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19-4972; Rev 1; 12/09
TION KIT EVALUA BLE AVAILA
Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
General Description
The MAX16838 is a dual-channel LED driver that integrates both the DC-DC switching boost regulator and two 150mA current sinks. A current-mode switching DC-DC controller provides the necessary voltage to both strings of HB LEDs. The MAX16838 accepts a wide 4.75V to 40V input voltage range and directly withstands automotive load-dump events. For a 5V Q10% input voltage, connect VIN to VCC. The wide input range allows powering HB LEDs for small-to-medium-sized LCD displays in automotive and display backlight applications. An internal current-mode switching DC-DC controller supports the boost or SEPIC topologies and operates in an adjustable frequency range between 200kHz and 2MHz. The current-mode control provides fast response and simplifies loop compensation. The MAX16838 also features an adaptive output-voltage adjustment scheme that minimizes the power dissipation in the LED current sink paths. The MAX16838 can be combined with the MAX15054 to achieve a buck-boost LED driver with two integrated current sinks. The channel current is adjustable from 20mA to 150mA using an external resistor. The external resistor sets both channel currents to the same value. The device allows connecting both strings in parallel to achieve a maximum current of 300mA in a single channel. The MAX16838 also features pulsed dimming control with minimum pulse widths as low as 1Fs, on both channels through a logic input (DIM). The MAX16838 includes an output overvoltage protection, open LED, shorted LED detection and overtemperature protection. The device operates over the -40NC to +125NC automotive temperature range. The MAX16838 is available in the 20-pin TSSOP and 4mm x 4mm, 20-pin TQFN packages.
Features
S Integrated, 2-Channel, 20mA to 150mA Linear LED Current Sinks S Boost or SEPIC Power Topologies for Maximum Flexibility S Adaptive Voltage Optimization to Minimize Power Dissipation in Linear Current Sinks S 4.75V to 40V or 5V 10% Input Operating Voltage Range S 5000:1 PWM Dimming at 200Hz S Open-Drain Fault Indicator Output S LED Open/Short Detection and Protection S Output Overvoltage and Overtemperature Protection S Programmable LED Current Foldback at Lower Input Voltages S 200kHz to 2MHz Resistor Programmable Switching Frequency with External Synchronization S Current-Mode Control Switching Stage with Internal Slope Compensation S Enable Input S Thermally Enhanced, 20-Pin TQFN (4mm x 4mm) and 20-Pin TSSOP Packages
MAX16838
Ordering Information
PART MAX16838ATP+ MAX16838ATP/V+ MAX16838AUP+ MAX16838AUP/V+ TEMP RANGE -40NC to +125NC -40NC to +125NC -40NC to +125NC -40NC to +125NC PIN-PACKAGE 20 TQFN-EP* 20 TQFN-EP* 20 TSSOP-EP* 20 TSSOP-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. /V denotes an automotive qualified part.
Simplified Schematic
4.75V TO 40V CIN L D COUT
Applications
Automotive Display Backlights LCD Display Backlights Automotive Lighting Applications
R2OV
IN EN CFB VCC DRV
DRAIN
OV NDRV GATE OUT1
R1OV
LED STRINGS
MAX16838
OUT2 ISET FLT
RISET
DIM COMP CCOMP RCOMP SGND PGND
CS RT LEDGND RRT RCS
Typical Operating Circuit and Pin Configurations appear at end of data sheet. _______________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
ABSOLUTE MAXIMUM RATINGS
IN, OUT_, DRAIN to SGND ...................................-0.3V to +45V EN to SGND ...............................................-0.3V to (VIN + 0.3V) PGND to SGND ....................................................-0.3V to +0.3V LEDGND to SGND ...............................................-0.3V to +0.3V DRV to PGND .......... -0.3V to the lower of (VIN + 0.3V) and +6V GATE to PGND ........................................................-0.3V to +6V NDRV to PGND .......................................-0.3V to (VDRV + 0.3V) VCC, FLT, DIM, CS, OV, CFB, to SGND .................-0.3V to +6V RT, COMP, ISET to SGND.........................-0.3V to (VCC + 0.3V) DRAIN and CS Continuous Current .................................. Q2.5A OUT_ Continuous Current ................................................175mA VDRV Short-Circuit Duration .....................................Continuous Continuous Power Dissipation (TA = +70NC) 20-Pin TQFN (derate 25.6mW/NC above +70NC) (Note 1).............................................................................2051mW Junction-to-Case Thermal Resistance (BJC) ............... +6NC/W Juction-to-Ambient Thermal Resistance (BJA) .......... +39NC/W 20-Pin TSSOP (derate 26.5mW/NC above +70NC) (Note 1) ......................................................................2122mW Junction-to-Case Thermal Resistance (BJC) ............... +2NC/W Junction-to-Ambient Thermal Resistance (BJA) ..... +37.7NC/W Operating Temperature Range ........................ -40NC to +125NC Junction Temperature .....................................................+150NC Storage Temperature Range............................ -65NC to +150NC Soldering Temperature (reflow) ......................................+260NC
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial.
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.
ELECTRICAL CHARACTERISTICS
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, DRAIN, COMP, OUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = 25NC.) (Note 2) PARAMETER Input Voltage Range Input Voltage Range Quiescent Supply Current Standby Supply Current Undervoltage Lockout Undervoltage Lockout Hysteresis DRV REGULATOR Output Voltage Dropout Voltage Short-Circuit Current Limit VCC Undervoltage Lockout Threshold VCC (UVLO) Hysteresis RT OSCILLATOR Switching Frequency Range Duty Cycle fSW DMAX fSW = 200kHz fSW = 2000kHz 200 87 83 90 85 2000 95 91 kHz % % UVLOVCC VDRV 5.75V < VIN < 10V, 0.1mA < ILOAD < 30mA 6.5V < VIN < 40V, 0.1mA < ILOAD < 3mA 4.75 5 0.11 97 3.4 4.0 123 4.4 5.25 0.5 V V mA V mV SYMBOL VIN VIN IQ ISH UVLOIN VIN = VCC VDIM = 5V VEN = SGND (Note 3) VIN rising, VDIM = 5V 4 CONDITIONS Internal LDO on MIN 4.75 4.55 3.1 15.5 4.3 177 TYP MAX 40 5.5 5 40 4.55 UNITS V V mA FA V mV
VDO VIN = 4.75V, IOUT = 30mA (VIN - VDRV) DRV shorted to GND VCC rising
2
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, DRAIN, COMP, OUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = 25NC.) (Note 2) PARAMETER Oscillator Frequency Accuracy Synchronization Logic-High Synchronization Logic-Low Logic-Level Before SYNC Capacitor Synchronization Pulse Width SYNC Frequency Range PWM COMPARATOR Leading-Edge Blanking Propagation Delay to NDRV SLOPE COMPENSATION Slope Compensation Peak Voltage per Cycle CS LIMIT COMPARATOR CS Threshold Voltage CS Limit Comparator Propagation Delay to NDRV CS Input Current ERROR AMPLIFIER OUT_ Regulation Voltage Transconductance No-Load Gain COMP Sink Current COMP Source Current MOSFET DRIVER NDRV On-Resistance Peak Sink Current Peak Source Current POWER MOSFET Power Switch On-Resistance Switch Leakage Current Switch Gate Charge ISWITCH = 0.5A, VGS = 5V VDRAIN = 40V, VGATE = 0V VDRAIN = 40V, VGS = 4.5V 0.15 0.003 3.1 0.35 1.2 I FA nC ISINK = 100mA, VIN > 5.5V ISOURCE = 100mA, VIN > 5.5V VNDRV = 5V VNDRV = 0V 1.5 1.5 0.8 0.8 4 4 I I A A Gm A ISINK ISOURCE (Note 4) VDIM = VOUT_ = 5V, VCOMP = 3V VDIM = 5V, VOUT_ = VCOMP = 0V VDIM = 5V 0.9 340 1 600 50 400 400 800 800 1.1 880 V FS dB FA FA ICS VCS_MAX VCOMP = 3V 10mV overdrive (including leading-edge blanking time) 0 P VCS P 0.35V -1.3 285 300 100 +0.5 315 mV ns FA Voltage ramp added to CS 0.12 V Including leading-edge blanking time 66 100 ns ns fSYNC SYMBOL VRT rising VRT falling 3.1 50 1.1 x fSW 1.5 x fSW CONDITIONS fSW = 200kHz to 2MHz MIN -7.5 1.8 2.5 3.8 TYP MAX +7.5 3.6 UNITS % V V V ns Hz
MAX16838
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3
Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, DRAIN, COMP, OUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = 25NC.) (Note 2) PARAMETER LED CURRENT SINKS OUT_ Current Range LED Strings Current Matching Maximum Peak-to-Peak Boost Ripple IOUT_ VDIM = 5V, VOUT_ = 1.0V IOUT_ = 100mA, RISET = 15kI 1% IOUT variation, IOUT = 100mA, RISET = 15kI IOUT_ = 100mA, RISET = 15kI TA = +25NC TA = -40NC to +125NC 97 95 18.7 0.3 100 100 20 1 1.23 0 P VCFB P 1.3V VENHI VEN_HYS VEN = 40V VIH VIL VDIM_HYS IDIM VDIM = 5V or 0 VDIM rising edge to 90% of set current VDIM falling edge to 10% of set current tR tF Rise time measured from 10% to 90% Fall time measured from 90% to 10% 3.1 3.55 6 6.8 -600 50 10 290 121 120 50 110 +100 1000 700 600 500 5.5 4.85 9.5 8.6 -500 2.1 0.8 VEN rising -0.3 1.1 1.24 71 +50 +700 +0.3 1.34 20 150 Q2 0.5 103 105 21.3 300 mA % V mA mA mA nA V FA V mV nA V V mV nA ns ns ns ns SYMBOL CONDITIONS MIN TYP MAX UNITS
Output Current Accuracy OUT_ Leakage Current Current Foldback Threshold Voltage CFB Input Bias Current ENABLE COMPARATOR (EN) Enable Threshold Enable Threshold Hysteresis Enable Input Current DIM LOGIC DIM Input Logic-High DIM Input Logic-Low Hysteresis DIM Input Current DIM to LED Turn-On Time DIM to LED Turn-Off Time IOUT_ Rise Time IOUT_ Fall Time LED FAULT DETECTION LED Shorted Fault Indicator Threshold LED String Shorted Shutoff Threshold Shorted LED Detection FLAG Delay
IOUT_ = 20mA, RISET TA = -40NC to +125NC = 75kI VDIM = 0V, VOUT_ = 40V
TA = +125NC TA = +125NC
4.2 7.7 6
V V Fs
4
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, DRAIN, COMP, OUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = 25NC.) (Note 2) PARAMETER FLT LOGIC Output-Voltage Low Output Leakage Current OVERVOLTAGE PROTECTION OV Trip Threshold OV Hysteresis OV Input Bias Current THERMAL SHUTDOWN Thermal Shutdown Thermal Shutdown Hysteresis 165 15
oC oC
MAX16838
SYMBOL VOL
CONDITIONS VIN = 4.75V and ISINK = 5mA VFILT = 5.5V VOV rising 0 P VOV P 1.3V
MIN
TYP
MAX 0.4
UNITS V nA V mV nA
-300 1.19 -100 1.23 70
+300 1.265 +100
Note 2: All devices are 100% tested at TA = +125NC. Limits over temperature are guaranteed by design, not production tested. Note 3: The shutdown current does not include currents in the OV and CFB resistive dividers. Note 4: Gain = DVCOMP/DVCS, 0.05V < VCS < 0.15V.
Typical Operating Characteristics
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, VDRAIN = VCOMP = VOUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = +25NC, unless otherwise noted.)
SWITCHING WAVEFORM AT 200Hz (50% DUTY CYCLE)
IIN vs. SUPPLY VOLTAGE
MAX16838 toc02
MAX16838 toc01
5.0 4.5
VLX
10V/div IIIN (mA) 0V 4.0 3.5 3.0 TA = -40C 2.5 2.0 1ms/div 4 8 12 16 20 24 28 SUPPLY VOLTAGE (V) VIN = 12V 32 36 TA = +125C TA = +25C
ILED
100mA/div 0A
VOUT
20V/div 0V
40
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5
Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
Typical Operating Characteristics (continued)
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, VDRAIN = VCOMP = VOUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = +25NC, unless otherwise noted.)
IIN vs. FREQUENCY
MAX16838 toc03
SWITCHING FREQUENCY vs. TEMPERATURE
359 SWITCHING FREQUENCY (kHz) 358 357 356 355 354 353 352 351 350 VIN = 12V -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
MAX16838 toc04
10 9 8 7 IIN (mA) 6 5 4 3 2 1 0 VIN = 12V
360
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 FREQUENCY (MHz)
VISET vs. TEMPERATURE
MAX16838 toc05
VISET vs. ILED
VIN = 12V 1.230 1.230 1.229 1.229 1.229
MAX16838 toc06
1.223 1.222 1.221 VISET (V) 1.220 1.219 1.218 1.217 1.216 1.215 VIN = 12V VDIM = 0V
1.230
VISET (V)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
20
40
60
80
100
120
140
160
ILED (mA)
VEN_TH vs. TEMPERATURE
MAX16838 toc07
EN LEAKAGE CURRENT vs. TEMPERATURE
MAX16838 toc08
1.30
300 250 EN LEAKAGE CURRENT (nA) 200 150 100 50 VEN = 12V VEN = 40V
1.25 VEN_TH (V)
VEN RISING
1.20
VEN FALLING
1.15
1.10 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
0 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
6
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
Typical Operating Characteristics (continued)
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, VDRAIN = VCOMP = VOUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = +25NC, unless otherwise noted.)
MAX16838
DRV LINE REGULATION
MAX16838 toc09
DRV LOAD REGULATION
5.005 5.000 4.995 VDRV (V) 4.990 4.985 4.980 TA = -40C TA = +25C TA = +125C
MAX16838 toc10
5.010 5.005 DRV VOLTAGE (V) 5.000 4.995 4.990 4.985 4.980 4.975 0 10 20 30 40 TA = -40C TA = +125C TA = +25C
5.010
4.975 4.970 4.965 50 0 5 10 15 LOAD (mA) 20 VIN = 12V 25 30
INPUT VOLTAGE (V)
FREQUENCY vs. RRT
VIN = 12V 2.0 FREQUENCY(MHz) 1.5 1.0 0.5 0 0 4 8 12 16 20 24 28 32 36 40 RRT (kI)
MAX16838 toc11
LODIM MODE RESPONSE
MAX16838 toc12
2.5
VIN VDIM
10V/div 0V 5V/div 0V 100mA/div
IOUT_
0A 20V/div
VLED_
DIM ON-TIME < 5 x fSW 20ms/div
0V
LED SWITCHING WITH DIM AT 200Hz (50% DUTY CYCLE)
MAX16838 toc13
ILED vs. RISET
10mA/div 0A ILED (mA) 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 RISET (kI) VIN = 12V
MAX16838 toc14
IOUT1
IOUT2
100mA/div 0A
VDIM
5V/div 0V 2ms/div
10 15 20 25 30 35 40 45 50 55 60 65 70 75
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
Typical Operating Characteristics (continued)
(VIN = VEN = 12V, RRT = 12.2kI, RISET = 15kI, CVCC = 1FF, VCC = VDRV = VCFB, VDRAIN = VCOMP = VOUT_, FLT = unconnected, VOV = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, VGATE = VNDRV, TA = +25NC, unless otherwise noted.)
COMP LEAKAGE CURRENT vs. TEMPERATURE
3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
MAX16838 toc15
OUT_ LEAKAGE CURRENT vs. TEMPERATURE
55 50 45 40 35 30 25 20 15 10 5 0 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C) VIN = 12V VEN = HIGH
MAX16838 toc16 MAX16838 toc18
60 VOUT = 40V VOUT = 12V
COMP LEAKAGE CURRENT (nA)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
OUT_ LEAKAGE CURRENT (nA)
VIN = 12V VEN = HIGH VCOMP = 2V VDIM = LOW
OV LEAKAGE CURRENT vs. TEMPERATURE
MAX16838 toc17
POWER MOSFET RDSON vs. TEMPERATURE
0.50 0.45 POWER MOSFET RDSON (I) 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 VIN = 12V
2.0 1.5 OV LEAKAGE CURRENT (nA) 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 VIN = 12V VEN = HIGH
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
8
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
Pin Description
PIN TQFN 1 TSSOP 4 NAME NDRV FUNCTION Gate Drive for Switching MOSFET. Connect NDRV to GATE directly or through a resistor to control the rise and fall times of the gate drive. 5V Regulator Output. MOSFET gate-driver supply input. Bypass DRV to PGND with a minimum of 1FF ceramic capacitor. Place the capacitor as close as possible to DRV and PGND. Internal Circuitry Supply Voltage. Bypass VCC to SGND with a minimum of 0.1FF ceramic capacitor. Place the capacitor as close as possible to VCC and SGND. Supply Input. Connect a 4.75V to 40V supply to IN. Bypass IN to PGND with a minimum of 1FF ceramic capacitor. For a 5V Q10% supply voltage, connect VIN to VCC. Enable/Undervoltage Lockout (UVLO) Threshold Input. EN is a dual-function input. Connect EN to VIN through a resistor-divider to program the UVLO threshold. Signal Ground. SGND is the current return path connection for the low-noise analog signals. Connect SGND, LEDGND, and PGND at a single point. Current Foldback Reference Input. Connect a resistor-divider between IN, CFB, and ground to set the current foldback threshold. When the voltage at CFB goes below 1.23V, the LED current starts reducing linearly. Connect to VCC to disable the current foldback feature. Overvoltage Threshold Adjust Input. Connect a resistor-divider from the switching converter output to OV and SGND. The OV comparator reference is internally set to 1.23V. LED Current Adjust Input. Connect a resistor RISET from ISET to SGND to set the current through each LED string (ILED) according to the formula ILED = 1512V/RISET. Open-Drain, Active-Low Flag Output. FLT asserts when there is an open/short-LED condition at the output or when there is a thermal shutdown event. LED String Cathode Connection 2. OUT2 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT2. OUT2 sinks up to 150mA. LED Ground. LEDGND is the return path connection for the linear current sinks. Connect SGND, LEDGND, and PGND at a single point. LED String Cathode Connection 1. OUT1 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT1. OUT1 sinks up to 150mA. Oscillator Timing Resistor Connection. Connect a timing resistor (RRT) from RT to SGND to program the switching frequency. Apply an AC-coupled external clock at RT to synchronize the switching frequency with an external clock source. Switching Converter Compensation Input. Connect an RC network from COMP to SGND (see the Feedback Compensation section).
MAX16838
2
5
DRV
3 4 5 6
6 7 8 9
VCC IN EN SGND
7
10
CFB
8 9 10
11 12 13
OV ISET FLT
11
14
OUT2
12
15
LEDGND
13
16
OUT1
14
17
RT
15
18
COMP
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9
Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
Pin Description (continued)
PIN TQFN 16 17 18 19 20 TSSOP 19 20 1 2 3 NAME DIM CS DRAIN GATE PGND Digital PWM Dimming Input Current-Sense Input. CS is the current-sense input for the switching regulator and is also connected to the source of the internal power MOSFET. Connect a sense resistor from CS to PGND to set the switching current limit. Internal Switching MOSFET Drain Output Internal Switching MOSFET Gate Input. Connect GATE to NDRV directly or through a resistor to control the rise and fall times of the gate drive. Power Ground. PGND is the high-switching current return path connection. Connect SGND, LEDGND, and PGND at a single point. Exposed Pad. EP is internally connected to SGND. Connect EP to a large-area contiguous ground plane for effective power dissipation. Connect EP to SGND. Do not use as the only ground connection. FUNCTION
--
--
EP
10
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
Simplified Functional Diagram
FLT DRAIN DRV POK SHDN DIM FLAG LOGIC CS GATE NDRV PGND COMP RT 1.8V MINIMUM STRING VOLTAGE GM ILIM 0 1 1 0 1.0V SOFTSTART DAC 1.17V OV COMPARATOR IN DIM DUTY TOO LOW OV LOGIC ARRAY = 2 OUT_ DRIVER PWM LOGIC SHORT-LED DETECTOR OPEN-LED DETECTOR
MAX16838
PWM COMP
RT OSCILLATOR
120mV SLOPE COMPENSATION
0.3V
CS
CS BLANKING
DIM
UVLO DRV VCC 1 UVLO THERMAL SHUTDOWN 0 VBG VCC
5V LDO
BANDGAP
EN
IN SHDN
POK
VBG VBG
MAX16838
SGND
LEDGND
OV
CFB
ISET
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11
Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
Detailed Description
The MAX16838 high-efficiency, HB LED driver integrates all the necessary features to implement a highperformance backlight driver to power LEDs in small-tomedium-sized displays for automotive as well as general applications. The device provides load-dump voltage protection up to 40V in automotive applications. The MAX16838 incorporates a DC-DC controller with peak current-mode control to implement a boost, coupled inductor boost-buck, or SEPIC-type switched-mode power supply and a 2-channel LED driver with 20mA to 150mA constant current-sink capability per channel. The MAX16838 can be combined with the MAX15054 to achieve boost-buck topology without a coupled inductor (see Figure 5). The MAX16838 features a constant-frequency peak current-mode control with internal slope compensation to control the duty cycle of the PWM controller. The DC-DC converter generates the required supply voltage for the LED strings from a wide input supply range. Connect LED strings from the DC-DC converter output to the 2-channel constant current sinks that control the current through the LED strings. A single resistor connected from ISET to ground sets the forward current through both LED strings. The MAX16838 features adaptive LED voltage control that adjusts the converter output voltage depending on the forward voltage of the LED strings. This feature minimizes the voltage drops across the constant currentsinks and reduces power dissipation in the device. The MAX16838 provides a very wide PWM dimming range where a dimming pulse as narrow as 1Fs is possible at a 200Hz dimming frequency. A logic input (EN) shuts down the device when pulled low. The device includes an internal 5V LDO to power up the internal circuitry and drive the internal switching MOSFET. The MAX16838 includes output overvoltage protection that limits the converter output voltage to the programmed OV threshold in the event of an open-LED condition. The device also features an overtemperature protection that shuts down the controller if the die temperature exceeds +165NC. In addition, the MAX16838 has a shorted LED string detection and an open-drain FLT signal to indicate open LED, shorted LED, and overtemperature conditions. The MAX16838 uses current-mode control to provide the required supply voltage for the LED strings. The internal MOSFET is turned on at the beginning of every switching cycle. The inductor current ramps up linearly until it is turned off at the peak current level set by the feedback loop. The peak inductor current is sensed from the voltage across the current-sense resistor, RCS, connected from the source of the internal MOSFET to PGND. A PWM comparator compares the current-sense voltage plus the internal slope compensation signal with the output of the transconductance error amplifier. The controller turns off the internal MOSFET when the voltage at CS exceeds the error amplifier's output voltage. This process repeats every switching cycle to achieve peak current-mode control. Error Amplifier The internal error amplifier compares an internal feedback (FB) signal with an internal reference voltage (VREF) and regulates its output to adjust the inductor current. An internal minimum string detector measures the minimum LED string cathode voltage with respect to SGND. During normal operation, this minimum VOUT_ voltage is regulated to 1V through feedback. The resulting DC-DC converter output voltage is 1V above the maximum required total LED voltage. The converter stops switching when LED strings are turned off during PWM dimming. The error amplifier is disconnected from the COMP output to retain the compensation capacitor charge. This allows the converter to settle to a steady-state level immediately when the LED strings are turned on again. This unique feature provides fast dimming response without having to use large output capacitors. If the PWM dimming on-pulse is less than five switching cycles, the feedback controls the voltage on OV such that the converter output voltage is regulated at 95% of the OV threshold. This mode ensures that narrow PWM dimming pulses are not affected by the response time of the converter. During this mode, the error amplifier remains continuously connected to the COMP output. Adaptive LED Voltage Control The MAX16838 reduces power dissipation using an adaptive LED voltage control scheme. The adaptive LED voltage control regulates the DC-DC converter output based on the operating voltage of the LED strings.
Current-Mode DC-DC Controller
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
The voltage at each of the current-sink outputs (OUT_) is the difference between the DC-DC regulator output voltage (VLED) and the total forward voltage of the LED string connected to the output (OUT_). The DC-DC converter then adjusts VLED until the output channel with the lowest voltage at OUT_ is 1V relative to LEDGND. As a result, the device minimizes power dissipation in the current sinks and still maintains LED current regulation. For efficient adaptive control functionality, use an equal number of HB LEDs of the same forward voltage rating in each string. Current Limit The MAX16838 includes a fast current-limit comparator to terminate the on-cycle during an overload or a fault condition. The current-sense resistor (RCS) connected between the source of the internal MOSFET and ground sets the current limit. The CS input has a 0.3V voltage trip level (VCS). Use the following equation to calculate RCS: RCS = (VCS)/IPEAK where IPEAK is the peak current that flows through the MOSFET. Undervoltage Lockout The MAX16838 features two undervoltage lockouts: UVLOIN and UVLOVCC. The undervoltage lockout threshold for VIN is 4.3V (typ) and the undervoltage lockout threshold for VCC is 4V (typ). Soft-Start The MAX16838 features a soft-start that activates during power-up. The soft-start ramps up the output of the converter in 64 steps in a period of 100ms typically, unless both strings reach regulation point, in which case the softstart would terminate to resume normal operation immediately. Once the soft-start is over, the internal soft-start circuitry is disabled and the normal operation begins. Oscillator Frequency/External Synchronization The MAX16838 oscillator frequency is programmable between 200kHz and 2MHz using one external resistor (RRT) connected between RT and SGND. The PWM MOSFET driver output switching frequency is the same as the oscillator frequency. The oscillator frequency is determined using the following formula: fSW = (7.342X109/RRT)(Hz) where RRT is in . Synchronize the oscillator with an external clock by AC-coupling the external clock to the RRT input. The capacitor used for the AC-coupling should satisfy the following relation:
MAX16838
9862 CSYNC - 0.144 x 10-3 (F) R T
where RRT is in I. The pulse width for the synchronization signal should satisfy the following relations:
tPW VS < 0.8 tCLK tPW VS + VS > 3.4 0.8 - tCLK
where tPW is the synchronization source pulse width, tCLK is the synchronization clock time period, and VS is the synchronization pulse voltage level. See Figure 1. 5V LDO Regulator (DRV) The internal LDO regulator converts the input voltage at IN to a 5V output voltage at DRV. The LDO regulator output supports up to 30mA current, enough to provide power to the internal control circuitry and the gate driver.
VS
tPW tCLK
Figure 1. Synchronizing External Clock Signal
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
Connect a 4.7I resistor from VCC to DRV to power the rest of the chip from the VCC pin with the 5V internal regulator. Bypass DRV to PGND with a minimum of 1FF ceramic capacitor as close as possible to the device. For input voltage range of 4.5V to 5.5V, connect IN to VCC. The MAX16838 features two identical constant-current sources used to drive multiple HB LED strings. The current through each of the channels is adjustable between 20mA and 150mA using an external resistor (RISET) connected between ISET and SGND. Select RISET using the following formula: RISET = 1512 IOUT _ () The duty cycle of the PWM signal applied to DIM also controls the DC-DC converter's output voltage. If the turn-on duration of the PWM signal is less than five oscillator clock cycles, then the boost converter regulates its output based on feedback from the OV input. During this mode, the converter output voltage is regulated to 95% of the OV threshold voltage. If the turn-on duration of the PWM signal is greater than or equal to six oscillator clock cycles, then the converter regulates its output such that the minimum voltage at OUT_ is 1V. The MAX16838 fault protections include cycle-by-cycle current limiting, DC-DC converter output overvoltage protection, open-LED detection, short-LED detection, and overtemperature detection. An open-drain LED fault flag output (FLT) goes low when an open-LED/short-LED or overtemperature condition is detected. Open-LED Management and Overvoltage Protection The MAX16838 monitors the drains of the current sinks (OUT_) to detect any open string. If the voltage at any output falls below 300mV and the OV threshold is triggered (i.e., even with OUT_ at the OV voltage the string is not able to regulate above 300mV), then the MAX16838 interprets that string to be open, asserts FLT, and disconnects that string from the operation loop. The MAX16838 features an adjustable overvoltage threshold input, OV. Connect a resistor-divider from the switching converter output to OV and SGND to set the overvoltage threshold level. Use the following formula to program the overvoltage threshold: R1 VOV = 1.23V x 1 + OV R2OV
40mA TO 300mA
LED Current Control (ISET)
Fault Protections
where IOUT_ is the desired output current for both channels in amps. For single-channel operation, connect channel 1 and channel 2 together. See Figure 2. The MAX16838 features LED brightness control using an external PWM signal applied at DIM. The device accepts a minimum pulse width of 1Fs. Therefore, a 5000:1 dimming ratio is achieved when using a PWM frequency of 200Hz. Drive DIM high to enable both LED current sinks and drive DIM low to disable both LED current sinks.
BOOST CONVERTER OUTPUT
LED Dimming Control
OUT1
MAX16838
OUT2
Figure 2. Configuration for Higher LED String Current
Short-LED Detection The MAX16838 features a two-level short-LED detection circuitry. If level 1 short is detected on any one of the strings, FLT is asserted. A level 1 short is detected if the difference between the total forward LED voltages of the two strings exceeds 4.2V (typ). If a level 2 short is detected on any one of the strings, the particular LED string with the short is turned off after 6Fs and FLT is asserted. A level 2 short is detected if the difference between the total forward LED voltages of the two strings exceeds 7.8V (typ). The strings are reevaluated on each DIM rising edge and FLT is deasserted if the short is removed.
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
EN is a logic input that completely shuts down the device when connected to logic-low, reducing the current consumption of the device to less than 15FA (typ). The logic threshold at EN is 1.24V (typ). The voltage at EN must exceed 1.24V before any operation can commence. There is a 71mV hysteresis on EN. The EN input also allows programming the supply input UVLO threshold using an external voltage-divider to sense the input voltage as shown in Figure 3. Use the following equation to calculate the value of R1EN and R2EN in Figure 3: VON R1EN = - 1 x R2EN VUVLOIN where VUVLOIN is the EN rising threshold, 1.24V, and VON is the desired input startup voltage. Choose an R2EN between 10kI and 50kI. Connect EN to IN if not used. Current Foldback The MAX16838 includes a current-foldback feature to limit the input current at low VIN. Connect a resistordivider between IN, CFB, and SGND to set the currentfoldback threshold. When the voltage at CFB goes below 1.23V, then the LED current starts reducing proportionally to VCFB. This feature can be used for analog dimming of the LEDs, too. Connect CFB to VCC to disable this feature.
Enable (EN)
MAX16838
VIN
MAX16838
EN
R1EN
R2EN
1.24V
Figure 3. Setting the MAX16838 Undervoltage Lockout Threshold
Inductor Selection in Boost Configuration Select the maximum peak-to-peak ripple on the inductor current (ILP-P). Use the following equations to calculate the maximum average inductor current (ILAVG) and peak inductor current (ILPEAK): ILAVG = ILED/(1 - DMAX) Assuming ILP-P is 40% of the average inductor current: ILP-P = ILAVG x 0.4 ILPEAK = ILAVG + ILP-P/2 Calculate the minimum inductance value LMIN with the inductor current ripple set to the maximum value. LMIN = VIN_MIN x DMAX/(fSW x ILP-P) Choose an inductor that has a minimum inductance greater than the calculated LMIN and current rating greater than ILPEAK. The recommended saturation current limit of the selected inductor is 10% higher than the inductor peak current. The ILP-P can be chosen to have a higher ripple than 40%. Adjust the minimum value of the inductance according to the chosen ripple. One fact that must be noted is that the slope compensation is fixed and has a 120mV peak per switching cycle. The dv/dt of the slope compensation ramp is 120fSWV/Fs, where fSW is in kHz. After selecting the inductance it is necessary to verify that the slope compensation is adequate to prevent subharmonic oscillations. In the case of the boost, the following criteria must be satisfied: 120fSW > RCS (VLED - 2VIN_MIN)/2L where L is the inductance value in FH, RCS is the current-sense resistor value in , VIN_MIN is the minimum input voltage in V, VLED is the output voltage, and fSW is the switching frequency in kHz. If the inductance value is chosen to keep the inductor in discontinuous conduction mode, the equation above does not need to be satisfied.
Applications Information
First, determine the required input supply voltage range, the maximum voltage needed to drive the LED strings including the minimum 1V across the constant LED current sink (VLED), and the total output current needed to drive the LED strings (ILED). Calculate the maximum duty cycle (DMAX) using the following equation: DMAX = (VLED + VD - VIN_MIN)/(VLED + VD) where VD is the forward drop of the rectifier diode, VIN_MIN is the minimum input supply voltage, and VLED is the output voltage. Select the switching frequency (fSW) depending on the space, noise, dynamic response, and efficiency constraints.
Boost-Circuit Design
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
Output Capacitor Selection in Boost Configuration For the boost converter, the output capacitor supplies the load current when the main switch is on. The required output capacitance is high, especially at higher duty cycles. Calculate the output capacitor (COUT) using the following equation: COUT > (DMAX x ILED)/(VLED_P-P x fSW) where VLED_P-P is the peak-to-peak ripple in the LED supply voltage. Use a combination of low-ESR and highcapacitance ceramic capacitors for lower output ripple and noise. Input Capacitor Selection in Boost Configuration The input current for the boost converter is continuous and the RMS ripple current at the input capacitor is low. Calculate the minimum input capacitor CIN using the following equation: CIN = ILP-P/(8 x fSW x VIN_P-P) where VIN_P-P is the peak-to-peak input ripple voltage. This equation assumes that input capacitors supply most of the input ripple current. Rectifier Diode Selection Using a Schottky rectifier diode produces less forward drop and puts the least burden on the MOSFET during reverse recovery. A diode with considerable reverse-recovery time increases the MOSFET switching loss. Select a Schottky diode with a voltage rating 20% higher than the maximum boost-converter output voltage and current rating greater than that calculated in the following equation: IL AVG ID = 1.2 x 1- D MAX (A) Feedback Compensation The voltage feedback loop needs proper compensation for stable operation. This is done by connecting a resistor RCOMP and capacitor CCOMP in series from COMP to SGND. RCOMP is chosen to set the high-frequency integrator gain for fast transient response while CCOMP is chosen to set the integrator zero to maintain loop stability. For optimum performance, choose the components using the following equations: RCOMP = where fZRHP x RCS x ILED 5 x FP1x GMCOMP x VLED x (1 - DMAX )
fZRHP =
VLED (1 - DMAX )2 2 x L x ILED
is the right half plane zero for the boost regulator. RCS is the current-sense resistor in series with the source of the internal switching MOSFET. ILED is the total LED current that is the sum of the LED currents in both the channels. VLED is the output voltage of the boost regulator. DMAX is the maximum duty cycle that occurs at minimum input voltage. GMCOMP is the transconductance of the error amplifier. FP1 = ILED 2 x x VLED x COUT
is the output pole formed by the boost regulator. Set the zero formed by RCOMP and CCOMP a decade below the crossover frequency. Using the value of RCOMP from above, the crossover frequency is at fZRHP/5. 50 CCOMP = 2 x RCOMP x fZRHP
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
4.75V TO 40V L1 Cs D COUT L2 R2OV CIN
R1OV R2EN IN EN R1EN CVCC RDRV DRV CDRV FLT RCOMP CCOMP DIM COMP SGND PGND LEDGND CS RT RRT RCS MAX16838 CFB VCC DRAIN OV NDRV GATE OUT1 OUT2 RISET ISET
LED STRINGS
Figure 4. SEPIC Configuration
Figure 4 shows a SEPIC application circuit using the MAX16838. The SEPIC topology is necessary to keep the output voltage of the DC-DC converter regulated when the input voltage can rise above and drop below the output voltage. Figure 5 shows a boost-buck configuration with the MAX16838 and the MAX15054. LED driver circuits based on the MAX16838 device use a high-frequency switching converter to generate the voltage for LED strings. Take proper care while laying out the circuit to ensure proper operation. The switchingconverter part of the circuit has nodes with very fast voltage changes that could lead to undesirable effects on the sensitive parts of the circuit. Follow these guidelines to reduce noise as much as possible:
SEPIC Operation
1)
Boost-Buck Configuration
Connect the bypass capacitor on VCC and DRV as close as possible to the device and connect the capacitor ground to the analog ground plane using vias close to the capacitor terminal. Connect SGND of the device to the analog ground plane using a via close to SGND. Lay the analog ground plane on the inner layer, preferably next to the top layer. Use the analog ground plane to cover the entire area under critical signal components for the power converter. Have a power ground plane for the switching-converter power circuit under the power components (input filter capacitor, output filter capacitor, inductor, MOSFET, rectifier diode, and current-sense resistor). Connect PGND to the power ground plane as close to PGND as possible. Connect all other ground connections to the power ground plane using vias close to the terminals.
PCB Layout Considerations
2)
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17
Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
3) There are two loops in the power circuit that carry high-frequency switching currents. One loop is when the MOSFET is on--from the input filter capacitor positive terminal, through the inductor, the internal MOSFET, and the current-sense resistor, to the input capacitor negative terminal. The other loop is when the MOSFET is off--from the input capacitor positive terminal, through the inductor, the rectifier diode, output filter capacitor, to the input capacitor negative terminal. Analyze these two loops and make the loop areas as small as possible. Wherever possible, have a return path on the power ground plane for the switching currents on the top layer copper traces, or through power components. This reduces the loop area considerably and provides a low-inductance path for the switching currents. Reducing the loop area also reduces radiation during switching. 4) Connect the power ground plane for the constantcurrent LED driver part of the circuit to LEDGND as close as possible to the device. Connect SGND to PGND at the same point.
D1
VIN C1
VDD
MAX15054
BST
CBST Q1
HDRV LX
GND
HI
L
D3
COUT
D2 CIN R1EN IN EN R2EN CVCC RDRV DRV CDRV FLT DIM COMP RCOMP CCOMP SGND PGND LEDGND CS RT RRT RCS MAX16838 CFB VCC GATE NDRV DRAIN OV OUT1 OUT2 RISET ISET
R1OV LED STRINGS R2OV
Figure 5. Boost-Buck Configuration
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
Pin Configurations
GATE DIM CS
MAX16838
TOP VIEW
PGND
DRAIN
20
19
18
17
16
DRAIN 1 15 14 COMP RT OUT1 LEDGND OUT2 GATE 2 PGND 3 NDRV 4 DRV 5 VCC 6 IN 7 EN 8 SGND 9 CFB 10
NDRV DRV VCC IN EN
1 2 3 4 5
MAX16838
13 12 *EP 11
6 SGND
7 CFB
8 OV
9 ISET
10 FLT
TQFN
*EXPOSED PAD
Typical Operating Circuit
4.75V TO 40V CIN L D COUT
R2OV
R1OV R2EN IN EN R1EN CVCC RDRV DRV CDRV FLT RCOMP CCOMP DIM COMP SGND PGND LEDGND CS RT RRT RCS MAX16838 CFB VCC DRAIN OV NDRV GATE OUT1 OUT2 RISET ISET
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+ MAX16838
*EP
20 CS 19 DIM 18 COMP 17 RT 16 OUT1 15 LEDGND 14 OUT2 13 FLT 12 ISET 11 OV
+
TSSOP
LED STRINGS
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16838
Chip Information
PROCESS: BiCMOS DMOS
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a "+", "#", or "-" in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE 20 TQFN-EP 20 TSSOP-EP PACKAGE CODE T2044+3 U20E+1 DOCUMENT NO. 21-0139 21-0108
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Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
Revision History
REVISION NUMBER 0 1 REVISION DATE 9/02 12/09 Initial release Added /V part number, updated soldering temperature DESCRIPTION PAGES CHANGED -- 1, 2
MAX16838
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
(c)
21
2009 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.

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