MAX16821BATI+ ,High-Power Synchronous HB LED Drivers with Rapid Current PulsingFeatures● Up to 30A Output CurrentThe MAX16821A, MAX16821B, and MAX16821C pulse-● True-Differential ..
MAX16823ATE+ ,High-Voltage, 3-Channel Linear High-Brightness LED Driver with Open LED Detectionfeatures include an ● Overtemperature Shutdownactive-high, open-drain LEDGOOD output for open LED ..
MAX16823AUE+ ,High-Voltage, 3-Channel Linear High-Brightness LED Driver with Open LED DetectionFeaturesThe MAX16823 three-channel LED driver operates from ● 5.5V to 40V Operating Rangea 5.5V to ..
MAX16823AUE+T ,High-Voltage, 3-Channel Linear High-Brightness LED Driver with Open LED DetectionElectrical Characteristics(V = 12V, C = 0.1μF, I = 0, C = 15nF, V = V = V = 3.4V, R = 2.85Ω from CS ..
MAX16826ATJ+ ,Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detectionapplications such as industrial or desktopWhite, RGB, or RGB Plus Amber LED Configurationsmonitors ..
MAX1682EUK+T ,Switched-Capacitor Voltage DoublersApplications Ordering InformationSmall LCD PanelsTEMP PIN- SOTPARTRANGE PACKAGE TOP MARKCell Phones ..
MAX4472ESD+ ,Single/Dual/Quad, +1.8V/750nA, SC70, Rail-to-Rail Op Amps
MAX4472EUD ,Single/Dual/Quad / !.8V/750nA / SC70 / Rail-to-Rail Op AmpsFeaturesThe MAX4464/MAX4470/MAX4471/MAX4472/MAX4474 Ultra-Low 750nA Supply Current Per Amplifierfa ..
MAX4472EUD+ ,Single/Dual/Quad, +1.8V/750nA, SC70, Rail-to-Rail Op Amps
MAX4473ESA ,Low-Cost / Low-Voltage / PA Power Control Amplifier for GSM Applications in 8-Pin MAXApplications in 8-Pin µMAXABSOLUTE MAXIMUM RATINGSV to GND ...+7V Continuous Power Dissipation (T = ..
MAX4473EUA ,Low-Cost / Low-Voltage / PA Power Control Amplifier for GSM Applications in 8-Pin MAXELECTRICAL CHARACTERISTICS(V = +2.7V to +6.5V, SHDN > +2.4V, MAX4473 test circuit, RG1 = RG2 = 1kΩ ..
MAX4473EUA ,Low-Cost / Low-Voltage / PA Power Control Amplifier for GSM Applications in 8-Pin MAXApplicationsPART TEMP. RANGE PIN-PACKAGEGSM Cellular PhonesMAX4473EUA -40°C to +85°C 8 µMAXCordless ..
MAX16821AATI+-MAX16821BATI+
High-Power Synchronous HB LED Drivers with Rapid Current Pulsing
General DescriptionThe MAX16821A, MAX16821B, and MAX16821C pulse-
width-modulation (PWM) LED driver controllers provide
high output-current capability in a compact package with a
minimum number of external components. The
MAX16821A–MAX16821C are suitable for use in synchro-
nous and nonsynchronous step-down (buck), boost, buck-
boost, SEPIC, and Cuk LED drivers. A logic input (MODE)
allows the devices to switch between synchronous buck
and boost modes of operation. These devices are the first
high-power drivers designed specifically to accommodate
common-anode HB LEDs.
The ICs offer average current-mode control that enable
the use of MOSFETs with optimal charge and on-
resistance figure of merit, thus minimizing the need for
external heatsinking even when delivering up to 30A of
LED current.
The differential sensing scheme provides accurate control
of the LED current. The ICs operate from a 4.75V to 5.5V
supply range with the internal regulator disabled (VCC
connected to IN). These devices operate from a 7V to 28V
input supply voltage with the internal regulator enabled.
The MAX16821A–MAX16821C feature a clock output
with 180° phase delay to control a second out-of-phase
LED driver to reduce input and output filter capacitor size
and to minimize ripple currents. The wide switching fre-
quency range (125kHz to 1.5MHz) allows the use of small
inductors and capacitors.
Additional features include programmable overvoltage
protection and an output enable function.
Applications●Front Projectors/Rear-Projection TVs●Portable and Pocket Projectors●LCD TVs and Display Backlight
Features●Up to 30A Output Current●True-Differential Remote Output Sensing●Average Current-Mode Control●4.75V to 5.5V or 7V to 28V Input Voltage Range●0.1V/0.03V LED Current-Sense Options Maximize
Efficiency (MAX16821B/MAX16821C)●Thermal Shutdown●Nonlatching Output-Overvoltage Protection●Low-Side Buck Mode with or without Synchronous
Rectification●High-Side Buck and Low-Side Boost Mode with or
without Synchronous Rectification●125kHz to 1.5MHz Programmable/Synchronizable
Switching Frequency●Integrated 4A Gate Drivers●Clock Output for 180° Out-of-Phase Operation for
Second Driver●-40°C to +125°C Operating Temperature Range
Typical Operating Circuit and Selector Guide appears at
end of data sheet.+Denotes lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
PARTTEMP RANGEPIN-PACKAGE
MAX16821AATI+-40°C to +125°C28 TQFN-EP*
MAX16821BATI+-40°C to +125°C28 TQFN-EP*
MAX16821CATI+-40°C to +125°C28 TQFN-EP*
HIGH-FREQUENCY
PULSE TRAIN
VLED
7V TO 28V
CSP
PGND
I.C.
OVI
CLP
MAX16821
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Simpliied Diagram
Ordering Information
EVALUATION KIT AVAILABLE
IN to SGND ...........................................................-0.3V to +30V
BST to SGND ........................................................-0.3V to +35V
BST to LX ................................................................-0.3V to +6V
DH to LX .......................................-0.3V to (VBST - VLX) + 0.3V
DL to PGND .............................................-0.3V to (VDD + 0.3V)
VCC to SGND ..........................................................-0.3V to +6V
VCC, VDD to PGND .................................................-0.3V to +6V
SGND to PGND ....................................................-0.3V to +0.3V
VCC Current .....................................................................300mA
All Other Pins to SGND ............................-0.3V to (VCC + 0.3V)
Continuous Power Dissipation (TA = +70°C)
28-Pin TQFN 5mm x 5mm (derate 34.5mW/°C
above +70°C) ...........................................................2758mW
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
(VCC = 5V, VDD = VCC, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSInput-Voltage RangeVINInternal LDO on728Internal LDO off (VCC connected to VIN)4.755.50
Quiescent Supply CurrentIQVEN = VCC or SGND, no switching2.75.5mA
LED CURRENT REGULATORDifferential Set Value
(VSENSE+ to VSENSE-)
(Note 2)
VIN = VCC = 4.75V to 5.5V, fSW = 500kHz
(MAX16821A)0.5940.6000.606
VIN = 7V to 28V, fSW = 500kHz
(MAX16821A)0.5940.6000.606
VIN = VCC = 4.75V to 5.5V,
fSW = 500kHz (MAX16821B)0.0980.1000.102
VIN = 7V to 28V, fSW = 500kHz
(MAX16821B)0.0980.1000.102
VIN = VCC = 4.75V to 5.5V,
fSW = 500kHz (MAX16821C)0.0280.0300.032
VIN = 7V to 28V, fSW = 500kHz
(MAX16821C)0.0280.0300.032
Soft-Start TimetSS1024Clock
Cycles
STARTUP/INTERNAL REGULATORVCC Undervoltage Lockout
(UVLO)UVLOVCC rising4.14.34.5V
UVLO HysteresisVCC falling200mV
VCC Output VoltageVIN = 7V to 28V, ISOURCE = 0 to 60mA4.855.105.30V
MOSFET DRIVEROutput Driver ImpedanceLow or high output, ISOURCE/SINK = 20mA1.13Ω
Output Driver Source/Sink
CurrentIDH, IDL4A
Nonoverlap TimetNOCDH/DL = 5nF35ns
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Absolute Maximum RatingsStresses 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.
Electrical Characteristics
(VCC = 5V, VDD = VCC, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
OSCILLATORSwitching Frequency Range1251500kHz
Switching FrequencyfSW
RT = 500kΩ120125130
kHzRT = 120kΩ495521547
RT = 39.9kΩ151516201725
Switching Frequency Accuracy120kΩ < RT ≤ 500kΩ-5+5%40kΩ ≤ RT ≤ 120kΩ-8+8
CLKOUT Phase Shift with
Respect to DH (Rising Edges)
fSW = 125kHz, MODE connected
to SGND180
degrees
CLKOUT Phase Shift with
Respect to DL (Rising Edges)
fSW = 125kHz, MODE connected
to VCC180
CLKOUT Output-Voltage LowVOLISINK = 2mA0.4V
CLKOUT Output-Voltage HighVOHISOURCE = 2mA4.5V
SYNC Input High Pulse WidthtSYNC200ns
SYNC Input Clock High
ThresholdVSYNCH2V
SYNC Input Clock Low
ThresholdVSYNCL0.4V
SYNC Pullup CurrentISYNC_
OUTVRT/SYNC = 0V250500µA
SYNC Power-Off LevelVSYNC_
OFF0.4V
INDUCTOR CURRENT LIMITAverage Current-Limit ThresholdVCLCSP to CSN26.427.533.0mV
Reverse Current-Limit ThresholdVCLRCSP to CSN-2.0mV
Cycle-by-Cycle Current LimitCSP to CSN60mV
Cycle-by-Cycle OverloadVCSP to VCSN = 75mV260ns
CURRENT-SENSE AMPLIFIERCSP to CSN Input ResistanceRCS4kΩ
Common-Mode RangeVCMR(CS)VIN = 7V to 28V05.5V
Input Offset VoltageVOS(CS)0.1mV
Ampliier Voltage GainAV(CS)34.5V/V
3dB Bandwidthf3dB4MHz
CURRENT-ERROR AMPLIFIER (TRANSCONDUCTANCE AMPLIFIER)Transconductancegm550µS
Open-Loop GainAVL(CE)50dB
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Electrical Characteristics (continued)
(VCC = 5V, VDD = VCC, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
LED CURRENT SIGNAL DIFFERENTIAL VOLTAGE AMPLIFIER (DIFF)Common-Mode Voltage RangeVCMR
(DIFF)01.0V
DIFF Output VoltageVCMVSENSE+ = VSENSE- = 0V0.6V
Input Offset VoltageVOS(DIFF)MAX16821A-3.7+3.7mVMAX16821B/MAX16821C-1.5+1.5
Ampliier Voltage GainAV(DIFF)
MAX16821A0.99211.008
V/VMAX16821B5.8566.1
MAX16821C18.52021.5
3dB Bandwidthf3dB
MAX16821A, CDIFF = 20pF1.7MHz
MAX16821B, CDIFF = 20pF1600kHzMAX16821C, CDIFF = 20pF550
SENSE+ to SENSE- Input
ResistanceRVS
MAX16821A50100MAX16821B3060
MAX16821C1020
OUTV AMPLIFIERGain-Bandwidth ProductVOUTV = 2V4MHz
3dB BandwidthVOUTV = 2V1MHz
Output Sink Current30µA
Output Source Current80µA
Maximum Load Capacitance50pF
OUTV to (CSP - CSN) Transfer
Function4mV ≤ CSP – CSN ≤ 32mV132.5135137.7V/V
Input Offset Voltage1mV
VOLTAGE-ERROR AMPLIFIER (EAOUT)Open-Loop GainAVOLEA70dB
Unity-Gain BandwidthfGBW3MHz
EAN Input Bias CurrentIB(EA)VEAN = 2V-0.2+0.03+0.2µA
Error Ampliier Output
Clamping Voltage
VCLAMP
(EA)With respect to VCM905930940mV
INPUTS (MODE AND OVI)MODE Input-Voltage High2V
MODE Input-Voltage Low0.8V
MODE Pulldown Current456µA
OVI Trip ThresholdOVPTH1.2441.2761.308V
OVI HysteresisOVIHYS200mV
OVI Input Bias CurrentIOVIVOVI = 1V0.2µA
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Electrical Characteristics (continued)
(VCC = 5V, VDD = VCC, TA = 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 +25°C. Limits over temperature are guaranteed by design.
Note 2: Does not include an error due to finite error amplifier gain. See the Voltage-Error Amplifier section.
(VIN = 12V, VDD = VCC = 5V, TA = +25°C, unless otherwise noted.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
ENABLE INPUT (EN)EN Input-Voltage HighEN rising2.4372.52.562V
EN Input Hysteresis0.28V
EN Pullup CurrentIEN13.51516.5µA
THERMAL SHUTDOWNThermal Shutdown165°C
Thermal-Shutdown Hysteresis20°C
SUPPLY CURRENT vs. TEMPERATUREMAX16821A toc02
TEMPERATURE (°C)
SUPPLY CURRENT (mA)3510-15
VIN = 12V
CDH/DL = 22nF
DRIVER FALL TIME
vs. DRIVER LOAD CAPACITANCEMAX16821A toc05
(ns)15510
VCC LOAD REGULATION vs. VIN
MAX16821A toc03
VCC LOAD CURRENT (mA)
(V)
VIN = 24V
VIN = 7V
VIN = 12V
HIGH-SIDE DRIVER (DH) SINK
AND SOURCE CURRENTMAX16821A toc06
2A/div
CLOAD = 22nF
VIN = 12V
SUPPLY CURRENT (IQ) vs. FREQUENCYMAX16821A toc01
FREQUENCY (kHz)
SUPPLY CURRENT (mA)
VIN = 24V
EXTERNAL CLOCK
NO DRIVER LOAD
VIN = 5V
VIN = 12V
DRIVER RISE TIME
vs. DRIVER LOAD CAPACITANCEMAX16821A toc04
(ns)15510
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Electrical Characteristics (continued)
Typical Operating Characteristics
(VIN = 12V, VDD = VCC = 5V, TA = +25°C, unless otherwise noted.)
HIGH-SIDE DRIVER (DH) RISE TIMEMAX16821A toc08
2V/div
40ns/div
VIN = 12V
DH RISING
HIGH-SIDE DRIVER (DH) FALL TIMEMAX16821A toc09
2V/div
40ns/div
CLOAD = 22nF
VIN = 12V
LOW-SIDE DRIVER (DL) RISE TIMEMAX16821A toc10
2V/div
40ns/div
CLOAD = 22nF
VIN = 12V
LOW-SIDE DRIVER (DL) FALL TIMEMAX16821A toc11
2V/div
40ns/div
CLOAD = 22nF
VIN = 12V
FREQUENCY vs. RTMAX16821A toc12
RT (k)
(kHz)
10,000
VIN = 12V
FREQUENCY vs. TEMPERATUREMAX16821A toc13
fSW
(kHz)202551015
VIN = 12V
LOW-SIDE DRIVER (DL) SINK
AND SOURCE CURRENTMAX16821A toc07
3A/div
100ns/div
CLOAD = 22nF
VIN = 12V
SYNC, CLKOUT, AND DH WAVEFORMSMAX16821A toc14
RT/SYNC
5V/div
CLKOUT
5V/div
5V/div
1s/div
MODE = SGND
SYNC, CLKOUT, AND DL WAVEFORMSMAX16821A toc15
RT/SYNC
5V/div
CLKOUT
5V/div
5V/div
1s/div
MODE = VCC
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Typical Operating Characteristics (continued)
PINNAMEFUNCTIONPGNDPower-Supply Ground
2, 7N.C.No Connection. Not internally connected.DLLow-Side Gate-Driver OutputBST
Boost-Flying Capacitor Connection. Reservoir capacitor connection for the high-side MOSFET driver
supply. Connect a ceramic capacitor between BST and LX.LXHigh-Side MOSFET Source ConnectionDHHigh-Side Gate-Driver Output
8, 22, 25SGNDSignal Ground. SGND is the ground connection for the internal control circuitry. Connect SGND and
PGND together at one point near the IC.CLKOUTOscillator Output. If MODE is low, the rising edge of CLKOUT phase shifts from the rising edge of DH by
180°. If MODE is high, the rising edge of CLKOUT phase shifts from the rising edge of DL by 180°.MODEBuck/Boost Mode Selection Input. Drive MODE low for low-side buck mode operation. Drive MODE high
for boost or high-side buck mode operation. MODE has an internal 5µA pulldown current to ground.ENOutput Enable. Drives EN high or leave unconnected for normal operation. Drive EN low to shut down
the power drivers. EN has an internal 15µA pullup current.RT/SYNCSwitching Frequency Programming. Connect a resistor from RT/SYNC to SGND to set the internal
oscillator frequency. Drive RT/SYNC to synchronize the switching frequency with an external clock.OUTVInductor Current-Sense Output. OUTV is an ampliier output voltage proportional to the inductor current.
The voltage at OUTV = 135 x (VCSP - VCSN).I.C.Internally Connected. Connect to SGND for proper operation.OVI
Overvoltage Protection. When OVI exceeds the programmed output voltage by 12.7%, the low-side and
the high-side drivers are turned off. When OVI falls 20% below the programmed output voltage, the
drivers are turned on after power-on reset and soft-start cycles are completed.CLPCurrent-Error-Ampliier Output. Compensate the current loop by connecting an RC network to ground.EAOUTVoltage-Error-Ampliier Output. Connect EAOUT to the external gain-setting network.EANVoltage-Error-Ampliier Inverting InputDIFFDifferential Remote-Sense Ampliier Output. DIFF is the output of a precision ampliier with SENSE+
and SENSE- as inputs.CSNCurrent-Sense Differential Ampliier Negative Input. The differential voltage between CSN and CSP is ampliied internally by the current-sense ampliier (Gain = 34.5) to measure the inductor current.CSPCurrent-Sense Differential Ampliier Positive Input. The differential voltage between CSP and CSN is ampliied internally by the current-sense ampliier (Gain = 34.5) to measure the inductor current.SENSE-Differential LED Current-Sensing Negative Input. Connect SENSE- to the negative side of the LED
current- sense resistor or to the negative feedback point.SENSE+Differential LED Current-Sensing Positive Input. Connect SENSE+ to the positive side of the LED
current- sense resistor, or to the positive feedback point.INSupply Voltage Input. Connect IN to VCC, for a 4.75V to 5.5V input supply range.VCCInternal +5V Regulator Output. VCC is derived from VIN. Bypass VCC to SGND with 4.7µF and 0.1µF
ceramic capacitors.VDDLow-Side Driver Supply VoltageEPExposed Pad. EP is internally connected to SGND. Connect EP to a large-area ground plane for
effective power dissipation. Connect EP to SGND. Do not use as a ground connection.
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Pin Description
Detailed DescriptionThe MAX16821A, MAX16821B, and MAX16821C are
high-performance average current-mode PWM control-
lers for high-power and high-brightness LEDs (HB LEDs).
The average current-mode control technique offers inher-
ently stable operation, reduces component derating and
size by accurately controlling the inductor current. The
devices achieve high efficiency at high currents (up to
30A) with a minimum number of external components.
A logic input (MODE) allows the LED driver to switch
between buck and boost modes of operation.
The MAX16821A–MAX16821C feature a CLKOUT output
180° out-of-phase with respect to either the high-side
or low-side driver, depending on MODE’s logic level.
CLKOUT provides the drive for a second out-of-phase
LED driver for applications requiring reduced input capac-
itor ripple current while operating another LED driver.
The MAX16821A–MAX16821C consist of an inner aver-
age current regulation loop controlled by an outer loop.
The combined action of the inner current loop and outer
voltage loop corrects the LED current errors by adjusting
the inductor current resulting in a tightly regulated LED
current. The differential amplifier (SENSE+ and SENSE-
inputs) senses the LED current using a resistor in series
with the LEDs and produces an amplified version of the
sense voltage at DIFF. The resulting amplified sensed
voltage is compared against an internal 0.6V reference at
the error amplifier input.
Input VoltageThe MAX16821A–MAX16821C operate with a 4.75V to
5.5V input supply range when the internal LDO is disabled
(VCC connected to IN) or a 7V to 28V input supply range
when the internal LDO is enabled. For a 7V to 28V input
voltage range, the internal LDO provides a regulated 5V
output with 60mA of sourcing capability. Bypass VCC to
SGND with 4.7µF and 0.1µF low-ESR ceramic capacitors.
The MAX16821A–MAX16821C’s VDD input pro-
vides supply voltage for the low-side and the high-
side MOSFET drivers. Connect VDD to VCC using an
R-C filter to isolate the analog circuits from the MOSFET
drivers. The internal LDO powers up the MAX16821A–
MAX16821C. For applications utilizing a 5V input volt-
age, disable the internal LDO by connecting IN and VCC
together. The 5V power source must be in the 4.75V to
5.5V range of for proper operation of the MAX16821A–
MAX16821C.
Undervoltage Lockout (UVLO)The MAX16821A–MAX16821C include UVLO and a
2048 clock-cycle power-on-reset circuit. The UVLO rising
threshold is set to 4.3V with 200mV hysteresis. Hysteresis
at UVLO eliminates chattering during startup. Most of the
internal circuitry, including the oscillator, turns on when the
input voltage reaches 4V. The MAX16821A–MAX16821C
draw up to 3.5mA of quiescent current before the input
voltage reaches the UVLO threshold.
Soft-StartThe MAX16821A–MAX16821C include an internal soft-
start for a glitch-free rise of the output voltage. After 2048
power-on-reset clock cycles, a 0.6V reference voltage
connected to the positive input of the internal error ampli-
fier ramps up to its final value after 1024 clock cycles.
Soft-start reduces inrush current and stress on system
components. During soft-start, the LED current will ramp
monotonically towards its final value.
Internal OscillatorThe internal oscillator generates a clock with the fre-
quency inversely proportional to the value of RT (see
the Typical Operating Circuit). The oscillator frequency is
adjustable from 125kHz to 1.5MHz range using a single
resistor connected from RT/SYNC to SGND. The fre-
quency accuracy avoids the overdesign, size, and cost
of passive filter components like inductors and capaci-
tors. Use the following equation to calculate the oscillator
frequency:
For 120kΩ ≤ RT ≤ 500kΩ:
6.25 x 10f (Hz)R=
For 40kΩ ≤ RT ≤ 120kΩ:
6.40 x 10f (Hz)R=
The oscillator also generates a 2VP-P ramp signal for the
PWM comparator and a 180° out-of-phase clock signal
at CLKOUT to drive a second out-of-phase LED current
regulator.
SynchronizationThe MAX16821A–MAX16821C synchronize to an exter-
nal clock connected to RT/SYNC. The application of an
external clock at RT/SYNC disables the internal oscillator.
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
MAX16821A
MAX16821B
MAX16821CVCM
VCM
0.12 x VREF
OVP
COMPARATOR
PWM
COMPARATOR
ENABLE
UVLO
VREF = 0.6V
VCM
0.5 x VCC
+5V LDO
VCC
SGND
RT/SYNC
CLKOUT
SENSE-
DIFF
SENSE+
VCC
I.C.
CLP
CSP
CSN
OUTV
BST
VDD
PGND
MODE
VTH
TO INTERNAL CIRCUIT
VCLAMP HIGH
AV = 4VCLAMP LOW
CLK
2 x fS
UVLO
POR
TEMP SEN
OSCILLATOR
RAMP
GENERATOR
SOFT-
START
AV = 34.5
DIFF
AMP
EAN
EAOUT
OVI
ERROR
AMP
MUXQ
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Once the MAX16821A–MAX16821C are synchronized to
an external clock, the external clock cannot be removed if
reliable operation is to be maintained.
Control LoopThe MAX16821A–MAX16821C use an average current-
mode control scheme to regulate the output current
(Figure 2). The main control loop consists of an inner
current regulation loop for controlling the inductor current
and an outer current regulation loop for regulating the
LED current. The inner current regulation loop absorbs
the double pole of the inductor and output capacitor com-
bination reducing the order of the outer current regulation
loop to that of a single-pole system. The inner current
regulation loop consists of a current-sense resistor (RS),
a current-sense amplifier (CSA), a current-error amplifier
(CEA), an oscillator providing the carrier ramp, and a
PWM comparator (CPWM) (Figure 2). The MAX16821A–
MAX16821C outer LED-current control loop consists of
a differential amplifier (DIFF), a reference voltage, and a
voltage-error amplifier (VEA).
Inductor Current-Sense AmpliierThe differential current-sense amplifier (CSA) provides
a 34.5V/V DC gain. The typical input offset voltage of
the current-sense amplifier is 0.1mV with a 0 to 5.5V
common-mode voltage range (VIN = 7V to 28V). The cur-
rent-sense amplifier senses the voltage across RS. The
maximum common-mode voltage is 3.2V when VIN = 5V.
Inductor Peak-Current ComparatorThe peak-current comparator provides a path for fast
cycle-by-cycle current limit during extreme fault condi-
tions, such as an inductor malfunction (Figure 3). Note the
average current-limit threshold of 27.5mV still limits the
output current during short-circuit conditions. To prevent
inductor saturation, select an inductor with a saturation
current specification greater than the average current limit.
The 60mV threshold for triggering the peak-current limit is
twice the full-scale average current-limit voltage threshold.
The peak-current comparator has only a 260ns delay.
Current-Error AmpliierThe MAX16821A–MAX16821C include a transconduc-
tance current-error amplifier with a typical gm of 550µS
and 320µA output sink and source capability. The current-
RIN
DIFF
SENSE+
SENSE-
EANEAOUTCSNCSPCLP
CCZ
CCP
RCFRLS
LED
STRING
COUT
DIFF
VREF
VEA
CEA
CPWM
DRIVER
MODE = SGND
VIN
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
error amplifier output (CLP) is connected to the inverting
input of the PWM comparator. CLP is also externally
accessible to provide frequency compensation for the
inner current regulation loop (Figure 2). Compensate CEA
so the inductor current negative slope, which becomes
the positive slope to the inverting input of the PWM com-
parator, is less than the slope of the internally generated
voltage ramp (see the Compensation section). In applica-
tions without synchronous rectification, the LED driver
can be turned off and on instantaneously by shorting or
opening the CLP to ground.
PWM Comparator and R-S Flip-FlopAn internal PWM comparator sets the duty cycle by
comparing the output of the current-error amplifier to a
2VP-P ramp signal. At the start of each clock cycle, an
R-S flip-flop resets and the high-side driver (DH) turns on
if MODE is connected to SGND, and DL turns on if MODE
is connected to VCC. The comparator sets the flip-flop as
soon as the ramp signal exceeds the CLP voltage, thus
terminating the ON cycle. See Figure 3.
Differential AmpliierThe differential amplifier (DIFF) allows LED current sens-
ing (Figure 2). It provides true-differential LED current
sensing, and amplifies the sense voltage by a factor of 1
(MAX16821A), 6 (MAX16821B), and 20 (MAX16821C),
while rejecting common-mode voltage errors. The VEA
provides the difference between the differential ampli-
fier output (DIFF) and the desired LED current-sense
voltage. The differential amplifier has a bandwidth of
1.7MHz (MAX16821A), 1.6MHz (MAX16821B), and
550kHz (MAX16821C). The difference between SENSE+
and SENSE- is regulated to +0.6V (MAX16821A), +0.1V
(MAX16821B), or +0.03V (MAX16821C).
Voltage-Error Ampliier (VEA)The VEA sets the gain of the voltage control loop, and
determines the error between the differential amplifier
Figure 3. MAX16821A–MAX16821C Phase Circuit
PWM
COMPARATOR
CLK
SHDN
RAMP
CSP
CLP
CSN
MODE = GNDAV = 34.5
60mV
gm = 550SQQ
PEAK-CURRENT
COMPARATOR
BST
VDD
PGND
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
output and the internal reference voltage. The VEA output
clamps to 0.93V relative to the internal common- mode
voltage, VCM (+0.6V), limiting the average maximum
current. The maximum average current-limit threshold is
equal to the maximum clamp voltage of the VEA divided by
the gain (34.5) of the current-sense amplifier. This results
in accurate settings for the average maximum current.
MOSFET Gate DriversThe high-side (DH) and low-side (DL) drivers drive the
gates of external n-channel MOSFETs. The drivers’ 4A
peak sink- and source-current capability provides ample
drive for the fast rise and fall times of the switching
MOSFETs. Faster rise and fall times result in reduced
cross-conduction losses. Size the high-side and low-side
MOSFETs to handle the peak and RMS currents during
overload conditions. The driver block also includes a logic
circuit that provides an adaptive nonoverlap time to pre-
vent shoot-through currents during transition. The typical
nonoverlap time is 35ns between the high-side and low-
side MOSFETs.
BSTThe MAX16821A–MAX16821C provide power to the low-
side and high-side MOSFET drivers through VDD. A boot-
strap capacitor from BST to LX provides the additional
boost voltage necessary for the high-side driver. VDD sup-
plies power internally to the low-side driver. Connect a
0.47µF low-ESR ceramic capacitor between BST and LX
and a Schottky diode from BST to VDD.
ProtectionThe MAX16821A–MAX16821C include output overvolt-
age protection (OVP). During fault conditions when the
load goes to high impedance (output opens), the control-
ler attempts to maintain LED current. The OVP disables
the MAX16821A–MAX16821C whenever the output volt-
age exceeds the OVP threshold, protecting the external
circuits from undesirable voltages.
Current LimitThe error amplifier (VEA) output is clamped between
-0.050V and +0.93V with respect to common-mode
voltage (VCM). Average current-mode control limits the
average current sourced by the converter during a fault
condition. When a fault condition occurs, the VEA output
clamps to +0.93V with respect to the common-mode volt-
age (0.6V) to limit the maximum current sourced by the
converter to ILIMIT = 0.0275 / RS.
Overvoltage ProtectionThe OVP comparator compares the OVI input to the
overvoltage threshold. The overvoltage threshold is typi-
cally 1.127 times the internal 0.6V reference voltage
plus VCM (0.6V). A detected overvoltage event trips the
comparator output turning off both high-side and low-side
MOSFETs. Add an RC delay to reduce the sensitivity of
the overvoltage circuit and avoid unnecessary tripping of
the converter (Figure 4). After the OVI voltage falls below
1.076V (typ.), high-side and low-side drivers turn on only
after a 2048 clock-cycle POR and a 1024 clock-cycle soft-
start have elapsed. Disable the overvoltage function by
connecting OVI to SGND.
Figure 4. Overvoltage-Protection Input Delay
MAX16821A
MAX16821B
MAX16821COVI
DIFF
EAN
EAOUT
COVI
VOUT
RIN
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
