NSC LM27964 White led driver system with i2c compatible brightness control Datasheet

LM27964
White LED Driver System with I2C Compatible
Brightness Control
General Description
Features
The LM27964 is a charge-pump-based white-LED driver that
is ideal for mobile phone display backlighting. The LM27964
can drive up to 6 LEDs in parallel along with multiple keypad
LEDs, with a total output current up to 180mA. Regulated
internal current sources deliver excellent current matching in
all LEDs.
n 87% Peak LED Drive Efficiency
n 0.2% Current Matching between Current Sinks
n Drives 6 LEDs with up to 30mA per LED in two distinct
groups, for backlighting two displays (main LCD and sub
LCD)
n Dedicated Keypad LED Driver with up to 80mA of drive
current
n Independent Resistor-Programmable Current Settings
n I2C Compatible Brightness Control Interface
n Adaptive 1x- 3/2x Charge Pump
n Extended Li-Ion Input: 2.7V to 5.5V
n Small low profile industry standard leadless package,
LLP 24 : (4mm x 4mm x 0.8mm)
n LM27964SQ-I LED PWM frequency = 10kHz,
LM27964SQ-A LED PWM frequency = 23kHz
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 and the second
group (2 LEDs) can be used to backlight a secondary display. A single Keypad LED driver can power up to 16 keypad
LEDs with a current of 5mA each. The LM27964 has an I2C
compatible interface that allows the user to independently
control the brightness on each bank of LEDs.
The LM27964 works off an extended Li-Ion input voltage
range (2.7V to 5.5V). 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 LM27964 is available in National’s small 24-pin Leadless Leadframe Package (LLP-24).
Applications
n
n
n
n
Mobile Phone Display Lighting
Mobile Phone Keypad Lighting
PDAs Backlighting
General LED Lighting
Typical Application Circuit
20138101
© 2005 National Semiconductor Corporation
DS201381
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LM27964 White LED Driver System with I2C Compatible Brightness Control
October 2005
LM27964
Connection Diagram
24 Pin Quad LLP Package
NS Package Number SQA24A
20138102
Pin Descriptions
Pin #s
Pin Names
24
VIN
Pin Descriptions
Input voltage. Input range: 2.7V to 5.5V.
23
POUT
Charge Pump Output Voltage
19, 22 (C1)
20, 21 (C2)
C1, C2
Flying Capacitor Connections
13, 14, 15, 16
D4A, D3A, D2A,
D1A
LED Drivers - GroupA
4, 5
D1B, D2B
LED Drivers - GroupB
6
DKEY
LED Driver - KEYPAD
17
ISETA
Placing a resistor (RSETA) between this pin and GND sets the full-scale LED
current for Group A LEDs. LED Current = 200 x (1.25V ÷ RSETA)
3
ISETB
Placing a resistor (RSETB) between this pin and GND sets the full-scale LED
current for Group B LEDs. LED Current = 200 x (1.25V ÷ RSETB)
12
ISETK
Placing a resistor (RSETK) between this pin and GND sets the total LED
current for the KEYPAD LEDs. Keypad LED Current = 800 x (1.25V ÷ RSETK)
1
SCL
Serial Clock Pin
2
SDIO
Serial Data Input/Output Pin
7
VIO
Serial Bus Voltage Level Pin
9, 10, 18, DAP
GND
Ground
8, 11
NC
No Connect
Ordering Information
Order Information
LM27964SQ-I
LM27964SQX-I
LM27964SQ-A
LM27964SQX-A
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Current Source
PWM Frequency
Package
10kHz.
SQA24 LLP
23kHz.
SQA24 LLP
2
Supplied As
1000 Units, Tape & Reel
4500 Units, Tape & Reel
1000 Units, Tape & Reel
4500 Units, Tape & Reel
Operating Rating
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN pin voltage
-0.3V to
(VPOUT+0.3V)
w/ 6.0V max
Continuous Power Dissipation
(Note 3)
Internally Limited
Junction Temperature (TJ-MAX)
150oC
Storage Temperature Range
-65oC to +150o C
Maximum Lead Temperature
(Soldering)
(Note 4)
ESD Rating (Note 5)
Human Body Model - IDxx Pins:
Human Body Model - All other
Pins:
Electrical Characteristics
2.7V to 5.5V
LED Voltage Range
2.0V to 4.0V
-30˚C to +100˚C
Ambient Temperature (TA)
Range(Note 6)
-0.3V to (VIN+0.3V)
w/ 6.0V max
IDxx Pin Voltages
Input Voltage Range
Junction Temperature (TJ) Range
-0.3V to 6.0V
SCL, SDIO, VIO pin voltages
(Notes 1, 2)
-30˚C to +85˚C
Thermal Properties
41.3˚C/W
Juntion-to-Ambient Thermal
Resistance (θJA), SQA24A Package
(Note 7)
1.0kV
2.0kV
(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; VDxA = 0.4V; VDxB = 0.4V; VDKEY = 0.4V; RSETA = RSETB = RSETK = 16.9kΩ; BankA,
BankB, and DKEY = Fullscale Current; ENA, ENB, ENK Bits = “1”; C1=C2=1.0µF, CIN=COUT=2.2µF; Specifications related to
output current(s) and current setting pins (IDxx and ISETx) apply to BankA, BankB and DKEY. (Note 9)
Symbol
Parameter
Condition
3.0V ≤ VIN ≤ 5.5V
BankA or BankB Full-Scale
ENA or ENB = "1", ENK = “0”
Output Current Regulation
BankA or BankB Enabled
IDxx
ROUT
VDxTH
Min
Typ
Max
Units
13.77
(-10%)
15.3
16.83
(+10%)
mA
(%)
3.0V ≤ VIN ≤ 5.5V
BankA or BankB Half-Scale
ENA or ENB = "1", ENK = “0”
7.5
mA
2.7V ≤ VIN ≤ 3.0V
BankA or BankB Full-Scale
ENA or ENB = "1", ENK = “0”
15
mA
Output Current Regulation
Keypad Driver Enabled
3.0V ≤ VIN ≤ 5.5V
DKEY Full-Scale
ENA = ENB = “0”, ENK = “1”
Output Current Regulation
BankA and DKEY Enabled
(Note 10)
3.2V ≤ VIN ≤ 5.5V
RSETA = 8.3kΩ, RSETK = 16.9kΩ
VLED = 3.6V
BankA and DKEY Full-Scale
ENA = ENK = “1”, ENB = “0”
Open-Loop Charge Pump Output
Resistance
Gain = 3/2
VDxx 1x to 3/2x Gain Transition
Threshold
VDxA and/or VDxB Falling
52.8
(-12%)
60
mA
60
DKEY
1
3
mA
(%)
30
DxA
2.75
Gain = 1
67.2
(+12%)
375
Ω
mV
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LM27964
Absolute Maximum Ratings (Notes 1, 2)
LM27964
Electrical Characteristics (Notes 2, 8)
(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; VDxA = 0.4V; VDxB = 0.4V; VDKEY = 0.4V; RSETA = RSETB = RSETK = 16.9kΩ; BankA,
BankB, and DKEY = Fullscale Current; ENA, ENB, ENK Bits = “1”; C1=C2=1.0µF, CIN=COUT=2.2µF; Specifications related to
output current(s) and current setting pins (IDxx and ISETx) apply to BankA, BankB and DKEY. (Note 9)
Symbol
VHR
Parameter
Condition
Min
Current Source Headroom Voltage IDxx = 95% xIDxx (nom.)
Requirement
(IDxx (nom) ≈ 15mA)
BankA and/or BankB Full-Scale
(Note 11)
Gain = 3/2, ENA and/or ENB = "1"
IDxx-MATCH LED Current Matching
Typ
Max
Units
180
mV
IDKEY = 95% xIDKEY (nom.)
(IDKEY (nom) ≈ 60mA)
DKEY Full-Scale
Gain = 3/2, ENK = "1"
180
(Note 12)
0.2
2
%
IQ
Quiescent Supply Current
Gain = 1.5x, No Load
1.3
1.7
mA
ISD
Shutdown Supply Current
All ENx bits = "0"
3.0
5
µA
VSET
ISET Pin Voltage
2.7V ≤ VIN ≤ 5.5V
1.25
IDxA-B /
ISETA-B
Output Current to Current Set
Ratio BankA and BankB
200
IDKEY /
ISETK
Output Current to Current Set
Ratio DKEY
800
fSW
Switching Frequency
tSTART
Start-up Time
POUT = 90% steady state
250
fPWM
Internal Diode Current PWM
Frequency
LM27964SQ-I
10
LM27964SQ-A
23
500
D.C. Step Diode Current Duty Cycle Step
700
V
900
kHz
µs
kHz
1/16
Fullscale
2
I C Compatible Interface Voltage Specifications (SCL, SDIO, VIO)
VIO
Serial Bus Voltage Level
1.8
VIN
V
VIL
Input Logic Low "0"
2.7V ≤ VIN ≤ 5.5V
0
0.27 x
VIO
V
VIH
Input Logic High "1"
2.7V ≤ VIN ≤ 5.5V
0.73 x
VIO
VIO
VOL
Output Logic Low "0"
ILOAD = 2mA
400
V
mV
I2C Compatible Interface Timing Specifications (SCL, SDIO, VIO)(Note 13)
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
20138113
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(Continued)
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 = 170˚C (typ.) and disengages at TJ =
165˚C (typ.).
Note 4: For detailed soldering specifications and information, please refer to National Semiconductor Application Note 1187: Leadless Leadframe Package
(AN-1187).
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 = 100˚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 x 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 1187:
Leadless Leadframe Package (AN-1187).
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: The maximum total output current for the LM27964 should be limited to 180mA. The total output current can be split among any of the three banks (IDxA
= IDxB = 30mA Max., IDKEY = 80mA 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 11: For each IDxx output pin, headroom voltage is the voltage across the internal current sink connected to that pin. For Group A and B outputs, VHR = VOUT
-VDxx. If headroom voltage requirement is not met, LED current regulation will be compromised.
Note 12: For the two groups of outputs on a part (BankA and BankB), 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 bank. The matching figure for a given part is considered to
be the highest matching figure of the two banks. The typical specification provided is the most likely norm of the matching figure for all parts.
Note 13: SCL and SDIO should be glitch-free in order for proper brightness control to be realized.
Block Diagram
20138103
5
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LM27964
Electrical Characteristics (Notes 2, 8)
LM27964
Typical Performance Characteristics
Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V, VLEDxB
= 3.6V; RSETA = RSETB = RSETK = 16.9kΩ; C1=C2=1µF , and CIN = CPOUT = 2.2µF.
LED Drive Efficiency vs Input Voltage
Charge Pump Output Voltage vs Input Voltage
20138117
20138123
Shutdown Current vs Input Voltage
Diode Current vs Input Voltage
20138119
20138124
BankA/BankB Diode Current vs Brightness Register
Code
BankA Diode Current vs BankA Headroom Voltage
20138118
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20138120
6
= 3.6V; RSETA = RSETB = RSETK = 16.9kΩ; C1=C2=1µF , and CIN = CPOUT = 2.2µF. (Continued)
BankB Diode Current vs BankB Headroom Voltage
Keypad Driver Current vs Input Voltage
20138121
20138115
Keypad Driver Current vs. Brightness Register Code
Keypad Diode Current vs Keypad Headroom Voltage
20138114
20138122
Keypad Driver Current vs Keypad RSET Resistance
20138116
7
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LM27964
Typical Performance Characteristics Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V, VLEDxB
LM27964
It is not recommended that any of the BankA or BankB
drivers be left disconnected if either bank will be used in the
application. If Dxx pin/s are left unconnected, the LM27964
will default to the gain of 3/2. If the BankA or BankB drivers
are not going to be used in the application, leaving the Dxx
pins is acceptable as long as the ENx bit in the general
purpose register is set to "0".
Circuit Description
OVERVIEW
The LM27964 is a white LED driver system based upon an
adaptive 1.5x/1x CMOS charge pump capable of supplying
up to 180mA of total output current. With three separately
controlled banks of constant current sinks, the LM27964 is
an ideal solution for platforms requiring a single white LED
driver for main and sub displays, as well as other general
purpose lighting needs. The tightly matched current sinks
ensure uniform brightness from the LEDs across the entire
small-format display.
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.
Each LED is configured in a common anode configuration,
with the peak drive current being programmed through the
use of external RSETx resistors. An I2C compatible interface
is used to enable and vary the brightness within the individual current sink banks. For BankA and BankB, 16 levels of
PWM brightness control are available, while 4 analog levels
are present for the DKEY driver.
CIRCUIT COMPONENTS
Charge Pump
The input to the 1.5x/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 LM27964 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.
20138106
FIGURE 1. Data Validity Diagram
A pull-up resistor between VIO and SDIO must be greater
than [ (VIO-VOL) / 2mA] 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. 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.
LED Forward Voltage Monitoring
The LM27964 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 within
BankA and BankB (DKEY is not monitored). At higher input
voltages, the LM27964 will operate in pass mode, allowing
the POUT voltage to track the input voltage. As the input
voltage drops, the voltage on the DXX pins will also drop
(VDXX = VPOUT – VLEDx). Once any of the active Dxx pins
reaches a voltage approximately equal to 375mV, 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 Dxx pins will be monitored. For example, if only
BankA is enabled, the LEDs in BankB will not affect the gain
transition point. If both banks are enabled, all diodes will be
monitored, and the gain transition will be based upon the
diode with the highest forward voltage. The DKEY pin is not
monitored as it is intended to be for keypad LEDs. Keypad
LEDs generally require lower current, resulting in lower forward voltage compared to the BankA and BankB LEDs that
have higher currents. In the event that only the DKEY driver
is enabled without either BankA or BankB, the charge pump
will default to 3/2 mode to ensure the DKEY driver has
enough headroom.
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20138111
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.
8
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 LM27964
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.
(Continued)
The master releases the SDIO line (HIGH) during the acknowledge clock pulse. The LM27964 pulls down the SDIO
line during the 9th clock pulse, signifying an acknowledge.
The LM27964 generates an acknowledge after each byte
has been received.
20138112
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, 36h for LM27964
I2C COMPATIBLE CHIP ADDRESS
The chip address for LM27964 is 0110110, or 36h.
20138108
FIGURE 5. General Purpose Register Description
Internal Hex Address: 10h
20138109
Note: ENA: Enables DxA LED drivers (Main Display)
ENB: Enables DxB LED drivers (Sub Display)
FIGURE 4. Chip Address
ENK: Enables Keypad Driver
INTERNAL REGISTERS OF LM27964
Internal Hex
Address
Power On
Value
General Purpose
Register
10h
0000 0000
Bank A and Bank
B Birghtness
Control Register
A0h
0000 0000
Register
20138107
KEYPAD
B0h
Brightness Control
FIGURE 6. General Purpose Register Example
0000 0000
9
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LM27964
Circuit Description
LM27964
Circuit Description
the LM27964 and GND. The DxA and DxB LED currents are
proportional to the current that flows out of the ISETA and
ISETB pins and are a factor of 200 times greater than the
ISETA/B currents. The DKEY current is proportional to the
current that flows out of the ISETK pin and is a factor of 800
times greater than the ISETK current. The feedback loops of
the internal amplifiers set the voltage of the ISETx pins to
1.25V (typ.). Separate RSETx resistor should be used on
each ISETx pin. The statements above are simplified in the
equations below:
IDxA/B = 200 x (VISET / RSETA/B)
RSETA/B = 200 x (1.25V / IDxA/B)
IDKEY = 800 x (VISET / RSETK)
RSETK = 800 x (1.25V / IDKEY)
Once the desired RSETx values have been chosen, the
LM27964 has the ability to internally dim the LEDs by Pulse
Width Modulating (PWM) the current. The PWM duty cycle is
set through the I2C compatible interface. LEDs connected to
BankA and BankB current sinks (DxA and DxB) can be
dimmed to 16 different levels/duty-cycles (1/16th of full-scale
to full-scale). The internal PWM frequency for BankA and
BankB is a fixed 10kHz (LM27964SQ-I) or 23kHz
(LM27964SQ-A) depending on the option.
The DKEY current sink uses an analog current scaling
method to control LED brightness. The brightness levels are
100% (Fullscale), 70%, 40%, and 20%. When connecting
multiple LEDs in parallel to the DKEY current sink, it is
recommended that ballast resistors be placed in series with
the LEDs. The ballast resistors help reduce the affect of LED
forward voltage mismatch, and help equalize the diode currents. Ballast resistor values must be carefully chosen to
ensure that the current source headroom voltage is sufficient
to supply the desired current.
Please refer to the I2C Compatible Interface section of this
datasheet for detailed instructions on how to adjust the
brightness control registers.
(Continued)
20138105
FIGURE 7. Brightness Control Register Description
Internal Hex Address: A0h
Note: DxA3-DxA0: Register Sets Current Level Supplied to DxA LED drivers
DxB3-DxB0: Register Sets Current Level Supplied to DxB LED drivers
Full-Scale Current set externally by the following equation:
IDxx = 200 x 1.25V / RSETx
Brightness Level Segments = 1/16th of Fullscale
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM27964 can drive 4 LEDs at 30mA each (BankA) and
12 keypad LEDs at 5mA each (60mA total at DKEY) 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
capabilities of the LM27964. The statement contains the key
application parameters that are required to validate an LEDdrive design using the LM27964: 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 LM27964:
ILED_MAX = [(1.5 x VIN) - VLED - (IADDITIONAL x ROUT)] /
[(Nx x ROUT) + kHRx] (eq. 1)
ILED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL x 2.75Ω)] /
[(Nx x 2.75Ω) + kHRx]
IADDITIONAL is the additional current that could be delivered
to the other LED banks.
ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage droop at
the pump output POUT. 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 LM27964 is typically 2.75Ω (VIN =
3.6V, TA = 25˚C). In equation form:
20138104
FIGURE 8. Brightness Control Register Example
20138110
FIGURE 9. Internal Hex Address: B0h
Note: DKEY1-DKEY0: Sets Brightness for DKEY pin (KEYPAD Driver).
11=Fullscale
Bit7 to Bit 2: Not Used
Full-Scale Current set externally by the following equation:
IDKEY = 800 x 1.25V / RSETx
Brightness Level are= 100% (Fullscale), 70%, 40%, 20%
Application Information
SETTING LED CURRENT
The current through the LEDs connected to DxA, DxB and
DKEY can be set to a desired level simply by connecting an
appropriately sized resistor (RSETx) between the ISETx pin of
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DxB pins unused if either diode bank is going to be used
during normal operation. Leaving DxA and/or DxB pins unconnected will force the charge-pump into 3/2x mode over
the entire VIN range negating any efficiency gain that could
be achieve by switching to 1x mode at higher input voltages.
Care must be taken when selecting the proper RSETx value.
The current on any Dxx pin must not exceed the maximum
current rating for any given current sink pin.
(Continued)
VPOUT = (1.5 x VIN) – [(NAx ILEDA + NB x ILEDB + NK x
(eq. 2)
ILEDK) x ROUT]
kHR – Headroom constant. This parameter models the minimum 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 kHR of the LM27964 is
12mV/mA. In equation form:
(eq. 3)
(VPOUT – VLEDx) > kHRx x ILEDx
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 1.5x/1x charge pump, the
input current is equal to the charge pump gain times the
output current (total LED current). The efficiency of the
LM27964 can be predicted as follows:
PLEDTOTAL = (VLEDA x NA x ILEDA) +
(VLEDB x NB x ILEDB) + (VLEDK x NK x ILEDK)
PIN = VIN x IIN
Typical Headroom Constant Values
kHRA = 12mV/mA
kHRB = 12 mV/mA
kHRK = 3 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.
PIN = VIN x (GAIN x ILEDTOTAL + IQ)
E = (PLEDTOTAL ÷ PIN)
It is also worth noting that efficiency as defined here is in part
dependent on LED voltage. Variation in LED voltage does
not affect power consumed by the circuit and typically does
not relate to the brightness of the LED. For an advanced
analysis, it is recommended that power consumed by the
circuit (VIN x IIN) be evaluated rather than power efficiency.
Total Output Current Capability
The maximum output current that can be drawn from the
LM27964 is 180mA. Each driver bank 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
DKEY
80mA
POWER DISSIPATION
The power dissipation (PDISS) and junction temperature (TJ)
can be approximated with the equations below. PIN is the
power generated by the 1.5x/1x 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 LLP-24 package. VIN is the input voltage to the
LM27964, VLED is the nominal LED forward voltage, N is the
number of LEDs and ILED is the programmed LED current.
PDISS = PIN - PLEDA - PLEDB - PLEDK
PDISS= (GAIN x VIN x ILEDA + LEDB + LEDK) - (VLEDA x NA x
ILEDA) (VLEDB x NB x ILEDB) - (VLEDK x NK x ILEDK)
TJ = TA + (PDISS x θJA)
The junction temperature rating takes precedence over the
ambient temperature rating. The LM27964 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 100˚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 100˚C.
The 180mA load can be distributed in many different configurations. Special care must be taken when running the
LM27964 at the maximum output current to ensure proper
functionality.
PARALLEL CONNECTED OUTPUTS
Outputs D1A-4A or D1B-D2B may be connected together to
drive one or two LEDs at higher currents. In such a configuration, all four parallel current sinks (BankA) of equal value
can drive a single LED. The LED current programmed for
BankA should be chosen so that the current through each of
the outputs is programmed to 25% of the total desired LED
current. For example, if 60mA is the desired drive current for
a single LED, RSETA should be selected such that the current
through each of the current sink inputs is 15mA. Similarly, if
two LEDs are to be driven by pairing up the D1A-4A inputs
(i.e D1A-2A, D3A-4A), RSETA should be selected such that
the current through each current sink input is 50% of the
desired LED current. The same RSETx selection guidelines
apply to BankB diodes.
Connecting the outputs in parallel does not affect internal
operation of the LM27964 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 4-LED application circuit.
Both BankA and BankB utilize LED forward voltage sensing
circuitry on each Dxx pin 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 or
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM27964
when the junction temperature exceeds 170˚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 165˚C (typ.).
It is important that the board layout provide good thermal
conduction to keep the junction temperature within the specified operating ratings.
11
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LM27964
Application Information
LM27964
Application Information
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 LM27964.
The minimum voltage rating acceptable for all capacitors is
6.3V. The recommended voltage rating of the output capacitor is 10V to account for DC bias capacitance losses.
(Continued)
CAPACITOR SELECTION
The LM27964 requires 4 external capacitors for proper operation (C1 = C2 = 1µF, CIN = COUT = 2.2µF). 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 LM27964 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
LM27964. 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).
PCB LAYOUT CONSIDERATIONS
The LLP is a leadframe based Chip Scale Package (CSP)
with very good thermal properties. This package has an
exposed DAP (die attach pad) at the center of the package
measuring 2.6mm x 2.5mm. The main advantage of this
exposed DAP is to offer lower thermal resistance when it is
soldered to the thermal land on the PCB. For PCB layout,
National highly recommends a 1:1 ratio between the package and the PCB thermal land. To further enhance thermal
conductivity, the PCB thermal land may include vias to a
ground plane. For more detailed instructions on mounting
LLP packages, please refer to National Semiconductor Application Note AN-1187.
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM27964. 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
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12
inches (millimeters) unless otherwise noted
SQA24: 24 Lead LLP
X1 = 4.0mm
X2 = 4.0mm
X3 = 0.8mm
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.
For the most current product information visit us at www.national.com.
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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 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 labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor manufactures products and uses packing materials that 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.
Leadfree products are RoHS compliant.
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LM27964 White LED Driver System with I2C Compatible Brightness Control
Physical Dimensions