VIN pin voltage
SCL, SDIO, VIO,
ADDR, SYNC pin voltages
IDx Pin Voltages
Continuous Power Dissipation
(Note 3)
Junction Temperature (TJ-MAX)
Storage Temperature Range
Maximum Lead Temperature
(Soldering)
ESD Rating(Note )
Human Body Model
Input Voltage Range
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range
(Note 6)
-0.3V to 6.0V
-0.3V to (VIN+0.3V)
w/ 6.0V max
-0.3V to (VPOUT+0.3V)
w/ 6.0V max
Internally Limited
2.7V to 5.5V
-30°C to 105°C
-30°C to +85°C
Thermal Properties
Juntion-to-Ambient Thermal
Resistance (θJA), TMD18AAA
Package (Note 7)
150°C
-65°C to +150°C
(Note 4)
Electrical Characteristics
LM2755
Absolute Maximum Ratings (Notes 1, 2)
56°C/W
ESD Caution Notice
National Semiconductor
recommends that all integrated circuits be handled with
appropriate ESD precautions. Failure to observe proper ESD
handling techniques can result in damage to the device.
2.5kV
(Notes 2, 8)
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; VD1 = 0.4V; VD2 = 0.4V; VD3 = 0.4V; RSET = 12.5kΩ; D1, D2, and D3 = Fullscale Current; EN1,
EN2, and EN3 Bits = “1”; CLK bit = '0'; C1=C2= 0.47µF, CIN=COUT= 1µF; Specifications related to output current(s) and current
setting pins (IDx and ISET) apply to D1, D2 and D3. (Note 9)
Symbol
Parameter
Condition
Min
Typ
Max
Units
18.7
20.7
22.7
mA
IDx
Output Current Regulation
3.0V ≤ VIN ≤ 5.5V
IMATCH
Output Current Matching
3.0V ≤ VIN ≤ 5.5V
(Note 10)
IQ
Quiescent Supply Current
Gain = 3/2
D1-3 = OPEN, RSET = OPEN
ISD
Shutdown Supply Current
3.0V ≤ VIN ≤ 5.5V
EN1 = EN2 = EN3 = 0
VSET
ISET Pin Voltage
3.0V ≤ VIN ≤ 5.5V
IDX / ISET
Output Current to Current Set Ratio (Note 11)
200
VDxTH
VDx 1x to 3/2x Gain Transition
Threshold
350
mV
VHR
Current Source Headroom Voltage
Requirement
(Note 12)
200
mV
fSW
Switching Frequency
tSTART
Start-up Time
fPWM
Internal Diode Current PWM
Frequency
fSYNC
Maximum External Sync Frequency
VHWEN
HWEN Voltage Thresholds
1
%
1.0
1.3
mA
5
9.5
µA
1.25
VD1 and/or VD2 and/or VD3Falling
V
IDx = 95% ×IDx (nom.)
(IDx (nom) ≈ 20mA)
Gain = 3/2
0.975
POUT = 90% steady state
1.25
1.525
µs
20
kHz
1.0
2.7V ≤ VIN ≤ 5.5V
Reset
Normal Operation
MHz
300
MHz
0
0.5
1.23
VIN
1.44
VIN
V
V
V
I2C Compatible Interface Voltage Specifications (SCL, SDIO, VIO)
VIO
Serial Bus Voltage Level
2.7V ≤ VIN ≤ 5.5V(Note 13)
VIL
Input Logic Low "0"
2.7V ≤ VIN ≤ 5.5V, VIO = 3.0V
0
0.35 ×
VIO
VIH
Input Logic High "1"
2.7V ≤ VIN ≤ 5.5V, VIO = 3.0V
0.65 ×
VIO
VIO
V
VOL
Output Logic Low "0"
ILOAD = 3mA
400
mV
I2C Compatible Interface Timing Specifications (SCL, SDIO, VIO)(Note 14)
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LM2755
Symbol
Parameter
Condition
Min
Typ
Max
Units
t1
SCL (Clock Period)
2.5
µs
t2
Data In Setup Time to SCL High
100
ns
t3
Data Out stable After SCL Low
0
ns
t4
SDIO Low Setup Time to SCL Low
(Start)
100
ns
t5
SDIO High Hold Time After SCL
High (Stop)
100
ns
20180905
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 TJ = 160°C (typ.) and disengages at TJ
= 155°C (typ.).
Note 4: For detailed soldering specifications and information, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip Scale
Package (AN-1112).
.
Note 5: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7)
Note 6: 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).
Note 7: 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 National Semiconductor Application Note
1112: Micro SMD Wafer Level Chip Scale Package (AN-1112).
Note 8: 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 9: CIN, CPOUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
Note 10: For the current sinks on a part, 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). Two matching numbers are calculated: (MAX-AVG)/AVG and (AVG-MIN)/AVG. The larger number of the two
(worst case) is considered the matching figure. The typical specification provided is the most likely norm of the matching figure for all parts
Note 11: The maximum total output current for the LM2755 should be limited to 90mA. The total output current can be split among any of the three banks (ID1 =
ID2 = ID3 = 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.
Note 12: For each IDx output pin, headroom voltage is the voltage across the internal current sink connected to that pin. For VHR = VOUT -VDxx. If headroom voltage
requirement is not met, LED current regulation will be compromised.
Note 13: SCL and SDIO signals are referenced to VIO and GND for minimum VIO voltage testing.
Note 14: SCL and SDIO should be glitch-free in order for proper brightness control to be realized.
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LED Drive Efficiency vs Input Voltage
Diode Current vs Input Voltage
20180910
20180909
Current Matching vs Input Voltage
3 LEDs
Diode Current vs Brightness Code
20180916
20180917
Shutdown Current vs Input Voltage
Quiescent Current vs Input Voltage
20180919
20180918
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LM2755
Typical Performance Characteristics Unless otherwise specified: TA = 25°C; VIN = 3.6V; VHWEN = VIN;
VD1 = VD2 = VD3 = 3.6V; RSET = 12.5kΩ; C1=C2= 0.47µF, CIN = CVOUT = 1µF; ENA = ENB = ENC = '1'.
LM2755
Square Wave Pattern with Delays
Triangle Wave Pattern
20180912
20180913
Trapezoid Wave Pattern
Slow Ramp-Up / Fast Ramp-Down Wave Pattern
20180911
20180914
Fast Ramp-Up / Slow Ramp-Down Wave Pattern
20180915
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LM2755
Block Diagram
20180920
Circuit Components
will be monitored, and the gain transition will be based upon
the diode with the highest forward voltage.
CHARGE PUMP
The input to the 3/2× - 1x 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 LM2755 is 3.0V 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. This allows the charge pump to stay
in the most efficient gain (1x) over as much of the input voltage
range as possible, reducing the power consumed from the
battery.
HWEN PIN
The LM2755 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.
SYNC PIN
The SYNC pin allows the LM2755 to use an external clock to
generate the timing within. This allows the LM2755's currentsinks to pulse-width modulate (PWM) and transition at a user
controlled frequency. The PWM frequency and the step-time
increment can be set by feeding a clock signal into the sync
pin and enabling bit '6' in the general purpose register (See
the I2C Compatible Interface section for more details.). The
maximum frequency allowed to ensure current level accuracy
is 1MHz. This external clock is divided down by 32x to create
the minimum time-step and PWM frequency. For a 1MHz external clock, the PWM frequency becomes 31.25KHz and the
minimum step time becomes 32 µseconds. If not used, it is
recommended that the SYNC pin be tied to ground.
LED FORWARD VOLTAGE MONITORING
The LM2755 has the ability to switch converter 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 LM2755 will operate in pass mode, allowing the POUT voltage to track the input voltage. As the input
voltage drops, the voltage on the Dx pins will also drop (VDX
= VPOUT – VLEDx). Once any of the active Dx pins reaches a
voltage approximately equal to 350mV, the charge pump will
then switch to the gain of 3/2. This switchover ensures that
the current through the LEDs never becomes pinched off due
to a lack of headroom on the current sources.
Only active Dx pins will be monitored. For example, if only D1
is enabled, the LEDs connected to D2 and D3 will not affect
the gain transition point. If all Dx pins are enabled, all diodes
ADDR PIN
The ADDR pin allows the user to chose between two different
I2C chip addresses for the LM2755. Tying the ADDR pin high
sets the chip address to hex 67 (0x67 or 67h), while tying the
ADDR pin low sets the chip address to hex 18(0x18 or 18h).
This feature allows multiple LM2755's to be used within a
system in addition to providing flexibility in the event another
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LM2755
chip in the system has a chip address similar to the default
LM2755 address (0x18).
can generate repeated START conditions. First START and
repeated START conditions are equivalent, function-wise.
The data on SDIO line must be stable during the HIGH period
of the clock signal (SCL). In other words, the state of the data
line can only be changed when CLK is LOW.
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 CLK is LOW.
20180907
FIGURE 2. Start and Stop Conditions
TRANSFERING DATA
Every byte put on the SDIO line must be eight bits long, with
the most significant bit (MSB) being 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 LM2755 pulls down the SDIO line
during the 9th clock pulse, signifying an acknowledge. The
LM2755 generates an acknowledge after each byte has been
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 LM2755 address
is 18h is ADR is tied low and 67h if ADR is tied high . 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.
20180906
FIGURE 1. Data Validity Diagram
A pull-up resistor between VIO and SDIO must be greater
than [ (VIO-VOL) / 3mA] 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
20180908
FIGURE 3. Write Cycle
w = write (SDIO = "0")
r = read (SDIO = "1")
ack = acknowledge (SDIO pulled down by either master or slave)
rs = repeated start
id = chip address, 18h if ADR = '0' or 67h if ADR = '1' for LM2755
I2C COMPATIBLE CHIP ADDRESS
The chip address for LM2755 is 0011000 (0x18) when ADR = '0' or 1100111(0x67) when ADR = '1'.
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LM2755
Dimming Waveform
20180903
FIGURE 4.
INTERNAL REGISTERS OF LM2755
Register Name
Internal Hex
Address
Power On Value
General Purpose
x10
0000 0000
Time Step
x20
1000 1000
D1 High Level
xA9
1110 0000
D1 Low Level
xA8
1110 0000
D1 Delay: tdelay
xA1
0000 0000
D1 Ramp-Up Step Time: trise
xA5
0000 0000
D1 Time High: thigh
xA3
0000 0000
D1 Ramp-Down Step Time: tfall
xA4
0000 0000
D1 Timing: tlow
xA2
0000 0000
D2 High Level
xB9
1110 0000
D2 Low Level
xB8
1110 0000
D2 Delay: tdelay
xB1
0000 0000
D2 Ramp-Up Step Time: trise
xB5
0000 0000
D2 Time High: thigh
xB3
0000 0000
D2 Ramp-Down Step Time: tfall
xB4
0000 0000
D2 Timing: tlow
xB2
0000 0000
D3 High Level
xC9
1110 0000
D3 Low Level
xC8
1110 0000
D3 Delay: tdelay
xC1
0000 0000
D3 Ramp-Up Step Time: trise
xC5
0000 0000
D3 Time High: thigh
xC3
0000 0000
D3 Ramp-Down Step Time: tfall
xC4
0000 0000
D3 Timing: tlow
xC2
0000 0000
General Purpose Register Description
• Bit 0: enable output D1 with high current level.
• Bit 1: enable output D2 with high current level.
• Bit 2: enable output D3 with high current level.
• Bit 3: enable dimming waveform on output D1.
• Bit 4: enable dimming waveform on output D2.
• Bit 5: enable dimming waveform on output D3.
• Bit 6: enable external clock. '1' = External Clock Sync, '0' = Internal Clock Used
• Bit 7: If Bit 7 = 0 the charge pump is powered on before any dimming waveform is enabled. If Bit7 = 1 the dimming waveform
can be enabled before charge pump is powered on.
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LM2755
where 0 ≤ nTrise/fall ≤ 255
Application Information
trise or fall Total = 50µs x (nhigh-nlow) when nTrise/fall = 0
SETTING FULL-SCALE LED CURRENT
The current through the LEDs connected to D1, D2 and D3
can be set to a desired level simply by connecting an appropriately sized resistor (RSET) between the ISET pin of the
LM2755 and GND. The LED currents are proportional to the
current that flows through the ISET pin and are a factor of 200
times greater than the ISET currents. The feedback loop of the
internal amplifier sets the voltage of the ISET pin to 1.25V
(typ.). The statement above is simplified in the equation below:
thigh or low = TSTEP × (nhigh/low + 1)
where 0 ≤ nThigh/low ≤ 255
SYNC PIN TIMING CONTROL
It is possible to replace the internal clock with an external one
placed on the external SYNC pin. Writing a '1' to bit6 in the
general purpose register switches the system clock from being internally generated to externally generated. The period
of the PWM modulating signal becomes:
fPWM = fSYNC / 32
IDx (Full-Scale) = 200 × (VISET / RSET)
The maximum recommended SYNC frequency is 1MHz. This
frequency yields a PWM frequency of 31.25KHz and the minimum step time of 32 µsec.
Please refer to the I2C Compatible Interface section of this
datasheet for detailed instructions on how to adjust the brightness control registers.
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM2755 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 LM2755. The statement contains the key application parameters that are required to validate an LEDdrive design using the LM2755: 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 LM2755:
BRIGHTNESS LEVEL CONTROL
Once the desired RSET value has been chosen, the LM2755
has the ability to internally dim the LEDs by modulating the
currents with an internally set 20kHz PWM signal. The PWM
duty cycle percentage is independently set for each LED
through the I2C compatible interface. The 32 brightness levels
follow a exponentially increasing pattern rather than a linearly
increasing one in order to better match the human eyes response to changing brightness. The brightness level response is modeled in the following equations.:
IDx LOW = (0.9)(31-nLOW) × IDx Fullscale
IDx HIGH = (0.9)(31-nHIGH) × IDx Fullscale
nHIGH and nLOW are numbers between 0 and 31 stored in the
brightness level registers. When the waveform enable bits are
set to '1', nHIGH and nLOW are the brightness level boundries.
These equations apply to all Dx outputs and their corresponding registers. A '0' code in the brightness control register sets
the current to an "off-state" (0mA).
ILED_MAX = [(1.5 x VIN) - VLED - (IADDITIONAL × ROUT)] ÷
[(Nx x ROUT) + kHRx] (eq. 1)
ILED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL × 2.4Ω)] ÷
[(Nx x 2.4Ω) + kHRx]
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 LM2755 is typically 2.4Ω (VIN = 3.6V, TA =
25°C). In equation form:
TIME STEP CONTROL
Bit 0-Bit 2: The value of the 3 bits is equal to N, which is used
in the timing control equations. 0 ≤ N ≤ 7. The minimum internal time step (N=0) is 50µs. Setting the time-step to N=7
results in a maximum time step of 12.8msec.
TSTEP = 50µsec × 2(N+1) if the internal clock is used.
TSTEP = 2(N+1) × (32 ÷ fSYNC ) if the external clock on the SYNC
pin is used.
Bit 3-Bit 7: Not used
VVOUT = (1.5 × VIN) – [( ILED1 + ILED2 + ILED3) × ROUT]
2)
DELAY CONTROL
The LM2755 allows the programmed current waveform on
each diode pin to independantly start with a delay upon enabling the waveform dimming bits in the general purpose
register. There are 256 delay levels available. The delay time
is set by the following equation:
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 LM2755 is 3.25mV/mA.
In equation form:
tdelay = TSTEP × ndelay
ndelay is stored in the Dx delay registers and N is stored in the
Time Step Control register.
By default, ndelay =0 with a range of 0 ≤ ndelay ≤255.
(VVOUT – VLEDx) > kHRx × ILEDx
(eq. 3)
Typical Headroom Constant Values
kHR1 = kHR2 = kHR3 = 10 mV/mA
The "ILED-MAX" equation (eq. 1) is obtained from combining the
ROUT equation (eq. 2) with the kHRx equation (eq. 3) 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.
TIMING CONTROL
TPWM INTERNAL =50µs, N is a value stored in the Time Step
register, and nTrise nTfall, nThigh, nTlow are numbers between 0
and 255, stored in the timing control registers. The durations
of the rise, high, fall and low times are given by:
trise/fall Total = TSTEP x (nhigh-nlow) x nTrise/fall
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(eq.
10
DRIVER TYPE
MAXIMUM Dx CURRENT
Dx
30mA per Dx Pin
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2755 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.
The 90mA load can be distributed in many different configurations. Special care must be taken when running the LM2755
at the maximum output current to ensure proper functionality.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM2755
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.
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 LM2755 can
be predicted as follow:
PLEDTOTAL = (VLEDA × NA × ILEDA) +
(VLEDB × NB × ILEDB) + (VLEDC × ILEDC)
CAPACITOR SELECTION
The LM2755 requires 4 external capacitors for proper operation (CIN = COUT = 1µF, C1 = C2 = 0.47µF). Surface-mount
multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent
series resistance (ESR <20mΩ typ.). Tantalum capacitors,
OS-CON capacitors, and aluminum electrolytic capacitors are
not recommended for use with the LM2755 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
LM2755. These capacitors have tight capacitance tolerance
(as good as ±10%) and hold their value over temperature
(X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to
85°C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2755. 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 LM2755.
The recommended voltage rating for the capacitors is
10V to account for DC bias capacitance losses.
PIN = VIN × IIN
PIN = VIN × (GAIN × ILEDTOTAL + IQ)
E = (PLEDTOTAL ÷ PIN)
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 LM2755 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
µSMD 18-bump package. VIN is the input voltage to the
LM2755, VLED is the nominal LED forward voltage, N is the
number of LEDs and ILED is the programmed LED current.
PDISS = PIN - PLED1 - PLED2 - PLED3
PDISS= (GAIN × VIN × ID1 + D2+ D3 ) - (VLED1 × ILED1) - (VLED2 ×
ILED2) - (VLED3 × ILED3)
11
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LM2755
TJ = TA + (PDISS x θJA)
Total Output Current Capability
The maximum output current that can be drawn from the
LM2755 is 90mA. Each driver Group has a maximum allotted
current per Dx sink that must not be exceeded.
LM2755
Physical Dimensions inches (millimeters) unless otherwise noted
TMD18AAA: 18 BUMP µSMD
X1 = 1.615mm
X2 = 1.807mm
X3 = 0.6mm
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12
LM2755
Notes
13
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LM2755 Charge Pump LED Controller with I2C Compatible Interface
Notes
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