View MAX16814AV_4666771.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

19-4722; Rev 3; 2/10
TION KIT EVALUA BLE AILA AV
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
General Description
The MAX16814 high-efficiency, high-brightness LED (HB LED) driver provides up to four integrated LED currentsink channels. An integrated current-mode switching DC-DC controller drives a DC-DC converter that provides the necessary voltage to multiple strings of HB LEDs. The MAX16814 accepts a wide 4.75V to 40V input voltage range and withstands direct automotive loaddump events. The wide input range allows powering HB LEDs for small to medium-sized LCD displays in automotive and general lighting applications. An internal current-mode switching DC-DC controller supports the boost, coupled-inductor boost-buck, or SEPIC topologies and operates in an adjustable frequency range between 200kHz and 2MHz. It can also be used for single-inductor boost-buck topology in conjunction with the MAX15054 and an additional MOSFET. The current-mode control with programmable slope compensation provides fast response and simplifies loop compensation. The MAX16814 also features an adaptive output-voltage control scheme that minimizes the power dissipation in the LED current-sink paths. The MAX16814 consists of four identical linear current source channels to drive four strings of HB LEDs. The channel current is adjustable from 20mA to 150mA with an accuracy of 3% using an external resistor. The external resistor sets all 4-channel currents to the same value. The device allows connecting multiple channels in parallel to achieve higher current per LED string. The MAX16814 also features pulsed dimming control, on all four channels through a logic input (DIM). In addition,the MAX16814A_ _ and MAX16814U_ _ include a unique feature that allows a very short minimum pulse width as low as 1s. The MAX16814 includes an output overvoltage, openLED detection and protection, programmable shorted LED detection and protection, and overtemperature protection. The device operates over the -40NC to +125NC automotive temperature range. The MAX16814 is available in the 6.5mm x 4.4mm, 20-pin TSSOP and 4mm x 4mm, 20-pin TQFN packages.
Features
S 4-Channel Linear LED Current Sinks with Internal MOSFETs Full-Scale LED Current Adjustable from 20mA to 150mA Drives One to Four LED Strings S Boost, SEPIC, or Coupled-Inductor Boost-Buck Current-Mode DC-DC Controller 200kHz to 2MHz Programmable Switching Frequency External Switching Frequency Synchronization S Adaptive Output-Voltage Optimization to Minimize Power Dissipation S 4.75V to 40V Operating Input Voltage Range S Less than 40A Shutdown Current S 5000:1 PWM Dimming at 200Hz (MAX16814A _ _ and MAX16814U_ _ Only) S Open-Drain Fault Indicator Output S Open-LED and LED Short Detection and Protection S Overtemperature Protection S Thermally Enhanced, 20-Pin TQFN and TSSOP Packages
MAX16814
Ordering Information
PART MAX16814ATP+ MAX16814ATP/V+ MAX16814AUP+ MAX16814BUTP+ MAX16814BUUP+ MAX16814UTP+ MAX16814UUP+ TEMP RANGE -40C to +125C -40C to +125C -40C to +125C 0C to +85C 0C to +85C 0C to +85C 0C to +85C PIN-PACKAGE 20 TQFN-EP* 20 TQFN-EP* 20 TSSOP-EP* 20 TQFN-EP* 20 TSSOP-EP* 20 TQFN-EP* 20 TSSOP-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. /V Denotes an automotive qualified part.
Applications
Automotive Displays LED Backlights Automotive RCL, DRL, Front Position, and Fog Lights LCD TV and Desktop Display LED Backlights Architectural, Industrial, and Ambient Lighting
_______________________________________________________________ Maxim Integrated Products 1 Typical Operating Circuit and Pin Configurations appear at end of data sheet.
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, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
ABSOLUTE MAXIMUM RATINGS
IN 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 OUT_ to LEDGND .................................................-0.3V to +45V VCC to SGND .......... -0.3V to the lower of (VIN + 0.3V) and +6V DRV, FLT, DIM, RSDT, OVP to SGND.....................-0.3V to +6V CS, RT, COMP, SETI to SGND ................. -0.3V to (VCC + 0.3V) NDRV to PGND .......................................-0.3V to (VDRV + 0.3V) NDRV Peak Current (< 100ns) ............................................. Q3A NDRV Continuous Current ............................................ Q100mA OUT_ Continuous Current ............................................. Q175mA VCC Short-Circuit Duration ........................................Continuous Continuous Power Dissipation (TA = +70NC) (Note 1) 20-Pin TQFN (derate 25.6mW/NC above +70NC) .......2051mW Junction-to-Case Thermal Resistance (BJC)..................6NC/W Junction-to-Ambient Thermal Resistance (BJA) ...........39NC/W 20-Pin TSSOP (derate 26.5mW/NC above +70NC) .....2122mW Junction-to-Case Thermal Resistance (BJC)...............2.0NC/W Junction-to-Ambient Thermal Resistance (BJA) ........37.7NC/W Operating Temperature Range MAX16814A_ _.............................................. -40NC to +125NC MAX16814U_ _and MAX16814BU_ _ ................0NC to +85NC Junction Temperature .....................................................+150NC Storage Temperature Range............................ -65NC to +150NC Lead Temperature (soldering, 10s) ................................+300NC
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer board. For detailed information on package thermal considerations, refer to http://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 = 12.25kI, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VRSDT = VDIM = VCC, VOVP = VCS = VLEDGND = VPGND = VSGND = 0V, TA = TJ = -40NC to +125NC for MAX16814A_ _ and TA = TJ = 0NC to +85NC for MAX16814U_ _ and MAX16814BU_ _, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER Operating Voltage Range Active Supply Current Standby Supply Current IN Undervoltage Lockout IN UVLO Hysteresis VCC REGULATOR Regulator Output Voltage Dropout Voltage Short-Circuit Current Limit VCC Undervoltage Lockout Threshold VCC UVLO Hysteresis RT OSCILLATOR Switching Frequency Range fSW 200 2000 kHz VCC 6.5V < VIN < 10V, 1mA < ILOAD < 50mA 10V < VIN < 40V, 1mA < ILOAD < 10mA VIN - VCC, VIN = 4.75V, ILOAD = 50mA VCC shorted to SGND VCC rising 4.75 5.0 200 100 4 100 5.25 500 V mV mA V mV SYMBOL VIN IIN MAX16814A_ _ and MAX16814U_ _ MAX16814BU_ _ only VEN = 0V VIN rising 3.975 CONDITIONS MIN 4.75 2.5 2.75 15 4.3 170 TYP MAX 40 5 5.5 40 4.625 UNITS V mA A V mV
2
______________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VEN = 12V, RRT = 12.25kI, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VRSDT = VDIM = VCC, VOVP = VCS = VLEDGND = VPGND = VSGND = 0V, TA = TJ = -40NC to +125NC for MAX16814A_ _ and TA = TJ = 0NC to +85NC for MAX16814U_ _ and MAX16814BU_ _ , unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER SYMBOL CONDITIONS fSW = 200kHz to 600kHz, MAX16814A_ _ and MAX16814U_ _ Maximum Duty Cycle fSW = 600kHz to 2000kHz, MAX16814A_ _ and MAX16814U_ _ fSW = 200kHz to 600kHz, MAX16814BU_ fSW = 600kHz to 2000kHz, MAX16814BU_ _ Oscillator Frequency Accuracy Sync Rising Threshold Minimum Sync Frequency PWM COMPARATOR PWM Comparator Leading-Edge Blanking Time PWM to NDRV Propagation Delay SLOPE COMPENSATION Peak Slope Compensation Current Ramp Magnitude CS LIMIT COMPARATOR Current-Limit Threshold CS Limit Comparator to NDRV Propagation Delay 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 Rise Time Fall Time ISINK = 100mA (nMOS) ISOURCE = 100mA (pMOS) VNDRV = 5V VNDRV = 0V CLOAD = 1nF CLOAD = 1nF 0.9 1.1 2.0 2.0 6 6 A A ns ns gM (Note 4) VOUT_ = 5V, VCOMP = 2.5V VOUT_ = 0V, VCOMP = 2.5V 160 160 340 1 600 75 375 375 800 800 880 V S dB A A (Note 3) 10mV overdrive, excluding leading-edge blanking time 396 416 10 437 mV ns Current ramp added to the CS input, MAX16814A_ _ only Current ramp added to the CS input, MAX16814U_ _ and MAX16814BU_ _ 44 45 49 50 54 A x fSW 55 Including leading-edge blanking time 60 90 ns ns fSW = 200kHz to 2MHz, MAX16814A_ _ and MAX16814U_ _ fSW = 200kHz to 2MHz, MAX16814BU_ _ MIN 85 82 90 86 -7.5 -7 4 1.1fSW TYP 89 86 94 90 MAX 93 90 98 94 +7.5 +7 V Hz % % UNITS
MAX16814
_______________________________________________________________________________________
3
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VEN = 12V, RRT = 12.25kI, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VRSDT = VDIM = VCC, VOVP = VCS = VLEDGND = VPGND = VSGND = 0V, TA = TJ = -40NC to +125NC for MAX16814A_ _ and TA = TJ = 0NC to +85NC for MAX16814U_ _ and MAX16814BU_ _, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER LED CURRENT SOURCES OUT_ Current-Sink Range Channel-to-Channel Matching VOUT_ = VREF IOUT_ = 100mA IOUT_ = 100mA Output Current Accuracy TA = +125C, MAX16814A_ _ only TA = -40C to +125C, MAX16814A_ _ only 20 150 2 3 mA 5 2.75 mA 4 1 1.125 1.144 1.23 1.23 50 VEN = 40V, MAX16814A_ _ only EN Input Current DIM Input High Voltage DIM Input Low Voltage DIM Hysteresis DIM Input Current DIM to LED Turn-On Delay DIM to LED Turn-Off Delay IOUT_ Rise and Fall Times FLT Output Low Voltage FLT Output Leakage Current LED Short Detection Threshold Short Detection Comparator Delay RSDT Leakage Current OVP Trip Threshold OVP Hysteresis OVP Leakage Current Thermal-Shutdown Threshold Thermal-Shutdown Hysteresis VOVP = 1.25V Temperature rising 165 15 Output rising 1.19 1.228 70 200 VIN = 4.75V and ISINK = 5mA VFLT = 5.5V Gain = 3 1.75 2.0 6.5 600 1.266 DIM rising edge to 10% rise in IOUT_ DIM falling edge to 10% fall in IOUT_ 100 100 200 0.4 1.0 2.25 250 2 VEN = 40V, MAX16814U_ _ and MAX16814BU_ _ 2.1 0.8 200 250 nA V V mV A ns ns ns V A V s nA V mV nA C C 1.335 1.316 V mV A mA % SYMBOL CONDITIONS MIN TYP MAX UNITS
OUT_ Leakage Current LOGIC INPUTS/OUTPUTS
TA = +25C, MAX16814U_ _ and IOUT_ = MAX16814BU_ _ 50mA to 150mA TA = 0C to +85C, MAX16814U_ _ and MAX16814BU_ _ VDIM = 0V, VOUT_ = 40V VEN rising, MAX16814A_ _ only
EN Reference Voltage EN Hysteresis
VEN rising, MAX16814U_ _ and MAX16814BU_ _
Note 2: All MAX16814A_ _ are 100% tested at TA = +125NC, while alll MAX16814U_ _ and MAX16814BU_ _ are 100% tested at TA = +25C. All limits overtemperature are guaranteed by design , not production tested. Note 3: CS threshold includes slope compensation ramp magnitude. Note 4: Gain = VCOMP/VCS, 0.05V < VCS < 0.15V. 4 ______________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller
Typical Operating Characteristics
(VIN = VEN = 12V, fSW = 300kHz, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VOVP = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, load = four strings of seven white LEDs, TA = +25NC, unless otherwise noted.)
SWITCHING WAVEFORM AT 5kHz (50% DUTY CYCLE) DIMMING
MAX16814 toc01
MAX16814
SUPPLY CURRENT vs. SUPPLY VOLTAGE
CNDRV = 13pF 3.6 3.4 IIN (mA) 3.2 3.0 2.8 2.6 2.4 5 10 15 20 25 VIN (V) 30 35 40 45 TA = -40NC TA = +25NC TA = +125NC
MAX16814 toc02
3.8
VLX 10V/div 0V IOUT1 100mA/div 0A VOUT 10V/div FIGURE 2 40Fs/div 0V
SUPPLY CURRENT vs. SWITCHING FREQUENCY
CNDRV = 13pF 4.2 4.0 IIN (mA) 3.8 3.6 3.4 3.2 3.0 200 400 600 800 1000 1200 1400 1600 1800 2000 fSW (kHz)
MAX16814 toc03
SWITCHING FREQUENCY vs. TEMPERATURE
308 SWITCHING FREQUENCY (kHz) 306 304 300 298 296 294 292 290 -50 -25 0 25 50 75 100 125 TEMPERATURE (NC) VSETI (V) 302
MAX16814 toc04
VSETI vs. TEMPERATURE
MAX16814 toc05
4.4
310
1.240 1.236 1.232 1.228 1.224 1.220 -50 -25 0 25 50 75 100
125
TEMPERATURE (NC)
VSETI vs. PROGRAMMED CURRENT
MAX16814 toc06
EN THRESHOLD VOLTAGE vs. TEMPERATURE
MAX16814 toc07
EN LEAKAGE CURRENT vs. TEMPERATURE
VEN = 2.5V
EN LEAKAGE CURRENT (nA)
MAX16814 toc08
1.234 1.233 1.232 VSETI (V) 1.231 1.230 1.229 1.228 20 46 72 98 124
1.30
150 120 90 60 30 0
EN THRESHOLD VOLTAGE (V)
1.25
VEN RISING
1.20 VEN FALLING 1.15
150
1.10 -50 -25 0 25 50 75 100 125 TEMPERATURE (NC)
-50
-25
0
25
50
75
100
125
LED STRING CURRENT (mA)
TEMPERATURE (NC)
_______________________________________________________________________________________
5
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16814
Typical Operating Characteristics (continued)
(VIN = VEN = 12V, fSW = 300kHz, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VOVP = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, load = four strings of seven white LEDs, TA = +25NC, unless otherwise noted.)
VCC LINE REGULATION
MAX16814 toc09
VCC LOAD REGULATION
5.08 5.06 5.04 VCC (V) 5.02 5.00 4.98 4.96 4.94 4.92 4.90
TA = -40NC TA = +125NC TA = +25NC
MAX16814 toc10
SWITCHING FREQUENCY vs. 1/RT
1.80 SWITCHING FREQUENCY (MHz) 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20
MAX16814 toc11
5.08 5.06 5.04 VCC (V) 5.02 5.00 4.98 4.96 5 10 15 20 25 30 35 TA = +125NC TA = +25NC TA = -40NC
5.10
2.00
40
0
20
VIN (V)
40 IVCC (mA)
60
80
0.02
0.06
0.10
0.14
0.18
0.22
0.26
0.30
1/RT (mS)
STARTUP WAVEFORM WITH DIM ON PULSE WIDTH < tSW
MAX16814 toc12
VIN 20V/div 0V VDIM 5V/div 0V IOUT_ 100mA/div 0A VLED 20V/div 0V
STARTUP WAVEFORM WITH DIM ON PULSE WIDTH = 10tSW
MAX16814 toc13
VIN 20V/div 0V VDIM 5V/div 0V IOUT1 100mA/div 0A VLED 10V/div
FIGURE 2 40ms/div
0V
40ms/div
STARTUP WAVEFORM WITH DIM CONTINOUSLY ON
MAX16814 toc14
MOSFET DRIVER ON-RESISTANCE vs. TEMPERATURE
MAX16814 toc15
VIN 20V/div 0V
ON-RESISTANCE (I)
1.5 1.3 pMOS 1.1 0.9 0.7 0.5 -50 -25 0 25 50 75 100
VDIM 5V/div 0V IOUT1 100mA/div 0A VLED 10V/div FIGURE 2 40ms/div
nMOS
0V
125
TEMPERATURE (NC)
6
______________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
Typical Operating Characteristics (continued)
(VIN = VEN = 12V, fSW = 300kHz, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VOVP = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, load = four strings of seven white LEDs, TA = +25NC, unless otherwise noted.)
LED CURRENT SWITCHING WITH DIM AT 5kHz AND 50% DUTY CYCLE
MAX16814 toc16
MAX16814
LED CURRENT RISING AND FALLING WAVEFORM
MAX16814 toc17
FIGURE 2
IOUT1 100mA/div 0A IOUT2 100mA/div 0A IOUT3 100mA/div 0A IOUT4 100mA/div 0A
VDIM 5V/div 0V
ILED 50mA/div 0A
FIGURE 2
100Fs/div
4Fs/div
OUT_ CURRENT vs. 1/RSETI
MAX16814 toc18
COMP LEAKAGE CURRENT vs. TEMPERATURE
VDIM = 0V
COMP LEAKAGE CURRENT (nA)
MAX16814 toc19
160 140 120 IOUT_ (mA) 100 80 60 40 20 0.010
1.0 0.8 0.6 VCOMP = 4.5V 0.4 VCOMP = 0.5V 0.2 0 -50 -25 0 25 50 75 100
0.025
0.040
0.055
0.070
0.085
0.100
125
1/RSETI (mS)
TEMPERATURE (NC)
OUT_ LEAKAGE CURRENT vs. TEMPERATURE
MAX16814 toc20
OVP LEAKAGE CURRENT vs. TEMPERATURE
MAX16814 toc21
RSDT LEAKAGE CURRENT vs. TEMPERATURE
MAX16814 toc22
100
VDIM = 0V VOUT = 40V
2.0 1.8 OVP LEAKAGE CURRENT (nA) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 -50 -25 0 25 50 75 100 VOVP = 1.25V
250
RSDT LEAKAGE CURRENT (nA)
OUT_ LEAKAGE CURRENT (nA)
200 VRSDT = 0.5V
10
150
1
100 VRSDT = 2.5V 50
0.1 -50 -25 0 25 50 75 100 125 TEMPERATURE (NC)
125
-50
-25
0
25
50
75
100
125
TEMPERATURE (NC)
TEMPERATURE (NC)
_______________________________________________________________________________________
7
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
Pin Description
PIN TQFN 1 2 3 TSSOP 4 5 6 NAME IN EN COMP FUNCTION Bias Supply Input. Connect a 4.75V to 40V supply to IN. Bypass IN to SGND with a ceramic capacitor. Enable Input. Connect EN to logic-low to shut down the device. Connect EN to logic-high or IN for normal operation. The EN logic threshold is internally set to 1.23V. Switching Converter Compensation Input. Connect the compensation network from COMP to SGND for current-mode control (see the Feedback Compensation section). Oscillator Timing Resistor Connection. Connect a timing resistor (RT) from RT to SGND to program the switching frequency according to the formula RT = 7.350 x 109/fsw (for the MAX16814A_ _ and the MAX16814U_ _) or to the formula RT = 7.72 x 109/fsw (for the MAX16814BU_ _). Apply an AC-coupled external clock at RT to synchronize the switching frequency with an external clock. Open-Drain Fault Output. FLT asserts low when an open LED, short LED, or thermal shutdown is detected. Connect a 10k pullup resistor from FLT to VCC. Overvoltage Threshold Adjust Input. Connect a resistor-divider from the switching converter output to OVP and SGND. The OVP comparator reference is internally set to 1.23V. LED Current Adjust Input. Connect a resistor (RSETI) from SETI to SGND to set the current through each LED string (ILED) according to the formula ILED = 1500/RSETI. LED Short Detection Threshold Adjust Input. Connect a resistive divider from VCC to RSDT and SGND to program the LED short detection threshold. Connect RSDT directly to VCC to disable LED short detection. The LED short detection comparator is internally referenced to 2V. Signal Ground. SGND is the current return path connection for the low-noise analog signals. Connect SGND, LEDGND, and PGND at a single point. Digital PWM Dimming Input. Apply a PWM signal to DIM for LED dimming control. Connect DIM to VCC if dimming control is not used. 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. If unused, connect OUT1 to LEDGND.
4
7
RT
5 6 7
8 9 10
FLT OVP SETI
8
11
RSDT
9 10
12 13
SGND DIM
11
14
OUT1
8
______________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
Pin Description (continued)
PIN TQFN 12 TSSOP 15 NAME FUNCTION 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. If unused, connect OUT2 to LEDGND. 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 3. OUT3 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT3. OUT3 sinks up to 150mA. If unused, connect OUT3 to LEDGND. LED String Cathode Connection 4. OUT4 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT4. OUT4 sinks up to 150mA. If unused, connect OUT4 to LEDGND. Current-Sense Input. CS is the current-sense input for the switching regulator. A sense resistor connected from the source of the external power MOSFET to PGND sets the switching current limit. A resistor connected between the source of the power MOSFET and CS sets the slope compensation ramp rate (see the Slope Compensation section). Power Ground. PGND is the switching current return path connection. Connect SGND, LEDGND, and PGND at a single point. Switching n-MOSFET Gate-Driver Output. Connect NDRV to the gate of the external switching power MOSFET. MOSFET Gate-Driver Supply Input. Connect a resistor between VCC and DRV to power the MOSFET driver with the internal 5V regulator. Bypass DRV to PGND with a minimum of 0.1F ceramic capacitor. 5V Regulator Output. Bypass VCC to SGND with a minimum of 1F ceramic capacitor as close as possible to the device. Exposed Pad. Connect EP to a large-area contiguous copper ground plane for effective power dissipation. Do not use as the main IC ground connection. EP must be connected to SGND.
MAX16814
OUT2
13
16
LEDGND
14
17
OUT3
15
18
OUT4
16
19
CS
17 18
20 1
PGND NDRV
19
2
DRV
20 --
3 --
VCC EP
_______________________________________________________________________________________
9
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller MAX16814
FLT RSDT
VREF
POKD
MAX16814
SHDN DRV NDRV PGND PWM LOGIC CLK SLOPE COMPENSATION RAMP/RT OSC 1.8V di ( dt = 50FA x fsw) CS COMP CS BLANKING
FAULT FLAG LOGIC
SHORT LED DETECTOR
OPEN-LED DETECTOR
UNUSED STRING DETECTOR
TSHDN
RT
ILIM 0.425V
MIN STRING VOLTAGE
OUT_
OVP COMP THERMAL SHUTDOWN TSHDN REF BANDGAP UVLO 5V LDO REGULATOR SS_REF UVLO TSHDN POK VREF SS_DONE LOGIC (REF/FB SELECTOR) gM FB VBG
R LOGIC
IN
SHDN LEDGND DIM
VBG = 1.235V
VCC
SOFT-START 100ms
POKD VBG
SHDN
P EN SHDN 1.23V TSHDN SGND SGND OVP SETI
Figure 1. Simplified Functional Diagram
10
_____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
VIN L2 10FH C5 47FF C1 1FF M1 IRF7468 R1 261kI L1 22FH D1 B160B-1 C2 1FF C7 33FF 7 HBLEDS PER STRING
C6 10FF
R2 24kI Q10% D2 R7 22I RCOMP 24kI RCS 0.1I
C8 1nF
IN EN
NDRV
CS
OVP
OUT1 VCC C3 1FF R5 4.7I VDRV C4 0.1FF OUT2 OUT3
MAX16814
OUT4 RSETI 15kI SETI
DIM FLT COMP RSDT RCOMP 14kI CCOMP 0.022FF
R6 50kI R3 30.1kI
VCC
RT RT 21kI
R4 20kI
SGND
PGND
LEDGND
Figure 2. Circuit Used for Typical Operating Characteristics
______________________________________________________________________________________
11
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
Detailed Description
The MAX16814 high-efficiency HB LED driver integrates all the necessary features to implement a highperformance backlight driver to power LEDs in small to medium-sized displays for automotive as well as general applications. The device provides load-dump voltage protection up to 40V in automotive applications. The MAX16814 incorporates two major blocks: a DC-DC controller with peak current-mode control to implement a boost, coupled-inductor boost-buck, or a SEPIC-type switched-mode power supply and a 4-channel LED driver with 20mA to 150mA constant current-sink capability per channel. Figure 1 is the simplified functional diagram and Figure 2 shows the circuit used for typical operating characteristics. The MAX16814 features a constant-frequency peak current-mode control with programmable slope compensation to control the duty cycle of the PWM controller. The high-current FET driver can provide up to 2A of current to the external n-channel MOSFET. The DC-DC converter implemented using the controller 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 4-channel constant current-sink drivers that control the current through the LED strings. A single resistor connected from the SETI input to ground adjusts the forward current through all four LED strings. The MAX16814 features adaptive voltage control that adjusts the converter output voltage depending on the forward voltage of the LED strings. This feature minimizes the voltage drop across the constant current-sink drivers and reduces power dissipation in the device. A logic input (EN) shuts down the device when pulled low. The device includes an internal 5V LDO capable of powering additional external circuitry. All the versions of the MAX16814 include PWM dimming. The MAX16814A_ and the MAX16814U_ versions, in particular, provide very wide (5000:1) PWM dimming range where a dimming pulse as narrow as 1s is possible at a 200Hz dimming frequency. This is made possible by a unique feature that detects short PWM dimming input pulses and adjusts the converter feedback accordingly. Advanced features include detection and string-disconnect for open-LED strings, partial or fully shorted strings and unused strings. Overvoltage protection clamps the converter output voltage to the programmed OVP threshold in the event of an open-LED condition. Shorted LED string detection and overvoltage protection thresholds are programmable using RSDT and OVP inputs, respectively. An open-drain FLT signal asserts to indicate open-LED, shorted LED, and overtemperature conditions. Disable individual current-sink channels by connecting the corresponding OUT_ to LEDGND. In this case, FLT does not assert indicating an open-LED condition for the disabled channel. The device also features an overtemperature protection that shuts down the controller if the die temperature exceeds +165NC. The peak current-mode controller allows boost, coupledinductor buck-boost, or SEPIC-type converters to generate the required bias voltage for the LED strings. The switching frequency can be programmed over the 200kHz to 2MHz range using a resistor connected from RT to SGND. Programmable slope compensation is available to compensate for subharmonic oscillations that occur at above 50% duty cycles in continuous conduction mode. The external 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 external MOSFET to PGND. The MAX16814 features leading-edge blanking to suppress the external MOSFET switching noise. A PWM comparator compares the current-sense voltage plus the slope compensation signal with the output of the transconductance error amplifier. The controller turns off the external 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) with an internal reference (REF) and regulates its output to adjust the inductor current. An internal minimum string detector measures the minimum currentsink voltage with respect to SGND out of the 4 constantcurrent-sink channels. During normal operation, this minimum OUT_ voltage is regulated to 1V through feedback. The error amplifier takes 1V as the REF and the minimum OUT_ voltage as the FB input. The amplified error at the COMP output controls the inductor peak current to regulate the minimum OUT_ voltage at 1V. The resulting DC-DC converter output voltage is the highest LED string voltage plus 1V.
Current-Mode DC-DC Controller
12
_____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
The converter stops switching when the 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 steady-state level almost immediately when the LED strings are turned on again. This unique feature provides fast dimming response, without having to use large output capacitors. For the MAX16814A_ _ and the MAX16814U_ _, if the PWM dimming on-pulse is less than or equal to five switching cycles, the feedback controls the voltage on OVP so that the converter output voltage is regulated at 95% of the OVP 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 connected to the COMP output continuously and the DC-DC converter continues switching. Undervoltage Lockout (UVLO) The MAX16814 features two undervoltage lockouts that monitor the input voltage at IN and the output of the internal LDO regulator at VCC. The device turns on after both VIN and VCC exceed their respective UVLO thresholds. The UVLO threshold at IN is 4.3V when VIN is rising and 4.15V when VIN is falling. The UVLO threshold at VCC is 4V when VCC is rising and 3.9V when VCC is falling. 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 40FA. The logic threshold at EN is 1.23V (typ). The voltage at EN must exceed 1.23V before any operation can commence. There is a 50mV 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 below. Use the following equation to calculate the value of R1 and R2 in Figure 3: soft-start, the DC-DC converter output ramps towards 95% of the OVP voltage and uses feedback from the OVP input. Soft-start terminates when the minimum current-sink voltage reaches 1V or when the converter output reaches 95% OVP. The typical soft-start period is 100ms. The 1V minimum OUT_ voltage is detected only when the LED strings are enabled by PWM dimming. Connect OVP to the boost converter output through a resistive divider network (see the Typical Operating Circuit). When there is an open-LED condition, the converter output hits the OVP threshold. After the OVP is triggered, openLED strings are disconnected and, at the beginning of the dimming PWM pulse, control is transferred to the adaptive voltage control. The converter output discharges to a level where the new minimum OUT_ voltage is 1V. Oscillator Frequency/External Synchronization The internal oscillator frequency is programmable between 200kHz and 2MHz using a resistor (RT) connected from the RT input to SGND. Use the equation below to calculate the value of RT for the desired switching frequency, fSW.
MAX16814
RT =
7.35 x 10 9 Hz fSW
7.72 x 10 9 fSW
Enable
(for the MAX16814A_ _ and the MAX16814U_ _). RT =
(for the MAX16814BU_ _). Synchronize the oscillator with an external clock by AC-coupling the external clock to the RT input. The capacitor used for the AC-coupling should satisfy the following relation:
9.862 C SYNC -0.144x10 -3 (F) RT
where RT is in .
VIN
V R1 = UVLO - 1 x R2 1.23V
where VUVLO is the desired undervoltage lockout level and 1.23V is the EN input reference. Connect EN to IN if not used. Soft-Start The MAX16814 provides soft-start with internally set timing. At power-up, the MAX16814 enters soft-start once unused LED strings are detected and disconnected (see the Open-LED Management and Overvoltage Protection section). During
MAX16814
EN
R1
R2 1.23V
Figure 3. Setting the MAX16814 Undervoltage Lockout Threshold 13
______________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
The pulse width for the synchronization pulse should satisfy the following relations: t PW VS < 0.5 t CLK t PW VS + VS > 3.4 0.8 - t CLK t CLK t PW < (t CI - 1.05 xt CLK ) t CI where tPW is the synchronization source pulse width, tCLK is the synchronization clock time period, tCI is the programmed clock period, and VS is the synchronization pulse voltage level. 5V LDO Regulator (VCC) The internal LDO regulator converts the input voltage at IN to a 5V output voltage at VCC. The LDO regulator supplies up to 50mA current to provide power to internal control circuitry and the gate driver. Connect a resistor between VCC and DRV to power the gate-drive circuitry; the recommended value is 4.7I. Bypass DRV with a capacitor to PGND. The external resistor and bypass capacitor provide noise filtering. Bypass VCC to SGND with a minimum of 1FF ceramic capacitor as close to the device as possible. PWM MOSFET Driver The NDRV output is a push-pull output with the on-resistance of the pMOS typically 1.1I and the on-resistance of the nMOS typically 0.9I. NDRV swings from PGND to DRV to drive an external n-channel MOSFET. The driver typically sources 2.0A and sinks 2.0A allowing for fast turn-on and turn-off of high gate-charge MOSFETs. The power dissipation in the MAX16814 is mainly a function of the average current sourced to drive the external MOSFET (IDRV) if there are no additional loads on VCC. IDRV depends on the total gate charge (QG) and operating frequency of the converter. Connect DRV to VCC with a 4.7I resistor to power the gate driver with the internal 5V regulator. The MAX16814 features four identical constant-current sources used to drive multiple HB LED strings. The current through each one of the four channels is adjustable between 20mA and 150mA using an external resistor (RSETI) connected between SETI and SGND. Select RSETI using the following formula: R SETI = 1500 IOUT_ where IOUT_ is the desired output current for each of the four channels. If more than 150mA is required in an LED string, use two or more of the current source outputs (OUT_) connected together to drive the string as shown in Figure 4. The MAX16814 features LED brightness control using an external PWM signal applied at DIM. A logic-high signal on the DIM input enables all four LED current sources and a logic-low signal disables them. For the MAX16814A_ _ and the MAX16814U_ _, 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 5 oscillator clock cycles (DIM pulse width decreasing) then the boost converter regulates its output based on feedback from the OVP input. During this mode, the converter output voltage is regulated to 95% of the OVP threshold voltage. If the turn-on duration of the PWM signal is greater than or equal to 6 oscillator clock cycles (DIM pulse width increasing), then the converter regulates its output so that the minimum voltage at OUT_ is 1V. Fault protections in the MAX16814 include cycle-bycycle current limiting using the PWM controller, DC-DC converter output overvoltage protection, open-LED detection, short LED detection and protection, and overtemperature shutdown. An open-drain LED fault flag output (FLT) goes low when an open-LED string is detected, a shorted LED string is detected, and during
BOOST CONVERTER OUTPUT
LED Dimming Control
Fault Protections
40mA TO 300mA PER STRING
LED Current Control
OUT1
MAX16814
OUT2 OUT3 OUT4
Figure 4. Configuration for Higher LED String Current
14
_____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
thermal shutdown. FLT is cleared when the fault condition is removed during thermal shutdown and shorted LEDs. FLT is latched low for an open-LED condition and can be reset by cycling power or toggling the EN pin. The thermal shutdown threshold is +165NC and has 15NC hysteresis. Open-LED Management and Overvoltage Protection On power-up, the MAX16814 detects and disconnects any unused current-sink channels before entering softstart. Disable the unused current-sink channels by connecting the corresponding OUT_ to LEDGND. This avoids asserting the FLT output for the unused channels. After soft-start, the MAX16814 detects open LED and disconnects any strings with an open LED from the internal minimum OUT_ voltage detector. This keeps the DC-DC converter output voltage within safe limits and maintains high efficiency. During normal operation, the DC-DC converter output regulation loop uses the minimum OUT_ voltage as the feedback input. If any LED string is open, the voltage at the opened OUT_ goes to VLEDGND. The DC-DC converter output voltage then increases to the overvoltage protection threshold set by the voltage-divider network connected between the converter output, OVP input, SGND. The overvoltage protection threshold at the DC-DC converter output (VOVP) is determined using the following formula: R1 (see the Typical Operating Circuit) VOVP = 1.23 x 1 + R2 where 1.23V (typ) is the OVP threshold. Select R1 and R2 such that the voltage at OUT_ does not exceed the absolute maximum rating. As soon as the DC-DC converter output reaches the overvoltage protection threshold, the PWM controller is switched off setting NDRV low. Any current-sink output with VOUT_ < 300mV (typ) is disconnected from the minimum voltage detector. Connect the OUT_ of all channels without LED connections to LEDGND before power-up to avoid OVP triggering at startup. When an open-LED overvoltage condition occurs, FLT is latched low. Short LED Detection The MAX16814 checks for shorted LEDs at each rising edge of DIM. An LED short is detected at OUT_ if the following condition is met: VOUT_ > VMINSTR + 3 x VRSDT where VOUT_ is the voltage at OUT_, VMINSTR is the minimum current-sink voltage, and VRSDT is the programmable LED short detection threshold set at the RSDT input. Adjust VRSDT using a voltage-divider resistive network connected at the VCC output, RSDT input, and SGND. Once a short is detected on any of the strings, the LED strings with the short are disconnected and the FLT output flag asserts until the device detects that the shorts are removed on any of the following rising edges of DIM. Connect RSDT directly to VCC to always disable LED short detection.
MAX16814
Applications Information
Three different converter topologies are possible with the DC-DC controller in the MAX16814, which has the ground-referenced outputs necessary to use the constant current-sink drivers. If the LED string forward voltage is always more than the input supply voltage range, use the boost converter topology. If the LED string forward voltage falls within the supply voltage range, use the boost-buck converter topology. Boost-buck topology is implemented using either a conventional SEPIC configuration or a coupled-inductor boost-buck configuration. The latter is basically a flyback converter with 1:1 turns ratio. 1:1 coupled inductors are available with tight coupling suitable for this application. Figure 6 shows the coupled-inductor boost-buck configuration. It is also possible to implement a single inductor boost-buck converter using the MAX15054 high-side FET driver. The boost converter topology provides the highest efficiency among the above mentioned topologies. The coupled-inductor boost-buck topology has the advantage of not using a coupling capacitor over the SEPIC configuration. Also, the feedback loop compensation for SEPIC becomes complex if the coupling capacitor is not large enough. A coupled-inductor boost-buck is not suitable for cases where the coupled-inductor windings are not tightly coupled. Considerable leakage inductance requires additional snubber components and degrades the efficiency.
DC-DC Converter
______________________________________________________________________________________
15
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
First select a converter topology based on the above factors. 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) as follows: ILED = I STRING x N STRING where ISTRING is the LED current per string in amperes and NSTRING is the number of strings used. Calculate the maximum duty cycle (DMAX) using the following equations: For boost configuration: D MAX = (VLED + VD1 - VIN_MIN ) (VLED + VD1 - VDS - 0.3V) and:
Power-Circuit Design
Use the following equations to calculate the maximum average inductor current (ILAVG) and peak inductor current (ILP) in amperes: IL AVG = ILED 1 - D MAX
Allowing the peak-to-peak inductor ripple DIL to be +30% of the average inductor current:
IL = IL AVG x 0.3 x 2
IL P = IL AVG +
IL 2
For SEPIC and coupled-inductor boost-buck-configurations: (VLED + VD1) D MAX = (VIN_MIN - VDS - 0.3V + VLED + VD1) where VD1 is the forward drop of the rectifier diode in volts (approximately 0.6V), VIN_MIN is the minimum input supply voltage in volts, and VDS is the drain-to-source voltage of the external MOSFET in volts when it is on, and 0.3V is the peak current-sense voltage. Initially, use an approximate value of 0.2V for VDS to calculate DMAX. Calculate a more accurate value of DMAX after the power MOSFET is selected based on the maximum inductor current. Select the switching frequency (fSW) depending on the space, noise, and efficiency constraints.
Calculate the minimum inductance value, LMIN, in henries with the inductor current ripple set to the maximum value: L MIN = (VINMIN - VDS - 0.3V) x D MAX fSW x IL
where 0.3V is the peak current-sense voltage. Choose an inductor that has a minimum inductance greater than the calculated LMIN and current rating greater than ILP. The recommended saturation current limit of the selected inductor is 10% higher than the inductor peak current for boost configuration. For the coupled-inductor boost-buck, the saturation limit of the inductor with only one winding conducting should be 10% higher than ILP. Power circuit design for the SEPIC configuration is very similar to a conventional boost-buck design with the output voltage referenced to the input supply voltage. For SEPIC, the output is referenced to ground and the inductor is split into two parts (see Figure 5 for the SEPIC configuration). One of the inductors (L2) takes LED current as the average current and the other (L1) takes input current as the average current. Use the following equations to calculate the average inductor currents (IL1AVG, IL2AVG) and peak inductor currents (IL1P, IL2P) in amperes: I x D MAX x 1.1 IL1AVG = LED 1 - D MAX
SEPIC Configuration
Inductor Selection
Boost and Coupled-Inductor Boost-Buck Configurations In all the three converter configurations, the average inductor current varies with the line voltage and the maximum average current occurs at the lowest line voltage. For the boost converter, the average inductor current is equal to the input current. Select the maximum peak-to-peak ripple on the inductor current (DIL). The recommended peak-to-peak ripple is 60% of the average inductor current.
16
_____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
The factor 1.1 provides a 10% margin to account for the converter losses: IL2 AVG = ILED Assuming the peak-to-peak inductor ripple DIL is Q30% of the average inductor current: IL1 = IL1AVG x 0.3 x 2 and: IL1P = IL1AVG + IL1 2 and: IL AVG = IL1AVG + IL2 AVG where ILAVG represents the total average current through both the inductors together for SEPIC configuration. Use these values in the calculations for SEPIC configuration in the following sections. Select coupling capacitor CS so that the peak-to-peak ripple on it is less than 2% of the minimum input supply voltage. This ensures that the second-order effects created by the series resonant circuit comprising L1, CS, and L2 does not affect the normal operation of the converter. Use the following equation to calculate the minimum value of CS: CS ILED x D MAX VIN_MIN x 0.02 x fSW For simplifying further calculations, consider L1 and L2 as a single inductor with L1 and L2 connected in parallel. The combined inductance value and current is calculated as follows: L MIN = L1MIN x L2 MIN L1MIN + L2 MIN
MAX16814
IL2 = IL2 AVG x 0.3 x 2 and: IL2 P = IL2 AVG + IL2 2
Calculate the minimum inductance values L1MIN and L2MIN in henries with the inductor current ripples set to the maximum value as follows: L1MIN = L2 MIN = (VINMIN - VDS - 0.3V) x D MAX fSW x IL1 (VINMIN - VDS - 0.3V) x D MAX fSW x IL2
where CS is the minimum value of the coupling capacitor in farads, ILED is the LED current in amperes, and the factor 0.02 accounts for 2% ripple. Slope Compensation The MAX16814 generates a current ramp for slope compensation. This ramp current is in sync with the switching frequency and starts from zero at the beginning of every clock cycle and rises linearly to reach 50FA at the end of the clock cycle. The slope-compensating resistor, RSCOMP, is connected between the CS input and the source of the external MOSFET. This adds a programmable ramp voltage to the CS input voltage to provide slope compensation.
where 0.3V is the peak current-sense voltage. Choose inductors that have a minimum inductance greater than the calculated L1MIN and L2MIN and current rating greater than IL1P and IL2P, respectively. The recommended saturation current limit of the selected inductor is 10% higher than the inductor peak current:
______________________________________________________________________________________
17
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
Use the following equation to calculate the value of slope compensation resistance, RSCOMP. For boost configuration:
R SCOMP =
PWM dimming, the amount of ceramic capacitors on the output are usually minimized. In this case, an additional electrolytic or tantalum capacitor provides most of the bulk capacitance. External MOSFET Selection The external MOSFET should have a voltage rating sufficient to withstand the maximum output voltage together with the rectifier diode drop and any possible overshoot due to ringing caused by parasitic inductances and capacitances. The recommended MOSFET VDS voltage rating is 30% higher than the sum of the maximum output voltage and the rectifier diode drop. The recommended continuous drain current rating of the MOSFET (ID), when the case temperature is at +70NC, is greater than that calculated below: ID RMS = IL AVG 2 x D MAX x 1.3 The MOSFET dissipates power due to both switching losses and conduction losses. Use the following equation to calculate the conduction losses in the MOSFET:
PCOND = IL AVG 2 x D MAX x RDS (ON)
(VLED - 2VIN_MIN ) x R CS x 3
L MIN x 50FA x fSW x 4
For SEPIC and coupled-inductor boost-buck:
R SCOMP =
( VLED - VIN_MIN ) x R CS x 3
L MIN x 50FA x fSW x 4
where VLED and VIN_MIN are in volts, RSCOMP and RCS are in ohms, LMIN is in henries and fSW is in hertz. The value of the switch current-sense resistor, RCS, can be calculated as follows: For boost:
0.396 x 0.9 = ILP xRCS +
(DMAX x (VLED - 2VIN_MIN)xRCS x 3)
4 xL MN x fSW
For SEPIC and boost-buck:
0.396 x 0.9 = ILP xRCS +
(DMAX x (VLED - VIN_MIN ) xRCS x 3)
4 xL MN x fSW
where RDS(ON) is the on-state drain-to-source resistance of the MOSFET. Use the following equation to calculate the switching losses in the MOSFET: PSW = IL AVG x VLED 2 x C GD x fSW 1 1 x + 2 I GON I GOFF
where 0.396 is the minimum value of the peak current-sense threshold. The current-sense threshold also includes the slope compensation component. The minimum current-sense threshold of 0.396 is multiplied by 0.9 to take tolerances into account. Output Capacitor Selection For all the three converter topologies, the output capacitor supplies the load current when the main switch is on. The function of the output capacitor is to reduce the converter output ripple to acceptable levels. The entire output-voltage ripple appears across constant currentsink outputs because the LED string voltages are stable due to the constant current. For the MAX16814, limit the peak-to-peak output voltage ripple to 200mV to get stable output current. The ESR, ESL, and the bulk capacitance of the output capacitor contribute to the output ripple. In most of the applications, using low-ESR ceramic capacitors can dramatically reduce the output ESR and ESL effects. To reduce the ESL and ESR effects, connect multiple ceramic capacitors in parallel to achieve the required bulk capacitance. To minimize audible noise during
where IGON and IGOFF are the gate currents of the MOSFET in amperes, with VGS at the threshold voltage in volts, when it is turned on and turned off, respectively. CGD is the gate-to-drain MOSFET capacitance in farads. 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 reverserecovery 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: ID = 1.2 x IL AVG 1 - D MAX
18
_____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
Feedback Compensation During normal operation, the feedback control loop regulates the minimum OUT_ voltage to 1V when LED string currents are enabled during PWM dimming. When LED currents are off during PWM dimming, the control loop turns off the converter and stores the steady-state condition in the form of capacitor voltages, mainly the output filter capacitor voltage and compensation capacitor voltage. For the MAX16814A_ _ and the MAX16814U_ _, when the PWM dimming pulses are less than five switching clock cycles, the feedback loop regulates the converter output voltage to 95% of the OVP threshold. The worst-case condition for the feedback loop is when the LED driver is in normal mode regulating the minimum OUT_ voltage to 1V. The switching converter small-signal transfer function has a right-half plane (RHP) zero for boost configuration if the inductor current is in continuous conduction mode. The RHP zero adds a 20dB/decade gain together with a 90N-phase lag, which is difficult to compensate. The worst-case RHP zero frequency (fZRHP) is calculated as follows: For boost configuration: fZRHP = VLED (1 - D MAX ) 2 2 x L x ILED For SEPIC and coupled-inductor boost-buck configurations: fP1 = ILED x D MAX 2 x x VLED x C OUT
MAX16814
where fP1 is in hertz, VLED is in volts, ILED is in amperes, and COUT is in farads. Compensation components, RCOMP and CCOMP, perform two functions. CCOMP introduces a low-frequency pole that presents a -20dB/decade slope to the loop gain. RCOMP flattens the gain of the error amplifier for frequencies above the zero formed by RCOMP and CCOMP. For compensation, this zero is placed at the output pole frequency fP1 so that it provides a -20dB/ decade slope for frequencies above fP1 to the combined modulator and compensator response. The value of RCOMP needed to fix the total loop gain at fP1 so that the total loop gain crosses 0dB with -20dB/ decade slope at 1/5 the RHP zero frequency is calculated as follows: For boost configuration: R COMP = fZRHP x R CS x ILED 5 x fP1 x GM COMP x VLED x (1 - D MAX )
For SEPIC and coupled-inductor boost-buck configurations: fZRHP = VLED (1 - D MAX ) 2 2 x L x ILED x D MAX
For SEPIC and coupled-inductor boost-buck configurations: R COMP = fZRHP x R CS x ILED x D MAX 5 x fP1 x GM COMP x VLED x (1 - D MAX )
where fZRHP is in hertz, VLED is in volts, L is the inductance value of L1 in henries, and ILED is in amperes. A simple way to avoid this zero is to roll off the loop gain to 0dB at a frequency less than one fifth of the RHP zero frequency with a -20dB/decade slope. The switching converter small-signal transfer function also has an output pole. The effective output impedance together with the output filter capacitance determines the output pole frequency fP1 that is calculated as follows: For boost configuration: fP1 = ILED 2 x x VLED x C OUT
where RCOMP is the compensation resistor in ohms, fZRHP and fP2 are in hertz, RCS is the switch currentsense resistor in ohms, and GMCOMP is the transconductance of the error amplifier (600FS). The value of CCOMP is calculated as follows: C COMP = 1 2 x R COMP x fP1
If the output capacitors do not have low ESR, the ESR zero frequency may fall within the 0dB crossover frequency. An additional pole may be required to cancel out this pole placed at the same frequency. This is usually implemented by connecting a capacitor in parallel with CCOMP and RCOMP. Figure 5 shows the SEPIC configuration and Figure 6 shows the coupled-inductor boost-buck configuration.
19
______________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
Connect a resistor RSETI2 to the SETI input as shown in Figure 7 for controlling the LED string current using an external control voltage. The MAX16814 applies a fixed 1.23V bandgap reference voltage at SETI and measures the current through SETI. This measured current multiplied by a factor of 1220 is the current through each one of the four constant current-sink channels. Adjust the current through SETI to get analog dimming functionality by connecting the external control voltage to
Analog Dimming Using External Control Voltage
SETI through the resistor RSETI2. The resulting change in the LED current with the control voltage is linear and inversely proportional. The LED current control range remains between 20mA to 150mA. Use the following equation to calculate the LED current set by the control voltage applied: I OUT = 1500 (1.23 - VC ) + x 1220 R SETI R SETI2
VIN 4.75V TO 40V L1 C1 CS D1 C2 UP TO 40V R1
N
L2
RSCOMP
RCS
R2
IN EN VCC C3 R5 DRV C4
NDRV
CS
OVP OUT1 OUT2
MAX16814
OUT3 OUT4 RSETI SETI VCC
DIM COMP
FLT R3 RSDT
RCOMP SGND CCOMP PGND LEDGND
RT RT
R4
Figure 5. SEPIC Configuration
20
_____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
PCB Layout Considerations
LED driver circuits based on the MAX16814 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 the guidelines below to reduce noise as much as possible: 1) Connect the bypass capacitor on VCC and DRV as close to the device as possible 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. 2) 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. 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 to the device as possible. Connect SGND to PGND at the same point.
MAX16814
______________________________________________________________________________________
21
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
VIN 4.75V TO 40V T1 (1:1) D1
C1
UP TO 40V R1 C2
N
RSCOMP
RCS
R2
IN EN VCC C3 R5 DRV C4
NDRV
CS
OVP OUT1 OUT2
MAX16814
OUT3 OUT4 RSETI SETI VCC
DIM COMP
FLT R3 RSDT
RCOMP SGND CCOMP PGND LEDGND
RT RT
R4
Figure 6. Coupled-Inductor Boost-Buck Configuration
MAX16814
1.23V SETI RSETI2
RSETI
VC
Figure 7. Analog Dimming with External Control Voltage 22 _____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
Pin Configurations
OUT4 OUT3 OUT2
MAX16814
TOP VIEW
NDRV 1 DRV 2 VCC 3 IN 4 EN 5 COMP 6 RT 7 FLT 8 OVP 9 SETI 10
+
20 PGND 19 CS 18 OUT4 CS 16 PGND 17 NDRV 18 DRV 19 VCC 20
15
14
13
12
OUT1 11 10 9 DIM SGND RSDT SETI OVP
TOP VIEW
MAX16814
17 OUT3 16 LEDGND 15 OUT2 14 OUT1 13 DIM 12 SGND
LEDGND
MAX16814
EP*
8 7 6 5 FLT
1 IN
2 EN
3 COMP
4 RT
EP*
11 RSDT
TSSOP
*EXPOSED PAD.
THIN QFN
Typical Operating Circuit
VIN 4.75V TO 40V L C1 D1 C2 UP TO 40V R1
N
RSCOMP
RCS
R2
IN EN VCC C3 R5 DRV C4
NDRV
CS
OVP OUT1 OUT2
MAX16814
OUT3 OUT4 RSETI SETI VCC
DIM COMP
FLT R3 RSDT
RCOMP SGND CCOMP PGND LEDGND
RT RT
R4
______________________________________________________________________________________
23
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
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 TSSOP-EP 20 TQFN-EP PACKAGE CODE U20E+1 T2044+3 DOCUMENT NO. 21-0108 21-0139
24
_____________________________________________________________________________________
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller
Revision History
REVISION NUMBER 0 1 2 3 REVISION DATE 7/09 9/09 11/09 2/10 Initial release Correction to slope compensation description and block diagram Correction to synchronization description frequency and minor edits Correction to CSYNC formula DESCRIPTION PAGES CHANGED -- 10, 18 1-4, 8, 12-20, 22, 25 13
MAX16814
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)
25
2010 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.

▲Up To Search▲

Price & Availability of MAX16814AV

	To Download MAX16814AV Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .