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MAX16826ATJ+
Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault DetectionEVALUATION KIT AVAILABLEEVALUATION KIT AVAILABLE
General DescriptionThe MAX16826 high-brightness LED (HB LED) driver is
designed for backlighting automotive LCD displays and
other display applications such as industrial or desktop
monitors and LCD televisions. The MAX16826 integrates
a switching regulator controller, a 4-channel linear cur-
rent sink driver, an analog-to-digital converter (ADC),
and an I2C interface. The IC is designed to withstand
automotive load dump transients up to 40V and can
operate under cold crank conditions.
The MAX16826 contains a current-mode PWM switching
regulator controller that regulates the output voltage to
the LED array. The switching regulator section is config-
urable as a boost or SEPIC converter and its switching
frequency is programmable from 100kHz to 1MHz.
The MAX16826 includes 4 channels of programmable,
fault-protected, constant-current sink driver controllers
that are able to drive all white, RGB, or RGB plus amber
LED configurations. LED dimming control for each chan-
nel is implemented by direct PWM signals for each of the
four linear current sinks. An internal ADC measures the
drain voltage of the external driver transistors and the
output of the switching regulator. These measurements
are then made available through the I2C interface to an
external microcontroller (µC) to enable output voltage
optimization and fault monitoring of the LEDs.
The amplitude of the LED current in each linear current-
sink channel and the switch-mode regulator output volt-
age is programmed using the I2C interface. Additional
features include: cycle-by-cycle current limit, shorted
LED string protection, and overtemperature protection.
The MAX16826 is available in a thermally enhanced,
5mm x 5mm, 32-pin thin QFN package and is specified
over the automotive -40°C to +125°C temperature range.
ApplicationsLCD Backlighting:
Automotive Infotainment Displays
Automotive Cluster Displays
Industrial and Desktop Monitors
LCD TVs
Automotive Lighting:
Adaptive Front Lighting
Low- and High-Beam Assemblies
Benefits and Features4 Channels of Programmable, Fault-Protected,
Constant-Current Source-Driver Controllers for All
White, RGB, or RGB Plus Amber LED ConfigurationsDrives One to Four LED StringsIndividual PWM Dimming Inputs per StringVery Wide Dimming Range
Configurable as a boost or SEPIC Converter with
Programmable 100kHz to 1MHz Switching Frequency
for Design FlexibilityIntegrated Boost/SEPIC ControllerExternal MOSFETs Allow Wide-Range LED
Current with Multiple LEDs per String200kHz to 2MHz Programmable Switching
Frequency for Optimizing Size vs. EfficiencyExternal Switching-Frequency Synchronization
I2C Allows the Current-Sink Drain Voltages to be
Read to Minimize Power Dissipation and Detect LED
String Fault Conditions7-Bit Internal ADC for LED Voltage Monitoring and
OptimizationDynamic Adjustment of LED String Currents and
Output VoltageStandby Mode
Protection Features Improve ReliabilityLED Short and Open DetectionOvervoltage and Overtemperature Protection
Typical Application Circuit and Pin Configuration appear at
end of data sheet.
Simplified DiagramMAX16826
DR4
DR1
DL1DL
BOOST LED DRIVERFB
GND
VIN
DIM1
DIM2
SDA
SCL
DIM3
DIM4
I2C
INTERFACE
DIMMING
INPUTS
CS1
DL4
CS4
Ordering Information appears at end of data sheet.
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
Absolute Maximum Ratings
Electrical Characteristics(VIN= 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_= 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values
are at TA= +25°C.) (Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
IN to GND (Continuous).........................................-0.3V to +30V
IN Peak Current (≤400ms)...............................................300mA
IN Continuous Current........................................................50mA
PGND to GND.......................................................-0.3V to +0.3V
All Other Pins to GND...............................................-0.3V to +6V
DL Peak Current (< 100ns)....................................................±3A
DL Continuous Current.....................................................±50mA
DL1, DL2, DL3, DL4 Peak Current ..................................±50mA
DL1, DL2, DL3, DL4 Continuous Current ........................±20mA
VCCContinuous Current.....................................................50mA
All Other Pins Current.......................................................±20mA
Continuous Power Dissipation (TA= +70°C)
32-Pin Thin QFN (derate 34.5mW/°C above +70°C)
Multilayer Board..........................................................2759mW
Operating Temperature Range.........................-40°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s)........…………………+300°C
Soldering Temperature (reflow).......................................+260°C
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSPower-Supply VoltageVINVSYNC = 3V4.7524V
Quiescent CurrentIINDL_ = unconnected; R19, C33 = open510mA
Shutdown CurrentIIN,SDVSYNC = 0V2075µA
Standby CurrentIIN,SBI2C standby activated3mA
I2C-COMPATIBLE I/O (SCL, SDA)Input High VoltageVIH1.5V
Input Low VoltageVIL0.5V
Input HysteresisVHYS25mV
Input High Leakage CurrentIIHVLOGIC = 5V-1+1µA
Input Low Leakage CurrentIILVLOGIC = 0V-1+1µA
Input CapacitanceCIN10pF
Output Low VoltageVOLIOL = 3mA0.4V
Output High CurrentIOHVOH = 5V1µA
2C-COMPATIBLE TIMINGSerial Clock (SCL) FrequencyfSCL400kHz
BUS Free Time Between STOP
and START ConditionstBUF1.3µs
START Condition Hold TimetHD:STA0.6µs
STOP Condition Setup TimetSU:STO0.6µs
Clock Low PeriodtLOW1.3µs
Clock High PeriodtHIGH0.6µs
Data Setup TimetSU:DAT0.3µs
Data In Hold TimetHD:DATIN0.030.9µs
Data Out Hold TimetHD:DATOUT0.3µs
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSMaximum Receive SCL/SDA Rise
TimetRCB = 400pF300ns
Minimum Receive SCL/SDA Rise
TimetRCB = 400pF60ns
Maximum Receive SCL/SDA Fall
TimetFCB = 400pF300ns
Minimum Receive SCL/SDA Fall
TimetFCB = 400pF60ns
Transmit SDA Fall TimetFCB = 400pF, IO = 3mA60250ns
Pulse Width of Suppressed SpiketSP50ns
INTERNAL REGULATORS (IN, VCC)VCC Output VoltageVVCC
0V < IVCC < 30mA (Note 2),
4.75V < VIN < 24V, DL, DL1 to DL4
unconnected
4.55.255.65V
VCC Undervoltage LockoutVVCC_UVLOVCC rising4.5V
VCC Undervoltage Lockout
HysteresisVVCC_HYS135175205mV
IN Shunt Regulation VoltageIIN = 250mA24.0526.027.5V
PWM GATE DRIVER (DL)Peak Source Current2A
Peak Sink Current2A
DL High-Side Driver ResistanceIDL = -100mA2.25Ω
DL Low-Side Driver ResistanceIDL = +100mA1.30Ω
Minimum DL Pulse Width40ns
PWM CONTROLLER, SOFT-START (FB, COMP, OVP)FB shorted to COMP; MAX16826 only1.2301.2501.260FB Voltage MaximumVFB,MAXFB shorted to COMP; MAX16826B only1.231.251.27V
FB shorted to COMP; MAX16826 only862876885FB Voltage MinimumVFB,MIN
FB shorted to COMP; MAX16826B only735750765
FB shorted to COMP; MAX16826 only2.94FB Voltage LSBFB shorted to COMP; MAX16826B only3.9mV
FB Input Bias CurrentIFB0V < VFB < 5.5V-1000+100nA
Feedback-Voltage Line
Regulation
Level to produce VCOMP = 1.25V,
4.5V < VVCC < 5.5V±0.25%/V
Soft-Start CurrentISSVCSS = 0.5VVCC3.26.010.4µA
OVP Input Bias CurrentIOVP0V < VOVP < 5.5V-1000+100nA
Slope CompensationISLOPE192632µA/µs
Electrical Characteristics (continued)(VIN= 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_= 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values
are at TA= +25°C.) (Note 1)
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
ERROR AMPLIFIER (FB, COMP)Open-Loop GainAOL80dB
Unity-Gain BandwidthBW2MHz
Phase MarginPM65Degrees
Sourcing, VCOMP = 3V1.9Error-Amplifier Output CurrentICOMPSinking, VCOMP = 2V0.9mA
COMP Clamp VoltageVCOMPVFB = 0V3.254.5V
COMP Short-Circuit CurrentICOMP_SC12mA
PWM CURRENT LIMIT (CS)Cycle-by-Cycle Current-Limit
ThresholdVCLVDL = 0V187200217mV
Cycle-by-Cycle Current-Limit
Propagation Time To DLtPROP, CL10mV overdrive80ns
Gross Current-Limit ThresholdVGCLVCSS = 0V250270280mV
Gross Current-Limit Propagation
Time To DLtPROP,GCL10mV overdrive80ns
Input Bias Current0V < VCS < 5.5V-1000+100nA
PWM OSCILLATOR (RTCT)RTCT Voltage Ramp (Peak to
Peak)VRAMP5.5V < VIN < 24V1.601.651.80V
RTCT Voltage Ramp ValleyVRAMP_VALLEY5.5V < VIN < 24V1.111.201.27V
Discharge CurrentIDISVRTCT = 2V7.88.49.1mA
Frequency RangefOSC5.5V < VIN < 24V1001000kHz
SYNCHRONIZATION (SYNC/ENABLE)Input Rise/Fall Time200ns
Input Frequency Range1001000kHz
Input High Voltage1.5V
Input Low Voltage0.5V
Input Minimum Pulse Width200ns
Input Bias Current0V < VSYNC < 5.5V-1000+100nA
Delay to ShutdownVSYNC = 0V133265µs
LED DIMMING (DIM1–DIM4)Input High VoltageVDIM,MAX1.5V
Input Low VoltageVDIM,MIN0.5V
Minimum Dimming FrequencyfDIMtON = 2µs (Note 3)45Hz
Input Bias CurrentIDIM0V < VDIM_ < 5.5V-1000+100nA
Electrical Characteristics (continued)(VIN= 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_= 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values
are at TA= +25°C.) (Note 1)
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
ADC (DR1–DR4, OVP)Maximum ErrorEMAX±50mV
ADC Single Bit Acquisition
Latency(Note 4)2µs
DR Channel Sample TimetDR,SMPL190ms
OVP Channel Sample TimetOVP,SMPL20µs
Full-Scale Input VoltageVFS1.2151.241.2550V
Least Significant BitVLSB9.76mV
DR Input Bias CurrentIDR0V < VDR_ < 5.5V-1000+100nA
DRAIN FAULT COMPARATORS (DR1–DR4) (Shorted LED String Comparator)Drain Fault Comparator
ThresholdVDFTHVoltage to drive DL1–DL4 low1.41.521.63V
Drain Fault Comparator DelaytDFD10mV overdrive1µs
LINEAR REGULATORS (DL1–DL4, CS1–CS4)TransconductanceGm∆I = -500µA75mS
Maximum Output CurrentIDLSourcing or sinking15mA
CS1–CS4 Input Bias CurrentICS0V < VCS < 5.5V-1000+100nA
CS_ = DL_, FB DAC full scale;
MAX16826 only306316324
CS1–CS4 Regulation Voltage
MaximumVCS,MAX
CS_ = DL_, FB DAC full scale;
MAX16826B only308318328
CS_ = DL_, FB DAC minus full scale;
MAX16826 only9097105
CS1–CS4 Regulation Voltage
MinimumVCS,MIN
CS_ = DL_, FB DAC minus full scale;
MAX16826B only9099109
CS1–CS4 Regulation Voltage LSBVCS,LSBCS_ = DL_, FB DAC 1-bit transition1.72mV
Electrical Characteristics (continued)(VIN= 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_= 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values
are at TA= +25°C.) (Note 1)
Note 1:All devices are 100% production tested at TJ= +25°C and TJ= +125°C. Limits to -40°C are guaranteed by design.
Note 2:ICCincludes the internal bias currents and the current used by the gate drivers to drive DL, DL1, DL2, DL3, and DL4.
Note 3:Minimum frequency to allow the internal ADC to complete at least one measurement. tONis the on-time with the LED current
in regulation.
Note 4:Minimum LED current pulse duration, which is required to correctly acquire 1 bit.
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
SUPPLY CURRENT
vs. SUPPLY VOLTAGEMAX16826 toc01
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)164812
CDL = 4700pF
SUPPLY CURRENT
vs. OSCILLATOR FREQUENCYMAX16826 toc02
OSCILLATOR FREQUENCY (kHz)
SUPPLY CURRENT (mA)
CDL = 4700pF
C33 FROM 680pF TO 8200pF
SUPPLY CURRENT
vs. TEMPERATUREMAX16826 toc03
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
CDL = 4700pF
OSCILLATOR FREQUENCY
vs. SUPPLY VOLTAGEMAX16826 toc04
SUPPLY VOLTAGE (V)
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY
vs. TEMPERATURE
MAX16826 toc05
TEMPERATURE (°C)
OSCILLATOR FREQUENCY (kHz)
LED OUTPUT CURRENT
vs. TEMPERATURE
MAX16826 toc06
TEMPERATURE (°C)
LED OUTPUT CURRENT (mA)604020
VCS = 0.32V
Typical Operating Characteristics(VIN= 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_= 0.01µF. TA= +25°C, unless otherwise noted.)
LED OUTPUT CURRENT
vs. INPUT VOLTAGEMAX16826 toc07
LED OUTPUT CURRENT (mA)126
DIM INPUT TO ILED OUTPUT WAVEFORM
MAX16826 toc08
2μs/div
5V/div
100mA/div
0mA
VDIM
ILED
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
ENABLE AND DISABLE RESPONSEMAX16826 toc09
40ms/div
5V/div
100mA/div
0mA
VSYNC/EN
ILED
VCC VOLTAGE
vs. LOAD CURRENTMAX16826 toc10
LOAD CURRENT (mA)
VOLTAGE (V)302010
VCC VOLTAGE
vs. TEMPERATURE
MAX16826 toc11
TEMPERATURE (°C)
VOLTAGE (V)604020
Typical Operating Characteristics (continued)
(VIN= 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_= 0.01µF. TA= +25°C, unless otherwise noted.)
VCC VOLTAGE
vs. SUPPLY VOLTAGEMAX16826 toc12
SUPPLY VOLTAGE (V)
CC
VOLTAGE (V)161284
SHUNT VOLTAGE
vs. SHUNT CURRENT
MAX16826 toc13
SHUNT CURRENT (mA)
SHUNT VOLTAGE (V)
SHUNT VOLTAGE
vs. TEMPERATURE
MAX16826 toc14
SHUNT VOLTAGE (V)
SHUNT REGULATOR LOAD DUMP RESPONSE
MAX16826 toc15
200ms/div
20V/div
10V/div
VSUPPLY
VSHUNT
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
Pin Description
PINNAMEFUNCTIONPGNDPower Ground
2, 3GNDAnalog GroundRTCT
Timing Resistor and Capacitor Connection. A resistor, R19 (in the Typical Application Circuit), from VCC to
RTCT and a capacitor C33, from RTCT to GND set the oscillator frequency. See the Oscillator section to
calculate RT and CT component values.SYNC/EN
Synchronization and Enable Input. There are three operating modes:
SYNC/EN = LOW: Low current shutdown mode with all circuits shut down except shunt regulator.
SYNC/EN = HIGH: All circuits active with oscillator frequency set by RTCT network.
SYNC/EN = CLOCKED: All circuits active with oscillator frequency set by SYNC clock input. Conversion
cycles initiate on the rising edge of external clock input. The frequency programmed by R19/C33 must be
10% lower than the input SYNC/EN signal frequency.CSS
Soft-Start Timing Capacitor Connection. Connect a capacitor from CSS to GND to program the required soft-
start time for the switching regulator output voltage to reach regulation. See the Soft-Start (CSS) section to
calculate CCSS.COMPSwitching Regulator Compensation Component Connection. Connect the compensation network between
COMP and FB.
8FB
Switching Regulator Feedback Input. Connect FB to the center of a resistor-divider connected between the
switching regulator output and GND to set the output voltage. FB is regulated to a voltage set by an internal
register. See the Setting Output Voltage section for calculating resistor values.OVP
Switching Regulator Overvoltage Input. Connect OVP to the center of a resistor-divider connected between the
switching regulator output and GND. For normal operation, configure the resistor-divider so that the voltage at
this pin does not exceed 1.25V. If operation under load dump conditions is also required, configure the resistor-
divider so that the voltage at OVP is less than 1.25V.RSC
Slope Compensation Resistor and PWM Comparator Input Connection. Connect a resistor, R17, from RSC to
the switching current-sense resistor to set the amount of the compensation ramp. See the Slope Compensation
(RSC) section for calculating the value.SDAI2C Serial Data Input/OutputSCLI2C Serial Clock InputDIM1
LED String 1 Logic-Level PWM Dimming Input. A high logic level on DIM1 enables the current sink to operate at
the maximum current as determined by its sense resistor and internal register value. A low logic level disables
the current source.DIM2
LED String 2 Logic-Level PWM Dimming Input. A high logic level on DIM2 enables the current sink to operate at
the maximum current as determined by its sense resistor and internal register value. A low logic level disables
the current source.DIM3
LED String 3 Logic-Level PWM Dimming Input. A high logic level on DIM3 enables the current sink to operate at
the maximum current as determined by its sense resistor and internal register value. A low logic level disables
the current source.DIM4
LED String 4 Logic-Level PWM Dimming Input. A high logic level on DIM4 enables the current sink to operate at
the maximum current as determined by its sense resistor and internal register value. A low logic level disables
the current source.CS1LED String 1 Current-Sense Input. CS1 is regulated to a value set by an internal register. The regulation voltage
can be set between 97mV and 316mV.
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
Pin Description (continued)
PINNAMEFUNCTIONDL1
LED String 1 Linear Current Source Output. DL1 drives the gate of the external FET on LED String 1 and has
approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL1 to GND to
compensate the internal transconductance amplifier as well as program the rise and fall times of the LED currents.DR1
LED String 1 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND
voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C
interface. Connect a voltage-divider to scale drain voltage as necessary.CS2LED String 2 Current-Sense Input. CS2 is regulated to a value set by an internal register. The regulation voltage
can be set between 97mV and 316mV.DL2
LED String 2 Linear Current Source Output. DL2 drives the gate of the external FET on LED String 2 and has
approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL2 to GND to
compensate the internal transconductance amplifier, as well as program the rise and fall times of the LED
currents.DR2
LED String 2 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND
voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C
interface. Connect a voltage-divider to scale drain voltage as necessary.CS3LED String 3 Current-Sense Input. CS3 is regulated to a value set by an internal register. The regulation voltage
can be set between 97mV and 316mV.DL3
LED String 3 Linear Current Source Output. DL3 drives the gate of the external FET on LED String 3 and has
approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL3 to GND to
compensate the internal transconductance amplifier, as well as program the rise and fall times of the LED currents.DR3
LED String 3 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND
voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C
interface. Connect a voltage-divider to scale drain voltage as necessary.CS4LED String 4 Current-Sense Input. CS4 is regulated to a value set by an internal register. The regulation voltage
can be set between 97mV and 316mV.DL4
LED String 4 Linear Current Source Output. DL3 drives the gate of the external FET on LED String 4 and has
approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL4 to GND to
compensate the internal transconductance amplifier, as well as program the rise and fall times of the LED currents.DR4
LED String 4 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND
voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C
interface. Connect a voltage-divider to scale drain voltage as necessary.IN
Power Supply. IN is internally connected to a 26V shunt regulator that sinks current. In conjunction with an
external resistor it allows time-limited load dump events as high as 40V to be safely handled by the IC. Bypass
IN to GND with a minimum 10µF capacitor.CSCurrent-Sense InputVCC
Gate Driver Regulator Output. Bypass VCC to GND with a minimum 4.7µF ceramic capacitor. Gate drive current
pulses come from the capacitor connected to VCC. Place the capacitor as close as possible to VCC. If IN is
powered by a voltage less than 5.5V, connect VCC directly to IN.DLSwitching Regulator Gate Driver Output
—EPExposed Pad. Connect the exposed pad to the ground plane for heatsinking. Do not use this pad as the only
ground connection to the IC.
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
Detailed DescriptionThe MAX16826 HB LED driver integrates a switching
regulator controller, a 4-channel linear current sink dri-
ver, a 7-bit ADC, and an I2Cinterface. The IC is
designed to operate from a 4.75V to 24V input voltage
range and can withstand automotive load dump tran-
sients up to 40V.
The current-mode switching regulator controller is con-
figurable as a boost or SEPIC converter to regulate the
voltage to drive the four strings of HB LEDs. Its program-
mable switching frequency (100kHz to 1MHz) allows the
use of a small inductor and filter capacitors. The four
current sink regulators use independent external current-
sense resistors to provide constant currents for each
string of LEDs. Four DIM inputs allow a very wide range
of independent pulsed dimming to each LED string. An
internal 7-bit ADC measures the drain voltage of the
external driver transistors to enable output voltage opti-
mization and fault monitoring of the LEDs. The
MAX16826 is capable of driving four strings of LEDs.
The number of LEDs in each string is only limited by the
topology of choice, the rating of the external compo-
nents, and the resolution of the ADC and internal DAC.
Simplified Block DiagramVCC
VCC
PGND
OVP
COMP
RTCT
GND
OVT
CURRENT-
MODE
PWM
BLOCK
SYNC/EN
CSS
DR47-BIT ADC
AND
SHORTED
STRING
FAULT
DECTECTION
DR3
DR2
DR1
LINEAR
CURRENT-
SINK
DRIVERS
CS4
CS3
CS2
CS1
DL4
DL3
DL2
DL1
I2C
STATE
MACHINE
DOUBLE-
BUFFERED
REGISTER
AND DACS
DIM4
DIM3
OVTOVT
DIM2
DIM1
SDA
SCL
26V
SHUNT
GND
OVT
REF
VCC
GND
RSC
MAX16826
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
The MAX16826 provides additional flexibility with an
internal I2Cserial interface to communicate with a
microcontroller (µC). The interface can be used to
dynamically adjust the amplitude of the LED current in
each LED string and the switch-mode regulator output
voltage. It can also be used to read the ADC drain volt-
age measurements for each string, allowing a µC to
dynamically adjust the output voltage to minimize the
power dissipation in the LED current sink FETs. The I2C
interface can also be used to detect faults such as LED
short or open.
Modes of OperationThe MAX16826 has six modes of operation: normal
mode, undervoltage lockout (UVLO) mode, thermal
shutdown (TSD) mode, shutdown (SHDN) mode,
standby (STBY) mode, and overvoltage protection
(OVP) mode.
The normal mode is the default state where each cur-
rent sink regulator is maintaining a constant current
through each of the LED strings. Digitized voltage feed-
back from the drains of the current sink FETs can be
used to establish a secondary control loop by using an
external µC to control the output of the switching stage
for the purpose of achieving low-power dissipation
across these FETs.
UVLO mode occurs when VVCCgoes below 4.3V. In
UVLO mode, each of the linear current sinks and the
switching regulator is shut down until the input voltage
exceeds the rising UVLO threshold.
TSD mode occurs when the die temperature exceeds
the internally set thermal limit (+160°C). In TSD mode,
each of the linear regulators and the switching regulator
is shut down until the die temperature cools by 20°C.
SHDN mode occurs when SYNC/EN is driven low. In
SHDN mode, all internal circuitry with the exception of
the shunt regulator is deactivated to limit current draw
to less than 50µA. SHDN mode disengages when
SYNC/EN is driven high or clocked.
STBY mode is initiated using the I2C interface. In STBY
mode, each of the linear current sinks and the switching
regulator is shut down. STBY mode is also deactivated
using the I2C interface. In STBY mode, the internal VCC
regulator and the shunt regulator remain active. Whenever
the MAX16826 enters a mode that deactivates the switch-
ing regulator, the soft-start capacitor is discharged so that
soft-start occurs upon reactivation.
OVP mode occurs when the voltage at OVP is higher than
the internal reference. In OVP mode, the switching regula-
tor gate-drive output is latched off and can only be
restored by cycling enable, power, or entering standby
mode.
Switching Preregulator StageThe MAX16826 features a current-mode controller that
is capable of operating in the frequency range of
100kHz to 1MHz. Current-mode control provides fast
response and simplifies loop compensation.
Output voltage regulation can be achieved in a two-
loop configuration. A required conventional control loop
can be set up by using the internal error amplifier with
its inverting input connected to FB. The bandwidth of
this loop is set to be as high as possible utilizing con-
ventional compensation techniques. The noninverting
input of this amplifier is connected to a reference volt-
age that is dynamically adjustable using the I2Cinter-
face. The optional slower secondary loop consists of
the external µC using the I2Cinterface reading out the
voltages at the drains of the current sink FETs and
adjusting the reference voltage for the error amplifier.
To regulate the output voltage, the error amplifier com-
pares the voltage at FB to the internal 1.25V (adjustable
down by using the I2Cinterface) reference. The output
of the error amplifier is compared to the sum of the cur-
rent-sense signal and the slope compensation ramp at
RSC to control the duty cycle at DL.
Two current-limit comparators also monitor the voltage
across the sense resistor using CS. If the primary cur-
rent-limit threshold is reached, the FET is turned off and
remains off for the reminder of the switching cycle. If
the current through the FET reaches the secondary cur-
rent limit, the switching cycle is terminated and the soft-
start capacitor is discharged. The converter then
restarts in soft-start mode preventing inductor current
runaway due to the delay of the primary cycle-by-cycle
current limit. The switching regulator controller also fea-
tures an overvoltage protection circuit that latches the
gate driver off if the voltage at OVP exceeds the inter-
nal 1.25V reference voltage.
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
Shunt RegulatorThe MAX16826 has an internal 26V (typ) shunt regula-
tor to provide the primary protection against an auto-
motive load dump. When the input voltage is below
26V, the shunt voltage at IN tracks the input voltage.
When the input voltage exceeds 26V, the shunt regula-
tor turns on to sink current, and the voltage at IN is
clamped to 26V. During a load dump, the input voltage
can reach 40V, and the shunt regulator through the
resistor connected to IN is forced to sink large amounts
of current for up to 400ms to limit the voltage that
appears at IN to the shunt regulation voltage. The sink-
ing current of the shunt regulator is limited by the value
of resistor (R1 in Figure 1) in series with IN. There are
two criteria that determine the value of R1: the maxi-
mum acceptable shunt current during load dump, and
the voltage drop on R1 under normal operating condi-
tions with low battery voltage. For example, with typical
20mA input current in normal operation, 250mA load
dump current limit, 40V maximum load dump voltage,
the R1 value is:
where VINMINis the minimum operating voltage and
VINREGis the minimum acceptable voltage at IN.
Use the following equation to verify that the current
through R1 is less than 250mA under a load-dump con-
dition:
For stable operation, the shunt regulator requires a min-
imum 10µF of ceramic capacitance from IN to GND.
VCCRegulatorThe 5.25V VCCregulator provides bias for the internal
circuitry including the bandgap reference and gate dri-
vers. Externally bypass VCCwith a minimum 4.7µF
ceramic capacitor. VCChas the ability to supply up to
50mA of current, but external loads should be mini-
mized so as not to take away drive capability for inter-
nal circuitry. If IN is powered by a voltage less than
5.5V, connect VCCdirectly to IN.
Switch-Mode ControllerThe MAX16826 consists of a current-mode controller
that is capable of operating in the 100kHz to 1MHz fre-
quency range (Figure 2). Current-mode control pro-
vides fast response and simplifies loop compensation.
The error amplifier compares the voltage at FB to 1.25V
and varies the COMP output accordingly to regulate.
The PWM comparator compares the voltage at COMP
with the voltage at RSC to determine the switching duty
cycle. The primary cycle-by-cycle current-limit com-
parator interrupts the on-time if the sense voltage is
larger than 200mV. When the sense voltage is larger
than 270mV, the secondary gross current-limit com-
parator is activated to discharge the soft-start capaci-
tor. This forces the IC to re-soft-start preventing
inductor current runaway due to the delay of the prima-
ry cycle-by-cycle current limit.
The switch-mode controller also features a low current
shutdown mode, adjustable soft-start, and thermal
shutdown protection.IVVmALDLD===−−2626
100140VV
INMININREG17555101003==−−Ω
MAX16826INR1
5V
REFERENCE
Figure 1. Shunt Regulator Block Diagram
MAX16826Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
OVP
CSS
SWR DAC
I2C BUS
SET
CLR
SET
CLR
6µA
1.25V
COMP
SHDN
STBY
OSCILLATOR
26µA/µs
SYNC
RTCT
RSC
SOFT-START COMPARATOR
OVP COMPARATOR
ERROR AMPLIFIER
PWM COMPARATOR
270mV
200mV
VCC
10µA
MAX16826
CURRENT-
RAMP
GENERATOR
VCC
CURRENT-LIMIT
COMPARATORS
ANALOG
MUX
Figure 2. Switch Regulator Controller Block Diagram
