LM3537
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SNVS634B – JUNE 2011 – REVISED MAY 2013
LM3537 8-Channel WLED Driver with Four Integrated LDOs
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FEATURES
1
•
•
Lighting:
2
•
•
•
•
•
•
•
•
•
•
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
Ggains 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
DESCRIPTION
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.
Typical Application Circuit
C1
1 PF
C2
1 PF
C1+ C1- C2+ C2VIN = 2.7 to 5.5V
VIN_A
CIN_A
1 PF
D2
VIN_B
GROUP A
CIN_B
COUT
1 PF
VOUT
D1
D3
100 nF
D4
VIN_C
CIN_C
2.2 PF
SCL
D6
SDA
D7/
INT
LM3537
HWEN
GROUP B
MCU
D5
D8
+
-
AMBIENT
LIGHT
SENSOR
CSEN
1 PF
PWM
LDO1
SBIAS
LDO2
GPO1
LDO3
CLDO1 1 PF
CLDO2 1 PF
CLDO3 1 PF
GPO2
ALS
LDO4
GNDs
CLDO4 1 PF
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2011–2013, Texas Instruments Incorporated
LM3537
SNVS634B – JUNE 2011 – REVISED MAY 2013
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DESCRIPTION (CONTINUED)
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 DSBGA package.
Connection Diagram
1
2
3
4
5
A
HWEN
PGND
C1-
C2+
C1+
B
SCL
PWM
SDA
C2-
VOUT
C
D2
D1
D8
D7/
INT
VIN_A
D
D3
D4
D5
D6
ALS
E
LDO2
GPO2
GPO1
SBIAS
VIN_B
F
LDO1
LDO4
GND
LDO3
VIN_C
Figure 1. 30–Bump DSBGA Package
Top View
2
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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.
F1
LDO1
Programmable VOUT of 1.2-3.3 V. Max load = 300 mA.
C3
D8
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
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.
Charge pump output
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
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LM3537
SNVS634B – JUNE 2011 – REVISED MAY 2013
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ABSOLUTE MAXIMUM RATINGS (1) (2) (3)
VIN_A, VIN_B , VIN_C pin voltage
-0.3V to 6.0V
Voltage on Logic Pins (SCL, SDA, GPO1, GPO2, HWEN, PWM)
LED driver (D1 to D8) Pin Voltages
-0.3V to (VOUT+0.3V) with 6.0V max
Voltage on All Other Pins
-0.3V to (VIN_A +0.3V) with 6.0V max
Continuous Power Dissipation (4)
Internally Limited
Junction Temperature (TJ-MAX)
150°C
Storage Temperature Range
-40°C to +150°C
ESD Rating (5) Human Body Model
(1)
(2)
(3)
(4)
(5)
-0.3V to (VIN_A+0.3V) with 6.0V max
2 kV
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is specified. Operating Ratings do not imply specified performance limits. For specified performance limits
and associated test conditions, see the Electrical Characteristics tables.
All voltages are with respect to the potential at the GND pins.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
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.).
The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. (MIL-STD-883 3015.7)
OPERATING RATINGS (1) (2)
VIN_A, VIN_B Input Voltage Range
2.7V to 5.5V
LED Voltage Range
2.0V to 4.0V
VIN_C Input Voltage Range (Note: must stay > VOUTLDO + 0.3V)
1.8V to VIN_B
−30°C to +110°C
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range (3)
(1)
(2)
(3)
−30°C to +85°C
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is specified. Operating Ratings do not imply specified performance limits. For specified performance limits
and associated test conditions, see the Electrical Characteristics tables.
All voltages are with respect to the potential at the GND pins.
In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP =
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).
THERMAL PROPERTIES (1)
Junction-to-Ambient Thermal Resistance (θJA), YFQ Package (2)
(1)
(2)
4
45°C/W
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
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 Texas
Instruments' Application Note AN-1112: DSBGA Wafer Level Chip Scale Package (Literature Number SNVA009).
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SNVS634B – JUNE 2011 – REVISED MAY 2013
CHARGE PUMP AND LED DRIVERS ELECTRICAL CHARACTERISTICS (1) (2)
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. (3)
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 (4)
3.2V ≤ VIN_A ≤ 5.5V
VLED = 3.6V
BankA current code = 1111101b, exp dimming
scale
LED Current Matching (5)
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 (6)
IDx = 95% ×IDx (nom.)
(IDx (nom) ≈ 20 mA)
100
mV
ROUT
Open-Loop Charge Pump Output
Resistance (7)
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
1.2
IQ
Quiescent Supply Current
ISB
Standby Supply Current
HWEN = 1.8V. All registers in factory defaults
state. Current through VIN_A pin.
ISD
Shutdown Supply Current
HWEN = 0V. Current through VIN_A pin.
fSW
Switching Frequency
tSTART
Startup Time
VALS
ALS Reference Voltage
RALS
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Internal ALS Resistor
Ω
mA
1.1
See (8)
µA
0.2
1.0
µA
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%
V
kΩ
All voltages are with respect to the potential at the GND pins.
Min and Max limits are specified by design, test, or statistical analysis. Typical numbers are not ensured, but do represent the most
likely norm.
CIN_X, COUT, CLDOX, CSEN, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.
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.
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.
For each Dxpin, 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.
Specified by design.
Turn-on time is measured from the moment the charge pump is activated until the VOUT crosses 90% of its target value.
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LOGIC INTERFACE CHARACTERISTICS (1) (2)
Symbol
Parameter
Condition
Min
Typ
Max
Units
I2C-Compatible Interface Timing Specifications (SCL, SDA) (3)
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)
See (4)
2.5
µs
100
ns
0
ns
100
ns
100
ns
2
I C-Compatible Interface Voltage Specifications (SCL, SDA)
VIL
Input Logic Low "0"
2.7V ≤ VIN_A ≤ 5.5V
0
0.45
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
V
Logic inputs HWEN and PWM
VHWEN
HWEN Voltage Thresholds
2.7V ≤ VIN_A ≤ 5.5V
VPWM
PWM Voltage Thresholds
2.7V ≤ VIN_A ≤ 5.5V
Reset
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 (5)
VOL
VOH
(1)
(2)
(3)
(4)
(5)
6
Output Low Level
Output High Level
IOUT = 3 mA
IOUT = −2 mA
0.3
VOUT_S
−0.5
VOUT_S
–0.3
V
All voltages are with respect to the potential at the GND pins.
Min and Max limits are specified by design, test, or statistical analysis. Typical numbers are not ensured, but do represent the most
likely norm.
SCL and SDA should be glitch-free in order for proper device control to be realized. See Figure 2 for timing specification details.
SCL is tested with a 50% duty-cycle clock.
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.
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VOLTAGE REGULATORS ELECTRICAL CHARACTERISTICS (1) (2)
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). (3)
Symbol
Parameter
Condition
Min
Typ
Max
Units
LDO1
VOUT
Output Voltage Accuracy
IOUTLDO = 1 mA, VOUTLDO = 2.80V
−2
+2
−3
+3
Default Output Voltage
IOUT
VDO
ΔVOUT
PSRR
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
Power Supply Ripple Rejection Ratio
f = 100Hz,
CLDO1 = 1 µF,
IOUTLDO = 20 mA
Output Voltage = 1.20V
65
%
V
300
mA
mA
300
mV
mV
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
Power Supply Ripple Rejection Ratio
f = 100 Hz,
CLDOX = 1µF,
IOUTLDO = 20 mA
Output Voltage = 1.20V
65
%
V
V
150
mA
mA
200
mV
mV
dB
LDO Combined Common Electrical Characteristics
IGND
tSTARTUP
TTransient
(1)
(2)
(3)
(4)
Ground Pin Current (GND and PGNDpin)
Turn-on Time from Shut-down (4)
Startup Transient Overshoot
Note: IOUTLDOX = 0mA
All LDOs Disabled
0.2
1
One LDO Enabled
70
130
Two LDOs Enabled
100
Three LDOs Enabled
130
Four LDOs Enabled
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
CLDOX = 1 µF, IOUTLDO = 150 mA
µA
µA
µs
30
mV
All voltages are with respect to the potential at the GND pins.
Min and Max limits are specified by design, test, or statistical analysis. Typical numbers are not ensured, but do represent the most
likely norm.
CIN_C, CLDOX : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.
Time needed for VOUTLDO to reach 95% of final value.
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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
(1)
(2)
SBIAS Output Current
SBIAS Output Voltage
Sensor Interface Quiescent Supply
Current (1) (2)
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%
2.4
+5%
3.3V ≤ VIN_A ≤ 5.5V. IOUT_S = 1.0 mA. 3.0V
option selected via register.
−5%
3.0
+5%
No Load
V
35
µA
In addition to Quiescent Supply Current (IQ) drawn by the charge pump. (See Charge Pump and LED Drivers Electrical Characteristics.)
Specified by design.
Figure 2. Timing Parameters
8
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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
Power Supply Rejection Ratio, VOUT = 1.20V, ILOAD = 20 mA
VIN_C is shorted to VIN_A, VIN_B
Power Supply Rejection Ratio, VOUT = 1.20V, ILOAD = 20 mA
Signal Applied on VIN_C, VIN_A and VIN_B Clear.
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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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
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.
10
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
Regulator 2,3,4 Short Circuit Current
VOUT setting = 1.80V
Regulator 1 Short Circuit Current
VOUT setting = 2.80V
Shutdown Supply Current
HWEN = 0V. Current through VIN_A pin
Standby Supply Current
HWEN = 1.8V. Current through VIN_A pin
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
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.
(1)
Quiescent Current
vs
Input Voltage
1× Gain
Quiescent Current
vs
Input Voltage 3/2× Gain
3/2× Gain
LED Current Matching Distribution.
6 Drivers on Group A, Output Set to 25 mA. (1)
Charge Pump 1.5x Efficiency
vs
Load Current
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.
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BLOCK DIAGRAM
1 PF
VIN
C1+
1 PF
C1-
C2+
C2VOUT
VIN_A
2.7V to 5.5V
COUT
1 PF
CHARGE PUMP
1X/1.5X
CIN
SOFTSTART
1 PF
1.3 MHz
OSC
GAIN
CONTROL
D1
VREF
1.25V
GROUP B
BRIGHTNESS
CTRL = 8 STEPS
SERIAL
DATA
D3
CURRENT SINKS
SCL
SDA
D2
REGISTERS
POR
HWEN
GROUP A
BRIGHTNESS
CTRL = 128 STEPS
D4
D5
D6
CONTROL
D7/
INT
PWM
D8
PWM
SBIAS
2.4V or
3.0V
SENSOR
POWER
THERMAL
SHUTDOWN
CSEN
1 PF
GPO1
+
-
AMBIENT
LIGHT
SENSOR
INT
GPO2
LDO1
ALS ENGINE
ALS
LDO 1
CLDO1
1 PF
LDO2
LDO 2
CLDO2
1 PF
LDO3
LDO 3
CLDO3
1 PF
RALS
VIN_B
VOLTAGE REFERENCE
WITH NOISE
SUPPRESSION FILTER
CIN_B
100 nF
VIN_C
LDO4
CIN_C
LDO 4
2.2 PF
CLDO4
1 PF
GNDs
12
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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 smallformat display.
Each LED is configured in a common anode configuration, with the peak drive current set to 25 mA. An I2Ccompatible 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 300-mA 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.
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.
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 '0' alerting the controller. Available resistor values are shown in
Table 1 below.
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Table 1. ALS Resistor Values
r_als[4]
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.
Table 2. Sensor Gain Control
REGISTER SETTING
OUTPUT PIN STATUS
GPO1
GPO2
Can be set to "1" or "0" with REG 52H, bit
gpo1
Can be set to "1" or "0" with REG 52H, bit
gpo2
autogain_en = "1" (enables autogain
functionality) LOW GAIN
0
1
autogain_en = "1" (enables autogain
functionality) HIGH GAIN
1
0
autogain_en = "0"
14
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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 compared 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 7-bit 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 Figure 3 below. Here, the
exponential dimming scheme is used.
Vals_ref
= 1V
Full
Scale
Zone 4
ZB3
ZB1
LED Current
Vsense
Zone 3
ZB2
Zone 2
Zone 1
ZB0
Zone 0
Z0T
Ambient Light (lux)
Z1T
Z2T
Z3T
Z4T
LED Driver Input Code (0-127)
Figure 3. ALS Zone to LED Brightness Mapping
ALS Zone transitions with AutoGain is shown in Figure 4. 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. High-gain 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.
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Z0
VSENSE (mV)
Z3
Z4
}
}
}
1000
t2-3
Z2
Z1
}
}
LIGHT INTENSITY INCREASES
HIGH-to-LOW gain
transition
750
t1-2
500
t0-1
250
t3-4
Z1
Z2
}
Z0
}
}
}
}
t3-2
0
Z3
LOW-to-HIGH gain
transition
LOW INTENSITY
Z4
LIGHT INTENSITY DECREASES
HIGH INTENSITY
The higher X-axis is for increasing light intensity, while the lower axis is for decreasing light intensity
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.
Figure 4. ALS Zone Transitions with AutoGain
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 5 for details.
Table 3. Countdown Timer
Pre-defined Countdown Timer Function
16
TIMER[1]
TIMER[0]
Timer Function
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 PERIOD
STARTED
ZONE4
ZONE3
TIMER PERIOD TIMER PERIOD
STARTED
STARTED
ZONE2
TIMER PERIOD
STARTED
ZONE1
THE END OF THE
COUNTDOWN PERIOD
ZONE0
5
10
20
15
25
30
35
40
45
THE END OF THE
COUNTDOWN
PERIOD
50
55
60
ELAPSED TIME (s)
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.
Figure 5. Countdown Timer Principle
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.
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.
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.
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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).
Table 4. Regulator Voltage Options
18
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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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 should be equal to VIN_A, and at least 0.3V
above the output voltage of the regulators.
VIN_C
PASS
ELEMENT
NOISE
SUPPRESSION
+
-
VIN_B
REGULATED
OUTPUT
VREF
VOLTAGE
CONTROL
VIN_B supplies internal circuitry. VIN_C, the power input voltage, is regulated to the fixed output voltage.
Figure 6. LDO Block Diagram
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 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.
Figure 7. Start and Stop Sequences
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 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.
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MSB
0
Bit 7
LSB
1
Bit 6
1
Bit 5
1
Bit 4
0
Bit 3
0
Bit 2
0
Bit 1
R/W
Bit 0
Serial Bus Slave Address (chip address)
Figure 8. Chip Address
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 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 9 is an example of a write sequence to the DIODE ENABLE register of the LM3537.
Figure 9. Write Sequence to the LM3537
20
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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
20
Register Name
MASTER ENABLE
DIODE ENABLE
CONFIGURATION
Bit(s)
Read/W
rite
Default Value
After Reset
Bit Mnemonic and Description
[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
[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).
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Table 5. Control Register Map (continued)
Hex
Addr.
30
40
50
51
22
Register Name
OPTIONS
ALS ZONE
READBACK
ALS CONTROL
ALS RESISTOR
Bit(s)
Read/W
rite
Default Value
After Reset
Bit Mnemonic and Description
[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.
[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 1 for details.
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Table 5. Control Register Map (continued)
Hex
Addr.
52
(1)
Register Name
ALS CONFIG
Bit(s)
Read/W
rite
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. (1)
[0]
R/W
xxxxxxx0
gpo2
'0' = GPO2 pin state is low. '1' = GPO2 pin state is high. Effective
only when the autogain is disabled. (1)
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
TP
[7:0]
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
ZONE#0
[6:0]
R/W
x0111100
z0b
Sets the Zone Brightness code for Zone0.
71
ALS BRIGHTNESS
ZONE#1
[6:0]
R/W
x1001101
z1b
Sets the Zone Brightness code for Zone1.
72
ALS BRIGHTNESS
ZONE#2
[6:0]
R/W
x1011001
z2b
Sets the Zone Brightness code for Zone2.
73
ALS BRIGHTNESS
ZONE#3
[6:0]
R/W
x1100110
z3b
Sets the Zone Brightness code for Zone3.
74
ALS BRIGHTNESS
ZONE#4
[6:0]
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.
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.
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Table 5. Control Register Map (continued)
Hex
Addr.
B0
C0
24
Register Name
GROUP B
BRIGHTNESS
LDO ENABLE
Bit(s)
Read/W
rite
Default Value
After Reset
Bit Mnemonic and Description
[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.
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.
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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 dimming curve current can be approximated by the following equation (where N =
the decimal value stored in the Group A Brightness register):
ILED (mA) | 25 x 0.85
[44 ± {(N+1)/2.91}]
(1)
Current vs. code is shown below.
Figure 10. 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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LM3537
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APPLICATION INFORMATION
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.
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 tables 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 important that the board layout provide good thermal conduction to keep the junction
temperature within the specified operating ratings.
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
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