TI LM3537TME

LM3537
LM3537 8-Channel WLED Driver with Four Integrated LDOs
Literature Number: SNVS634A
LM3537
8-Channel WLED Driver with Four Integrated LDOs
General Description
Features
The LM3537 is a highly integrated LED driver capable of driving 8 LEDs in parallel for single display backlighting applications. Independent LED control allows for a subset of the main
display LEDs to be selected for partial illumination applications.
I2C-compatible control allows full configurability of the backlighting function. The LM3537 provides multi-zone Ambient
Light Sensing allowing autonomous backlight intensity control
in the event of changing ambient light conditions. A PWM input is also provided to give the user a means to adjust the
backlight intensity dynamically based upon the content of the
display.
Four integrated LDOs are fully configurable through I2C capable of addressing point-of-load regulation needs for functions such as integrated camera modules. The LDOs can be
powered from main battery source, or by a fixed output voltage of an external buck converter (post regulation) leading to
higher conversion efficiency.
The LM3537 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.
LM3537 is offered in a tiny 30-bump micro-SMD package:
2.02 mm x 2.52 mm x 0.60 mm, 0.40 mm pitch.
Lighting:
■ 8-channel backlight capability
■ Internal ALS engine; PWM input to support CABC
■ Built-in power supply and gain control for ambient light
sensor
■ Up to 90% efficiency
■ Adaptive charge pump with 1x and 1.5x gains for
maximum efficiency
■ 128 dimming steps for group A, exponential or linear
dimming selectable by register setup
■ 8 linear dimming states for group B
LDOs:
■ 4 Programmable LDOs (300 mA/150 mA output currents)
■ Default startup voltage states
■ Low dropout voltage: 100 mV typ. at 150 mA load current
■ LDO input voltage = 1.8V to VIN_A
■ Overload protection
Combined Common Features:
■ Wide input voltage range: 2.7V to 5.5V
■ I2C-compatible serial interface
■ 2 general-purpose outputs
Applications
■ Smartphone lighting
■ MP3 players, gaming devices
■ Digital cameras
Typical Application Circuit
30108301
© 2011 National Semiconductor Corporation
301083
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LM3537 8-Channel WLED Driver with Four Integrated LDOs
October 17, 2011
LM3537
Connection Diagrams
30–Bump micro SMD Package
Top View
30108341
XY – Date Code
TT – Die Traceability
ABCD – Product Identification
30108302
Pin Descriptions
Bump
Name
Description
C5
VIN_A
Input voltage for LED driver and sensor interface. Input range: 2.7V to 5.5V.
E5
VIN_B
Input voltage for the regulators. This must be connected to the same voltage supply as
VIN_A
F5
VIN_C
Input voltage (power rail) for the LDO regulators. 1.8V ≤ VIN_C ≤ VIN_A
B1
SCL
Serial interface clock
B3
SDA
Serial interface data
A1
HWEN
B2
PWM
External PWM Input - Allows the current sinks to be turned on and off at a frequency
and duty cycle externally controlled. Minimum on-time pulse width = 15 µsec.
E4
SBIAS
Power supply for a light sensor. Leave unconnected if not used.
E3
GPO1
General purpose output. Can be used as a sensor gain control signal. When functioning
as a general purpose output, it is open drain and requires an external pullup. Leave
unconnected if not used.
E2
GPO2
General purpose output. Can be used as a sensor gain control signal. When functioning
as a general purpose output, it is open drain and requires an external pullup. Leave
unconnected if not used.
Hardware enable pin. High = normal operation, low = RESET
D5
ALS
Ambient Light Sensor input. Connect to ground if not used.
F3
GND
Regulator ground
A2
PGND
LED driver and charge pump ground
F2
LDO4
Programmable VOUT of 1.2-3.3 V. Max load = 150 mA.
F4
LDO3
Programmable VOUT of 1.2-3.3 V. Max load = 150 mA.
E1
LDO2
Programmable VOUT of 1.2-3.3 V. Max load = 150 mA.
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2
Name
LM3537
Bump
Description
F1
LDO1
C3
D8
Programmable VOUT of 1.2-3.3 V. Max load = 300 mA.
C4
D7/INT
D4
D6
LED driver
D3
D5
LED driver
D2
D4
LED driver
D1
D3
LED driver
C1
D2
LED driver
C2
D1
LED driver
B5
VOUT
Charge pump output
B4
C2-
Flying capacitor 2 negative terminal
A4
C2+
Flying capacitor 2 positive terminal
A3
C1-
Flying capacitor 1 negative terminal
A5
C1+
Flying capacitor 1 positive terminal
LED driver
LED driver/ ALS interrupt (mode of operation is selected via register). In ALS interrupt
mode, a pullup resistor is required. A '0’ means a change has occurred, while a ‘1’
means no ALS adjustment has been made.
Ordering Information
Order Information
LM3537TME
LM3537TMX
Package
TMD30AEA
3
Supplied As
250 Units, Tape & Reel
3000 Units, Tape & Reel
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LM3537
Absolute Maximum Ratings (Note 1, Note
Operating Rating
VIN_A, VIN_B Input Voltage Range
LED Voltage Range
VIN_C Input Voltage Range (Note: must
stay > VOUTLDO + 0.3V)
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range
(Note 6)
2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN_A, VIN_B , VIN_C pin voltage
Voltage on Logic Pins (SCL, SDA,
GPO1, GPO2, HWEN, PWM)
LED driver (D1 to D8) Pin Voltages
Voltage on All Other Pins
Continuous Power Dissipation
(Note 3)
Junction Temperature (TJ-MAX)
Storage Temperature Range
Maximum Lead Temperature
(Soldering)
ESD Rating (Note 5)
Human Body Model
(Note 1, Note 2)
-0.3V to 6.0V
-0.3V to
(VIN_A+0.3V)
with 6.0V max
-0.3V to
(VOUT+0.3V)
with 6.0V max
-0.3V to (VIN_A
+0.3V) with 6.0V
max
Internally Limited
2.7V to 5.5V
2.0V to 4.0V
1.8V to VIN_B
−30°C to +110°C
−30°C to +85°C
Thermal Properties
Junction-to-Ambient Thermal
Resistance (θJA),
TMD30 Package
(Note 7)
45°C/W
If Military/Aerospace specified devices are required, please
contact the National Semiconductor Sales/Office/Distributors
for availability and specifications.
150°C
-40°C to +150°C
(Note 4)
2 kV
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.
Charge Pump and LED Drivers Electrical Characteristics
(Note 2, Note 8) Limits in
standard typeface are for TJ = 25°C, and limits in boldface type apply over the operating ambient temperature range (−30°C to
+85°C). Unless otherwise specified: VIN_A = 3.6V; VHWEN = VIN_A; VDx = 0.4V; GroupA = GroupB = Fullscale Current; C1 = C2 =
CIN_A= COUT= 1.0 µF. (Note 9)
Symbol
IDx
IDx-
Min
Typ
Max
Units
Output Current Regulation
GroupA
Parameter
2.7V ≤ VIN_A ≤ 5.5V
8 LEDs in GroupA
−7.5%
25
+7.5%
mA
Output Current Regulation
GroupB
2.7V ≤ VIN_A ≤ 5.5V
4 LEDs in GroupB
−7.5%
25
+7.5%
mA
Output Current Regulation
All LED Drivers Enabled
All LED Drivers on BankA (Note 10)
3.2V ≤ VIN_A ≤ 5.5V
VLED = 3.6V
BankA current code = 1111101b, exp
dimming scale
LED Current Matching (Note 11)
2.7V ≤ VIN ≤ 5.5V
LED Current =
Fullscale current
MATCH
Condition
22.3
DxA
mA
GroupA (8 LEDs)
0.8
3
GroupB (4 LEDs)
0.4
3
%
VDxTH
VDx 1x to 3/2x Gain Transition Threshold VDx Falling
135
mV
VHR
Current sink Headroom Voltage
Requirement
(Note 12)
IDx = 95% ×IDx (nom.)
(IDx (nom) ≈ 20 mA)
100
mV
ROUT
Open-Loop Charge Pump Output
Resistance (Note 20)
Gain = 3/2
2.4
Gain = 1
0.5
Gain = 1.5x, No Load. Current through VIN_A
pin. Sensor Bias OFF
2.9
4.4
Gain = 1x, No Load. Current through VIN_A
pin. Sensor Bias OFF
1.1
2.4
IQ
Quiescent Supply Current
mA
ISB
Standby Supply Current
HWEN = 1.8V. All registers in factory defaults
state. Current through VIN_A pin.
1.2
ISD
Shutdown Supply Current
HWEN = 0V. Current through VIN_A pin.
0.2
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4
Ω
µA
1.0
µA
Parameter
fSW
Switching Frequency
tSTART
Startup Time
VALS
ALS Reference Voltage
RALS
Condition
1.1
(Note 16)
Internal ALS Resistor
Typ
Max
Units
1.3
1.6
MHz
250
µs
−6%
1.0
+6%
RALS register setting = 00010b
−6%
10.1
+6%
RALS register setting = 00100b
−6%
5.0
+6%
Min
Typ
Max
Logic Interface Characteristics
Symbol
Min
V
kΩ
(Note 2, Note 8)
Parameter
Condition
Units
I2C-Compatible Interface Timing Specifications (SCL, SDA) (Note 13)
t1
SCL (Clock Period)
t2
Data In Setup Time to SCL High
t3
Data Out stable After SCL Low
t4
SDA Low Setup Time to SCL Low
(Start)
t5
SDA High Hold Time After SCL High
(Stop)
(Note 14)
2.5
µs
100
ns
0
ns
100
ns
100
ns
I2C-Compatible Interface Voltage Specifications (SCL, SDA)
VIL
Input Logic Low "0"
2.7V ≤ VIN_A ≤ 5.5V
0
0.45
V
VIH
Input Logic High "1"
2.7V ≤ VIN_A ≤ 5.5V
1.25
VIN_A
V
VOL
Output Logic Low "0"
ILOAD = 3mA
400
mV
Logic inputs HWEN and PWM
VHWEN
HWEN Voltage Thresholds
2.7V ≤ VIN_A ≤ 5.5V
Reset
VPWM
PWM Voltage Thresholds
2.7V ≤ VIN_A ≤ 5.5V
0
0.45
1.2
VIN_A
LEDs Off
0
0.45
LEDs On
1.2
VIN_A
Normal Operation
V
V
ALS interrupt
VOL-INT
Interrupt Output Logic Low '0'
ILOAD = 3mA
400
mV
0.5
V
Logic outputs GPO1, GPO2 (Note 19)
VOL
Output Low Level
IOUT = 3 mA
VOH
Output High Level
IOUT = −2 mA
0.3
VOUT_S
−0.5
VOUT
_S −
0.3
V
Voltage Regulators Electrical Characteristics
(Note 2, Note 8) Unless otherwise noted, VIN_A=
VIN_B = VIN_C = 3.6V, CIN_A = 1 µF, CIN_B = 100 nF, CIN_C = 2.2 µF, CLDOX= 1 µF, HWEN = high. Limits in standard typeface are
for TJ = 25°C, and limits in boldface type apply over the operating ambient temperature range (-30°C to +85°C). (Note 17)
Symbol
Parameter
Condition
Min
Typ
Max
Units
LDO1
VOUT
Output Voltage Accuracy
IOUTLDO = 1 mA, VOUTLDO = 2.80V
Default Output Voltage
IOUT
VDO
ΔVOUT
−2
+2
−3
+3
2.80
Output Current
1.8V ≤ VIN_C ≤ 5.5V
Output Current Limit (short circuit)
VOUTLDO = 0V
600
Dropout Voltage
IOUTLDO = 300 mA
220
Line Regulation
VOUTLDO + 0.5V ≤ VIN_C ≤ 4.5V
IOUTLDO = 1 mA
2
Load Regulation
1 mA ≤ IOUTLDO ≤ 300 mA
20
5
%
V
300
mA
mA
300
mV
mV
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LM3537
Symbol
LM3537
Symbol
PSRR
Parameter
Condition
Min
f = 100Hz,
CLDO1 = 1 µF,
Power Supply Ripple Rejection Ratio
IOUTLDO = 20 mA
Output Voltage = 1.20V
Typ
Max
65
Units
dB
LDO2, LDO3, LDO4
Output Voltage Accuracy
VOUT
Default Output Voltage
IOUT
VDO
ΔVOUT
PSRR
IOUTLDO = 1 mA, VOUTLDO = 2.80V
−2
+2
−3
+3
LDO2
1.80
LDO3
1.80
LDO4
2.80
Output Current
1.8V ≤ VIN_C ≤ 5.5V
Output Current Limit (short circuit)
VOUTLDO = 0V
400
Dropout Voltage
IOUTLDO = 150 mA
100
Line Regulation
VOUTLDO + 0.5V ≤ VIN_C ≤ 4.5V
IOUTLDO = 1mA
2
Load Regulation
1mA ≤ IOUTLDO ≤ 150 mA
10
V
V
150
f = 100 Hz,
CLDOX = 1µF,
Power Supply Ripple Rejection Ratio
IOUTLDO = 20 mA
Output Voltage = 1.20V
%
mA
mA
200
mV
mV
65
dB
LDO Combined Common Electrical Characteristics
IGND
All LDOs Disabled
0.2
1
One LDO Enabled
70
130
Ground Pin Current (GND and PGNDNote: IOUTLDOX = 0mA Two LDOs Enabled
pin)
Three LDOs Enabled
100
tSTARTUP
Turn-on Time from Shut-down (Note
15)
TTransient Startup Transient Overshoot
µA
130
Four LDOs Enabled
µA
160
CLDOX = 1µF, IOUTLDO = 150 mA
VOUT = 2.8V. Enable of First LDO
130
CLDOX = 1 µF, IOUTLDO = 150 mA
VOUT = 2.8V. Enable of Each Subsequent LDO
after First Enabled
70
µs
CLDOX = 1 µF, IOUTLDO = 150 mA
30
mV
Sensor Interface Electrical Characteristics
Unless otherwise noted, VIN_A = 3.6V, CIN_A = 1 µF,
CIN_B = 100 nF, CIN_C = 2.2 µF, CSEN= 1 µF, HWEN = high. Limits in standard typeface are for TJ = 25°C, and limits in boldface
type apply over the operating ambient temperature range (−30°C to +85°C).
Symbol
Parameter
Condition
Min
Typ
Max
Units
20
mA
SBIAS
IOUT_S
VOUT_S
IQIF
SBIAS Output Current
SBIAS Output Voltage
Sensor Interface Quiescent Supply
Current (Note 18, Note 20)
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2.7V ≤ VIN_A ≤ 5.5V. VOUT_S < (VIN_A +0.3V)
2.7V ≤ VIN_A ≤ 5.5V. IOUT_S = 1.0 mA. 2.4V
option selected via register.
−5%
3.3V ≤ VIN_A ≤ 5.5V. IOUT_S = 1.0 mA. 3.0V
option selected via register.
−5%
No Load
2.4
V
3.0
35
6
+5%
+5%
µA
LM3537
30108313
FIGURE 1. Timing Parameters
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 pins.
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 100 pF capacitor discharged through a 1.5 kΩ 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 = 110°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_X, COUT, CLDOX, CSEN, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.
Note 10: The total output current can be split between the two groups (IDx = 25 mA Max). Under maximum output current conditions, special attention must be
given to input voltage and LED forward voltage to ensure proper current regulation. The maximum total output current for the LM3537 should be limited to 180
mA.
Note 11: For the two groups of current sinks on a part (group A and group B), 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.
Note 12: For each Dx pin, headroom voltage is the voltage across the internal current sink connected to that pin. For group A and B current sinks, VHRx = VOUT
-VLED. If headroom voltage requirement is not met, LED current regulation will be compromised.
Note 13: SCL and SDA should be glitch-free in order for proper device control to be realized. See Figure 1 for timing specification details.
Note 14: SCL is tested with a 50% duty-cycle clock.
Note 15: Time needed for VOUTLDO to reach 95% of final value.
Note 16: Turn-on time is measured from the moment the charge pump is activated until the VOUT crosses 90% of its target value.
Note 17: CIN_C, CLDOX : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.
Note 18: In addition to Quiescent Supply Current (IQ) drawn by the charge pump. (See Table Charge Pump and LED Drivers Electrical Characteristics.)
Note 19: VOUT_S = SBIAS pin output voltage. The voltage level of the GPOs depends on the sbias_en-bit: '1'; GPOs will behave as push-pull outputs and will
reference the high-side to the voltage of SBIAS. '0'; GPOs will act as open-drain outputs (default). In the open-drain configuration, they can be high-side referenced
to any voltage equal to, or less than, the VIN_A of the LM3537. Output High Level (VOH) specification is valid only for push-pull -type outputs.
Note 20: Guaranteed by design.
7
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LM3537
Typical Performance Characteristics
Unless otherwise specified: VIN_A,B,C = 3.6V, CIN_A = COUT = 1.0 µF, CIN_B = 0.1 µF, CIN_C = 4.7 µF, C1 = C2= 1.0 µF, CLDOx= 1.0
µF, TA = 25°C.
Regulator 1 (300 mA) Output Voltage vs Output Current
VSET = 2.80V
Regulator 2,3,4 (150 mA) Output Voltage vs Output Current
VSET = 1.80V
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30108320
Power Supply Rejection Ratio, VOUT = 1.20V, ILOAD = 20 mA Power Supply Rejection Ratio, VOUT = 1.20V, ILOAD = 20 mA
VIN_C is shorted to VIN_A, VIN_B
Signal Applied on VIN_C, VIN_A and VIN_B Clear.
30108321
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30108322
8
LM3537
Load Transient. VOUT setting = 1.80V
ILOAD 1mA to 150mA to 1mA; tRISE= tFALL= 5µs
Line Transient Response
VOUT setting = 1.80V,, ILOAD 1mA
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30108343
Regulator Enable Response; Enable of First Regulator
(1mA load, 1.80V) via Reg. Write
Regulator Enable Response; Enable of First Regulator
(150mA load, 2.80V) via Reg. Write
30108344
30108345
Regulator 2,3,4 Short Circuit Current
VOUT setting = 1.80V
Regulator 1 Short Circuit Current
VOUT setting = 2.80V
30108346
30108347
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LM3537
Shutdown Supply Current
HWEN = 0V. Current through VIN_A pin
Standby Supply Current
HWEN = 1.8V. Current through VIN_A pin
30108348
30108349
Quiescent Current vs Input Voltage
1× Gain
Quiescent Current vs Input Voltage 3/2× Gain
3/2× Gain
30108355
30108354
LED Current Matching Distribution.
6 Drivers on Group A, Output Set to 25 mA. (Note 11)
Charge Pump 1.5x Efficiency vs
Load Current
30108353
30108352
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10
LM3537
Block Diagram
30108303
11
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LM3537
'0' alerting the controller. Available resistor values are shown
in Table 1 below.
Circuit Description
OVERVIEW
The LM3537 is a white LED driver system based upon an
adaptive 3/2× - 1× CMOS charge pump capable of supplying
up to 180 mA of total output current. With two separately controlled groups of constant current sinks, the LM3537 is an
ideal solution for platforms requiring a single white LED driver
for main display and sub display (or keypad). 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 set to 25 mA. An I2C-compatible
interface is used to enable the device and vary the brightness
within the individual current sink groups. For group A, 128
brightness control levels are available (user defined linear or
exponential dimming curve). Group B has 8 linearly-spaced
analog brightness levels.
The LM3537 provides an input for an Ambient Light Sensor
to adaptively adjust the diode current based on ambient conditions, and a PWM pin to allow the diode current to be pulse
width modulated to work with a display driver utilizing dynamic
or content adjusted backlight control (DBC or CABC). Additionally, the device provides 20 mA power supply output for
the sensor. The GPOs can also be configured to serve as a
gain control interface for sensors with HW-controlled gain.
The LM3537 also integrates three 150-mA LDO and one 300mA LDO voltage regulators, which can be turned on/off using
separate enable bits on each LDO. Each LDO operates with
a power rail input voltage range between 1.8 V and 5.5V allowing them to be supplied from the battery or a step-down
converter. Furthermore, the regulated output voltages can be
adjusted through the serial bus.
TABLE 1. ALS Resistor Values
CIRCUIT COMPONENTS
Charge Pump
The input to the 3/2× - 1× charge pump is connected to the
VIN_A pin, and the regulated output of the charge pump is
connected to the VOUT pin. The operating input voltage range
of the LM3537 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
4.2V (typ.). The charge pump gain transitions are actively selected to maintain regulation based on LED forward voltage
and load requirements.
Diode Current Sinks
The matched current outputs are generated with a precision
current mirror that is biased off the charge pump output.
Matched currents are ensured with the use of tightly matched
internal devices and internal mismatch cancellation circuitry.
There are eight regulated current sinks configurable into 2
different lighting regions.
r_als
[3]
r_als
[2]
r_als
[1]
r_als
[0]
RALS
(typ)
Value
Unit
1
1
1
1
1
0.651
kΩ
1
1
1
1
0
0.672
kΩ
1
1
1
0
1
0.695
kΩ
1
1
1
0
0
0.720
kΩ
1
1
0
1
1
0.747
kΩ
1
1
0
1
0
0.776
kΩ
1
1
0
0
1
0.806
kΩ
1
1
0
0
0
0.840
kΩ
1
0
1
1
1
0.876
kΩ
1
0
1
1
0
0.916
kΩ
1
0
1
0
1
0.960
kΩ
1
0
1
0
0
1.01
kΩ
1
0
0
1
1
1.06
kΩ
1
0
0
1
0
1.12
kΩ
1
0
0
0
1
1.19
kΩ
1
0
0
0
0
1.26
kΩ
0
1
1
1
1
1.34
kΩ
0
1
1
1
0
1.44
kΩ
0
1
1
0
1
1.55
kΩ
0
1
1
0
0
1.68
kΩ
0
1
0
1
1
1.83
kΩ
0
1
0
1
0
2.02
kΩ
0
1
0
0
1
2.24
kΩ
0
1
0
0
0
2.52
kΩ
0
0
1
1
1
2.88
kΩ
0
0
1
1
0
3.36
kΩ
0
0
1
0
1
4.03
kΩ
0
0
1
0
0
5.00
kΩ
0
0
0
1
1
6.72
kΩ
0
0
0
1
0
10.1
kΩ
0
0
0
0
1
20.2
kΩ
0
0
0
0
0
HighZ
--
Automatic Gain Change
GPO pins of the LM3537 can be configured to serve as a gain
control interface for sensors with HW controlled gain, like
ROHM BH1600-series. Please see Table 2. LM3537 changes
sensor gain automatically based on ambient light intensity
changes.
Ambient Light Sensing (ALS) and Interrupt
The LM3537 provides an Ambient Light Sensing input for use
with ambient backlight control. Connecting the anode of a
photo diode to this pin and configuring the appropriate ALS
resistor, the LM3537 can be configured to adjust the LED
current to five unique settings corresponding to four adjustable light region trip points. Additionally, when the LM3537
determines that an ambient condition has changed, the interrupt pin, when connected to a pullup resistor will toggle to a
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r_als
[4]
12
OUTPUT PIN STATUS
GPO1
GPO2
autogain_en = "0" Can be set to "1" or Can be set to "1" or
"0" with REG 52H, "0" with REG 52H,
bit gpo1
bit gpo2
autogain_en =
"1" (enables
autogain
functionality)
LOW GAIN
0
1
autogain_en =
"1" (enables
autogain
functionality)
HIGH GAIN
1
0
The ambient light sensing circuit has 4 configurable Ambient
Light Boundaries (ZB0 – ZB3) programmed through the four
8-bit Zone Boundary Registers. These zone boundaries define 5 ambient brightness zones.
The ambient light sensor input has a 0 to 1V operational input
voltage range. The Typical Application Circuit shows the
LM3537 with an ambient light sensor (ROHM, BH1621FVC).
If the internal ALS Resistor Select Register is set to 0x14 (1.44
kΩ), this circuit will convert 0 to 1000 LUX light into approximately a 0 to 850 mV linear output voltage (high-gain mode).
The voltage at the active ambient light sensor input is com-
30108317
FIGURE 2. ALS Zone to LED Brightness Mapping
ALS Zone transitions with AutoGain is shown in Figure 3.
When the light intensity increases, the LM3537 configures the
sensor for low-gain mode. Transition from Zone2 to Zone3
triggers the shift to lower gain mode. When the light intensity
decreases, the LM3537 configures the sensor to high-gain
mode. The trip point to this transition is set by the ALS
LOW_to_HIGH_TP register, and it should be set lower than
the Zone2 to Zone3 transition, in order to have hysteresis.
Zone3 to Zone2 transition trip point must be set separately for
lower gain mode, by the ALS ZONE BOUNDARY Z3_to_Z2
register. This register value should be set higher than the ALS
LOW_to_HIGH_TP. In low-gain mode the sensor will have a
lower output current which helps save battery power. Highgain mode will allow better resolution, but will result higher
output current. Thus, there is a trade-off between increased
resolution and increased power consumption. High-gain
mode is the default mode of operation after enabling the autogain.
13
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LM3537
pared against the 8-bit values programmed into the Zone
Boundary Registers (ALS ZONE BOUNDARY#0 - ALS ZONE
BOUNDARY#3 ). When the ambient light sensor output
crosses one of the programmed thresholds the internal ALS
circuitry will smoothly transition the LED current to the new 7bit brightness level as programmed into the appropriate Zone
Target Register (ALS BRIGHTNESS ZONE#0 to ALS
BRIGHTNESS ZONE#4).
Ambient light sensor samples are averaged and then further
processed by the discriminator block to provide rejection of
noise and transient signals. The averager is configurable with
8 different averaging times to provide varying amounts of
noise and transient rejection. The discriminator block algorithm has a maximum latency of two averaging cycles; therefore, the averaging time selection determines the amount of
delay that will exist between a steady state change in the ambient light conditions and the associated change of the backlight illumination. For example, the A/D converter samples the
ALS inputs at 16 kHz. If the averaging time is set to 800 ms,
the averager will send the updated zone information to the
discriminator every 800 ms. This zone information contains
the average of approximately 12800 samples (800 ms × 16
kHz). Due to the latency of 2 averaging cycles, when there is
a steady state change in the ambient light, the LED current
will begin to transition to the appropriate target value after
approximately 1600 ms have elapsed.
ALS Zone to LED Brightness Mapping principle without AutoGain is shown in below. Here, the exponential dimming
scheme is used.
TABLE 2. Sensor Gain Control
REGISTER
SETTING
LM3537
30108350
FIGURE 3. ALS Zone transitions with AutoGain. The higher X-axis is for increasing light intensity, while the lower axis is
for decreasing light intensity (Note 21)
Note 21: There are some limits in Zone transitions when the autogain is enabled, for example a direct transition from the lowest Zone0 to the highest Zone4 (and
vice versa) is not possible, because the device must go through the gain change process first.
Countdown Timer
The ALS engine includes a pre-defined countdown timer function. This function is targeted to applications where it's favorable to only increase through the zones; i.e., the LM3537 will
stick to the highest zone reached, but won't allow transitions
to lower Zones until the countdown has completed. At the end
of every countdown, the timer sets the countdown timer flag
(reg 40H), and after that, any Zone transition to a lower Zone
re-loads the timer and starts the next timer period. See Table
3 and Figure 4 for details.
TABLE 3. Countdown Timer
Pre-defined Countdown Timer Function
TIMER[1]
TIMER[0]
0
0
Countdown timer is disabled
0
1
10s countdown timer is enabled
(stick to the highest zone for 10s).
1
0
Always stick to the highest zone
the ALS reached.
1
1
Always stick to the highest zone
the ALS reached.
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Timer Function
14
LM3537
30108315
FIGURE 4. Countdown timer principle. Solid line shows the ALS operation when the timer is disabled. Dashed line shows
the operation when the 10s timer is enabled. Dotted line shows the operation when the device sticks to the highest
zone.
PWM Input
A PWM (Pulse Width Modulation) pin is provided on the
LM3537 to allow a display driver utilizing dynamic backlight
control (DBC), to adjust the LED brightness based on the
content. The PWM input can be turned on or off (Acknowledge
or Ignore) and the polarity can be flipped (active high or active
low) through the I2C interface. The current sinks of the
LM3537 require approximately 15 µs to reach steady-state
target current. This turn-on time sets the minimum usable
PWM pulse width for DBC. The external PWM input is effective for group A LEDs only.
Configurable Gain Transition Delay
To optimize efficiency, the LM3537 has a user-selectable gain
transition delay that allows the part to ignore short duration
input voltage drops. By default, the LM3537 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 LM3537.
Hardware Enable (HWEN)
The LM3537 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.
LED Forward Voltage Monitoring
The LM3537 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 LM3537 will operate in pass mode, allowing the VOUT
voltage to track the input voltage. As the input voltage drops,
the voltage on the Dx pins will also drop (VDX = VVOUT –
VLEDx). Once any of the active Dx pins reaches a voltage approximately equal to 150 mV, 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 LM3537 will remain in the gain of 3/2 until an
I2C write to the part occurs. At that time, the LM3537 will
re-evaluate the LED conditions and select the appropriate gain.
Only active Dx pins will be monitored.
Low Dropout Voltage Regulators
The four low dropout voltage regulators are designed to operate with small-size ceramic input and output capacitors.
They can operate with power rail voltages down to 1.8V. The
LDOs 2, 3 and 4 offer a typical dropout voltage of 100 mV at
150 mA output current. The single, higher-current LDO 1 offers a typical dropout voltage of 220 mV at 300mA output
current. The LDOs are enabled by the EN_LDO1, EN_LDO2,
EN_LDO3 and EN_LDO4 bits (see Table 5 for details). summarizes the supported output voltages. At startup, the LDOs
are off but are preset to 1.8V (for LDO2 and LDO3) and 2.8V
(for LDO1 and LDO4).
15
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LM3537
TABLE 4. Regulator Voltage Options
LDOX_VOUT[4]
LDOX_VOUT[3]
LDOX_VOUT[2]
LDOX_VOUT[1]
LDOX_VOUT[0]
Output Voltage
(typ.)
1
1
1
1
1
3.30V
1
1
1
1
0
3.20V
1
1
1
0
1
3.10V
1
1
1
0
0
3.00V
1
1
0
1
1
2.95V
1
1
0
1
0
2.90V
1
1
0
0
1
2.85V
1
1
0
0
0
2.80V
1
0
1
1
1
2.75V
1
0
1
1
0
2.70V
1
0
1
0
1
2.65V
1
0
1
0
0
2.60V
1
0
0
1
1
2.55V
1
0
0
1
0
2.50V
1
0
0
0
1
2.40V
1
0
0
0
0
2.20V
0
1
1
1
1
2.00V
0
1
1
1
0
1.90V
0
1
1
0
1
1.85V
0
1
1
0
0
1.80V
0
1
0
1
1
1.75V
0
1
0
1
0
1.70V
0
1
0
0
1
1.65V
0
1
0
0
0
1.60V
0
0
1
1
1
1.55V
0
0
1
1
0
1.50V
0
0
1
0
1
1.45V
0
0
1
0
0
1.40V
0
0
0
1
1
1.35V
0
0
0
1
0
1.30V
0
0
0
0
1
1.25V
0
0
0
0
0
1.20V
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16
should be equal to VIN_A, and at least 0.3V above the output
voltage of the regulators.
30108314
FIGURE 5. LDO Block Diagram. VIN_B supplies internal circuitry.
VIN_C, the power input voltage, is regulated to the fixed output voltage.
and STOP conditions. The I2C bus is considered busy after a
START condition and free after a STOP condition. During data transmission, the I2C master can generate repeated
START conditions. A START and a repeated START conditions are equivalent function-wise. The data on SDA must be
stable during the HIGH period of the clock signal (SCL). In
other words, the state of SDA can only be changed when SCL
is LOW.
I2C-Compatible Interface
STOP AND START CONDITIONS
The LM3537 is controlled via an I2C-compatible interface.
START and STOP ) conditions classify the beginning and the
end of the I2C session. A START condition is defined as SDA
transitioning from HIGH to LOW while SCL is HIGH. A STOP
condition is defined as SDA transitioning from LOW to HIGH
while SCL is HIGH. The I2C master always generates START
30108356
FIGURE 6. Start and Stop Sequences
17
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LM3537
The power input voltage applied between VIN_C and GND
should be at least 0.3V above the output voltage of the regulators. The bias input voltage applied between VIN_B and GND
LM3537
WRITE and R/W = 1 indicates a READ. The second byte following the chip address selects the register address to which
the data will be written. The third byte contains the data for
the selected register.
I2C-COMPATIBLE CHIP ADDRESS
The chip address for the LM3537 is 0111000 (38h). After the
START condition, the I2C master sends the 7-bit chip address
followed by a read or write bit (R/W). R/W= 0 indicates a
30108357
FIGURE 7. Chip Address
pulse. The LM3537 pulls down SDA during the 9th clock
pulse, signifying an acknowledge. An acknowledge is generated after each byte has been received. Figure 8 is an example of a write sequence to the DIODE ENABLE register of the
LM3537.
TRANSFERRING DATA
Every byte on the SDA line must be eight bits long, with the
most significant bit (MSB) transferred first. Each byte of data
must be followed by an acknowledge bit (ACK). The acknowledge related clock pulse (9th clock pulse) is generated by the
master. The master releases SDA (HIGH) during the 9th clock
30108358
FIGURE 8. Write Sequence to the LM3537
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18
LM3537
Internal Registers of LM3537
The LM3537 is controlled by a set of registers through the
two-wire serial interface port. Table 5 below lists device registers and their addresses together with a short description.
TABLE 5. Control Register Map
Hex
Addr.
00
10
Register Name
Bit(s)
Read/
Write
Default Value
After Reset
Bit Mnemonic and Description
MASTER ENABLE [2]
R/W
xxxxx0xx
group_A_en
Master enable for all the LEDs, which are assigned to group A. '1'
= LEDs ON '0' = LEDs OFF.
[1]
R/W
xxxxxx0x
group_B_en
Master enable for all the LEDs, which are assigned to group B. '1'
= LEDs ON '0' = LEDs OFF.
[0]
W
xxxxxxx0
softw_rst
Writing = '1' to this register bit resets all the registers to factory
defaults. After writing, this bit is forced back to '0' automatically.
[7]
R/W
0xxxxxxx
enD8
ON/OFF Control for D8 output
[6]
R/W
x0xxxxxx
enD7
ON/OFF Control for D7 output
[5]
R/W
xx0xxxxx
enD6
ON/OFF Control for D6 output
[4]
R/W
xxx0xxxx
enD5
ON/OFF Control for D5 output
[3]
R/W
xxxx0xxx
enD4
ON/OFF Control for D4 output
[2]
R/W
xxxxx0xx
enD3
ON/OFF Control for D3 output
[1]
R/W
xxxxxx0x
enD2
ON/OFF Control for D2 output
[0]
R/W
xxxxxxx0
enD1
ON/OFF Control for D1 output
DIODE ENABLE
19
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LM3537
Hex
Addr.
20
30
40
Register Name
Bit(s)
Read/
Write
Default Value
After Reset
Bit Mnemonic and Description
CONFIGURATION [7]
R/W
0xxxxxxx
D7_int
Enables the Interrupt Pin. 1 = interrupt output enabled. 0 = interrupt
output disabled, LED driver operation. Reading the 0x40 register
clears the interrupt.
[6]
R/W
x0xxxxxx
lin
Selects between linear and exponential dimming curve. Effective
for Group A only. 1 = linear dimming curve. 0 = exponential
dimming curve.
[5]
R/W
xx1xxxxx
D8_A
Assign D8 diode to Group A Writing a '1' assigns D8 to BankA
(default) and a '0' assigns D8 to Group B.
[4]
R/W
xxx1xxxx
D7_A
Assign D7 diode to Group A Writing a '1' assigns D7 to BankA
(default) and a '0' assigns D7 to Group B.
[3]
R/W
xxxx1xxx
D6_A
Assign D6 diode to Group A Writing a '1' assigns D6 to BankA
(default) and a '0' assigns D6 to Group B.
[2]
R/W
xxxxx1xx
D5_A
Assign D5 diode toGroup A . Writing a '1' assigns D5 to BankA
(default) and a '0' assigns D5 to Group B.
[1]
R/W
xxxxxx0x
pwm_p
PWM input polarity. Writing a '0' = active high (default) and a '1' =
active low.
[0]
R/W
xxxxxxx0
pwm_en
PWM input enable. Writing a '1' = Enable, and a '0' = Ignore
(default).
[7:6]
R/W
00xxxxxx
gt
Charge pump gain transition filter. The value stored in this register
determines the filter time used to make a gain transition in the
event of an input line VIN_A step. Filter Times (typ.) = ‘00’ = 3-6ms,
‘01’ = 0.8-1.5ms, ‘10’ = 20µs, '11' = 1µs,
[5:3]
R/W
xx000xxx
rd
Diode current ramp down step time: ‘000’ = 6µs, ‘001’ = 0.77ms,
‘010’ = 1.5ms, ‘011’ = 3ms, ‘100’ = 6ms, ‘101’ = 12ms, ‘110’ =
25ms, ‘111’ = 50ms
[2:0]
R/W
xxxxx000
ru
Diode current ramp up step time : ‘000’ = 6µs, ‘001’ = 0.77ms, ‘010’
= 1.5ms, ‘011’ = 3ms, ‘100’ = 6ms, ‘101’ = 12ms, ‘110’ = 25ms,
‘111’ = 50ms
[7:6]
R
00xxxxxx
rev
Stores the silicon revision value. LM3537 = '00'
[5]
R
xx0xxxxx
als_gain
Gain_status indicator: '1' = high gain, '0' = low gain.
[4]
R
xxx0xxxx
timerflag
At the end of every countdown, the timer sets the timerflag ='1'.
The flag bit is cleared once the 0x40 register has been read.
[3]
R
xxxx0xxx
zoneflag
ALS transition flag. '1' = Transition has occurred. '0' = No transition.
The flag bit is cleared once the 0x40 register has been read.
[2:0]
R
xxxxx000
zone
ALS Zone information: '000’ = Zone0, ‘001’ = Zone1, ‘010’ =
Zone2, ‘011’ = Zone3, ‘100’ = Zone4. Other combinations not
used.
OPTIONS
ALS ZONE
READBACK
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20
50
51
Register Name
ALS CONTROL
ALS RESISTOR
Bit(s)
Read/
Write
Default Value
After Reset
Bit Mnemonic and Description
[7:5]
R/W
000xxxxx
ave
Sets averaging time for the ALS sampling. Need two to three
averaging periods to make transition decision.‘000’ = 25ms, ‘001’
= 50ms, ‘010’ = 100ms, ‘011’ = 200ms, ‘100’ = 400ms, ‘101’ =
800ms, ‘110’ = 1.6s, ‘111’ = 3.2s.
[4:3]
R/W
xxx00xxx
timer
Pre-defined countdown timer function.
'00' = countdown timer is disabled
'01' = 10s countdown timer is enabled (stick to the highest zone
for 10s)
'10' = Always stick to the highest zone the ALS reached
'11' = Always stick to the highest zone the ALS reached.
At the end of every countdown, the timer sets the countdown
timerflag (reg 40H), and after that, a Zone transition to a lower
Zone re-loads the timer and starts the next timer period.
[2]
R/W
xxxxx0xx
als_en
Enables ALS monitoring. Writing a '1' enables the ALS monitoring
circuitry and a '0' disables it. This feature can be enabled without
having the current sinks or charge pump active. The ALS value is
updated in register 0x40 ALS ZONE READBACK.
[1]
R/W
xxxxxx0x
als_en_a
Enable ALS on Group A. Writing a '1' enables ALS control of diode
current and a '0' (default) forces the Group A current to the value
stored in the Group A brightness register. The als_en bit must be
set to a '1' for the ALS block to control the Group A brightness.
[0]
R/W
xxxxxxx0
als_en_b
Enable ALS on Group B. Writing a '1' enables ALS control of diode
current and a '0' (default) forces the Group B current to the value
stored in the Group B brightness register. The als_en bit must be
set to a '1' for the ALS block to control the Group B brightness. The
ALS function for Group B is different than Group A in that the ALS
will only enable and disable the Group B diodes depending on the
ALS zone chosen by the user. Group A utilizes the 5 different zone
brightness registers (Addresses 0x70 to 0x74).
[4:0]
R/W
xxx00010
r_als
Sets the internal ALS resistor value. See Table 1for details.
21
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LM3537
Hex
Addr.
LM3537
Hex
Addr.
52
Register Name
ALS CONFIG
Bit(s)
Read/
Write
Default Value
After Reset
Bit Mnemonic and Description
[7]
R/W
0xxxxxxx
autogain_en
'1' = Enables autogain for the external ambient light sensor.
'0' = disables autogain and GPO's are controlled by the gpo1 and
gpo2 -bits. See Table 2 for details.
[6]
R/W
x0xxxxxx
sbias_en
'1' = External sensor power output enabled.
'0' = External sensor power output disbaled.
Note: '1' -> GPOs will behave as push-pull CMOS outputs
referenced to voltage on SBIAS. '0 '-> GPOs will act as open-drain
outputs (default).
[5]
R/W
xx0xxxxx
sbias_volt
Sensor bias output voltage selection.
'1' = 3.0V output voltage.
'0' = 2.4V output voltage.
[3]
R/W
xxxx0xxx
cp_en
Writing = '1' to this register bit enables the Charge-Pump block.
Forces the LM3537 to operate in the gain of 1.5x. This mode
DOES NOT require the Dx current sinks to be enabled for
operation.
[2]
R/W
xxxxx0xx
pass_en
Writing = '1' to this register bit forces the LM3537 to operate in the
gain of 1x (pass-mode). This mode DOES NOT require the Dx
current sinks to be enabled for operation. Note: 1.5x gain (cp_en
bit) has a higher priority.
[1]
R/W
xxxxxx0x
gpo1
'0' = GPO1 pin state is low. '1' = GPO1 pin state is high. Effective
only when the autogain is disabled.
(Note 19)
[0]
R/W
xxxxxxx0
gpo2
'0' = GPO2 pin state is low. '1' = GPO2 pin state is high. Effective
only when the autogain is disabled.
(Note 19)
60
ALS ZONE
BOUNDARY#0
[7:0]
R/W
00110011
zb0
Sets Zone0 to Zone1 transition trip point
61
ALS ZONE
BOUNDARY#1
[7:0]
R/W
01100110
zb1
Sets Zone1 to Zone2 transition trip point
62
ALS ZONE
BOUNDARY#2
[7:0]
R/W
10011001
zb2
Sets Zone2 to Zone3 transition trip point
63
ALS ZONE
BOUNDARY#3
[7:0]
R/W
11001100
zb3
Sets Zone3 to Zone4 transition trip point
64
ALS LOW to HIGH [7:0]
TP
R/W
00001011
LtoH
Sets the trip point for low gain to high gain transition. Effective only
when autogain = '1'.
65
ALS ZONE
BOUNDARY Z3 to
Z2
[7:0]
R/W
00010000
zb3to2
Zone3 to Zone2 transition trip point when the autogain is enabled.
70
ALS BRIGHTNESS [6:0]
ZONE#0
R/W
x0111100
z0b
Sets the Zone Brightness code for Zone0.
71
ALS BRIGHTNESS [6:0]
ZONE#1
R/W
x1001101
z1b
Sets the Zone Brightness code for Zone1.
72
ALS BRIGHTNESS [6:0]
ZONE#2
R/W
x1011001
z2b
Sets the Zone Brightness code for Zone2.
73
ALS BRIGHTNESS [6:0]
ZONE#3
R/W
x1100110
z3b
Sets the Zone Brightness code for Zone3.
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22
Register Name
Bit(s)
Read/
Write
Default Value
After Reset
LM3537
Hex
Addr.
Bit Mnemonic and Description
74
ALS BRIGHTNESS [6:0]
ZONE#4
R/W
x1110010
z4b
Sets the Zone Brightness code for Zone4.
A0
GROUP A
BRIGHTNESS
[6:0]
R/W
x0000000
dxa
Sets Brightness for Group A. 128 steps, 1111111=Fullscale.
B0
GROUP B
BRIGHTNESS
[5:3]
R/W
xx000xxx
alsZT
Sets the Brightness Zone boundary used to enable and disable
Group B diodes based upon ambient lighting conditions.
[2:0]
R/W
xxxxx000
dxb
Sets Brightness for Group B. 8 steps, 111 = Fullscale.
[3]
R/W
xxxx0xxx
en_ldo4
'1' = Regulator 4 enabled.
'0' = Regulator 4 disbaled.
[2]
R/W
xxxxx0xx
en_ldo3
'1' = Regulator 3 enabled.
'0' = Regulator 3 disbaled.
[1]
R/W
xxxxxx0x
en_ldo2
'1' = Regulator 2 enabled.
'0' = Regulator 2 disbaled.
[0]
R/W
xxxxxxx0
en_ldo1
'1' = Regulator 1 enabled.
'0' = Regulator 1 disbaled.
C0
LDO ENABLE
C1
LDO1 VOUT
[4:0]
R/W
xxx11000
ldo1_vout
Regulator 1 output voltage programming. See Table 4 for voltage
options.
C2
LDO2 VOUT
[4:0]
R/W
xxx01100
ldo2_vout
Regulator 2 output voltage programming.
C3
LDO3 VOUT
[4:0]
R/W
xxx01100
ldo3_vout
Regulator 3 output voltage programming.
C4
LDO4 VOUT
[4:0]
R/W
xxx11000
ldo4_vout
Regulator 4 output voltage programming.
23
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LM3537
dimming curve current can be approximated by the following
equation (where N = the decimal value stored in the Group A
Brightness register):
Current Control Registers
A0 GROUP A BRIGHTNESS
This is the LED driver current control register for Group A. The
register is effective when the ALS isn't used. The resolution
is 7 bits, so in linear dimming mode the step size from zero
up to full brightness is fixed (25.0mA/127) = 197 µA. Exponential dimming scheme provides a more fine-grained level
of control over low level LED currents. Group A exponential
30108304
Current vs. code is shown below.
30108351
FIGURE 9. LED current (typ.) vs. register code, exponential dimming curve
B0 GROUP B BRIGHTNESS
Bits [2:0] set the GroupB Brightness Levels, as shown in below:
TABLE 6. Group B Brightness Levels
dxb[2]
dxb[1]
dxb[0]
GroupB LED Current (typ.)
1
1
1
25.0 mA
1
1
0
17.5 mA
1
0
1
15.0 mA
1
0
0
12.5 mA
0
1
0
10.0 mA
0
1
0
7.5 mA
0
0
1
5.0 mA
0
0
0
2.5 mA
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24
LED CONFIGURATIONS
The LM3537 has a total of 8 current sinks capable of sinking
180mA of total diode current. These 8 current sinks are configured to operate in one or two independently controlled
lighting regions. GroupA has eight dedicated current sinks,
while GroupB has 0 by default. However, drivers D5 to D8 can
be assigned to either GroupA or GroupB one-by-one through
a setting in the configuration register. With this added flexibility, the LM3537 is capable of supporting applications requiring from 4 to 7 LEDs for main display lighting, while still
providing additional current sink(s) that can be used for a wide
variety of lighting functions.
CAPACITOR SELECTION
The LM3537 circuit requires 11 external capacitors for proper
operation. Surface-mount multi-layer ceramic capacitors are
recommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR <20 mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use
with the LM3537 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
LM3537. 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 LM3537. 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 LM3537.
Table 7 below lists recommended external capacitors from
some leading ceramic capacitor manufacturers. It is strongly
recommended that the LM3537 circuit be thoroughly evaluated early in the design-in process with the mass-production
capacitors of choice. This will help ensure that any variability
in capacitance does not negatively impact circuit performance.
PARALLEL CONNECTED AND UNUSED OUTPUTS
Connecting the outputs in parallel does not affect internal operation of the LM3537 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.
If some of the drivers are not going to be used, make sure that
the enable bits in the DIODE ENABLE register are set to '0'
to ensure optimal efficiency.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM3537
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
TABLE 7. Suggested Capacitors
Model
Type
Vendor
Voltage Rating
Package Size
1 µF for COUT , CLDO1, CLDO2, CLDO3, CLDO4, CSEN, C1, C2 and CIN_A (Note 22)
C1005X5R1A105K
Ceramic X5R
TDK
10V
0402
LMK105BJ105KV-F
Ceramic X5R
Taiyo Yuden
10V
0402
GRM155R61A105K
Ceramic X5R
Murata
10V
0402
GRM155R61A104K
Ceramic X5R
Murata
10V
0402
LMK105BJ104KV-F
Ceramic X5R
Taiyo Yuden
10V
0402
C1005X5R1A104K
Ceramic X5R
TDK
10V
0402
JMK105BJ225MV-F
Ceramic X5R
Taiyo Yuden
6.3V
0402
GRM155R60J225ME15D
Ceramic X5R
Murata
6.3V
0402
0.1 µF for CIN_B (Note 22)
2.2 µF for CIN_C
Note 22: The recommended voltage rating for these capacitors is 10V to account for DC bias capacitance losses.
25
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LM3537
important that the board layout provide good thermal conduction to keep the junction temperature within the specified
operating ratings.
Application Information
LM3537
Physical Dimensions inches (millimeters) unless otherwise noted
The dimension for X1, X2 and X3 are as given:
— X1 = 2,015mm ±0.030 mm
— X2 = 2,517mm ±0.030 mm
— X3 = 0.600 mm ±0.075 mm
TMD30: 30-Bump micro SMD
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26
LM3537
Notes
27
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LM3537 8-Channel WLED Driver with Four Integrated LDOs
Notes
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