Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - LED driver DC-DC converter QFN 56 EP Surface-mount

LT3746

3746fa

For more information www.linear.com/3746

Typical applicaTion

DescripTion

32-Channel 20mA LED

Driver with Buck Controller

The LT

3746 integrates a 32-channel LED driver with a 55V

buck controller. The LED driver lights up to 30mA/13V of

LEDs in series per channel, and the buck controller gener

ates an adaptive bus voltage supplying the parallel LED

strings. Each channel has individual 6-bit dot correction

current adjustment and 12-bit grayscale PWM dimming.

Both dot correction and grayscale are accessible via a

serial data interface in TTL/CMOS logic.

The LT3746 performs full diagnostic and protection

against open/short LED and overtemperature fault. The

fault status is sent back through the serial data interface.

The 30MHz fully-buffered, skew-balanced, cascadable

serial data interface makes the chip extremely suitable

for large screen LCD dynamic backlighting and mono-,

multi-, full-color LED displays.

32-Channel LED Driver, 1MHz Buck, 3 LEDs 10mA to 30mA per Channel, 500Hz 12-Bit Dimming

FeaTures

applicaTions

6V to 55V Power Input Voltage Range

32 Independent LED Outputs up to 30mA/13V

6-Bit Dot Correction Current Adjustment

12-Bit Grayscale PWM Dimming

0.5µs Minimum LED On Time

Adaptive LED Bus Voltage for High Efﬁciency

Cascadable 30MHz Serial Data Interface

Full Diagnostic and Protection: Individual Open/Short

LED and Overtemperature Fault

Large Screen Display LED Backlighting

Mono-, Multi-, Full-Color LED Displays

LED Billboards and Signboards

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

EN/UVLO

SYNC

SET

SCKI

SDI

LDI

SCKO

SDO

LDO

LED29

LED30

LED31

LED00

LED01

LED02

LT3746

100k

13V TO 42V

3V TO 5.5V

CAP

0.47µF

GATE

GND

ISP

ISN

4.7µF

80.6k

10k

2 × 47µF

22µH 33mΩ

PWMCK

TTL/CMOS

2.048MHz CLOCK

TTL/CMOS

3746 TA01

10µF

46.4k

10nF

60.4k

32.4k

Summary of content (28 pages)

PAGE 1
LT3746 32-Channel 20mA LED Driver with Buck Controller Features n n n n n n n n Description 6V to 55V Power Input Voltage Range 32 Independent LED Outputs up to 30mA/13V 6-Bit Dot Correction Current Adjustment 12-Bit Grayscale PWM Dimming 0.
PAGE 2
LT3746 Absolute Maximum Ratings Pin Configuration (Note 1) RT SS FB ISN ISP CAP GATE VIN TSET TOP VIEW ISET VIN ........................................................................... 57V CAP.......................................................... VIN – 8V to VIN GATE...............................................................CAP to VIN LED00 to LED31, ISP, ISN.......................................... 13V ISP..................................................
PAGE 3
LT3746 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, VCC = 3.3V, VEN/UVLO = 1.5V, VFB = 1.5V, VISP = VISN = 0V, RT = 105k, RISET = 60.4k, CCAP = 0.47µF to VIN, unless otherwise noted.
PAGE 4
LT3746 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, VCC = 3.3V, VEN/UVLO = 1.5V, VFB = 1.5V, VISP = VISN = 0V, RT = 105k, RISET = 60.4k, CCAP = 0.47µF to VIN, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VBIAS CAP Bias Voltage (VIN – VCAP) 7V < VIN < 55V 6.54 6.77 7.00 ICAP CAP Bias Current Limit (VIN – VCAP) = VBIAS – 0.
PAGE 5
LT3746 TIMING Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, VCC = 3V to 5.5V, VEN/UVLO = 1.5V, VFB = 1.5V, VISP = VISN = 5V, VLEDxx = 0.5V, RT = 105k, RISET = 60.4k, CCAP = 0.47µF to VIN, CSCKO = CSDO = CLDO = 27pF to GND, unless otherwise noted.
PAGE 6
LT3746 Electrical Characteristics Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3746E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature.
PAGE 7
LT3746 Typical Performance Characteristics 100Hz 8:1 GS Dimming TA = 25°C, unless otherwise noted.
PAGE 8
LT3746 Typical Performance Characteristics Buck Efficiency 5 95 24VIN, 12VOUT at 1MHz 90 0.43 4 0.42 T = 125°C 12VIN, 4VOUT at 500kHz 80 75 3 IVIN (mA) 85 IVIN (µA) EFFICIENCY (%) Quiescent IVIN vs VIN Shutdown IVIN vs VIN 100 2 48VIN, 4VOUT at 200kHz 70 T = –40°C 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 LOAD CURRENT (A) 0.39 0 10 20 30 VIN (V) 40 50 0.38 60 T = 125°C 10.30 T = 125°C 3.35 3.30 T = –40°C 3.5 4.0 4.5 VCC (V) 5.0 IVCC (mA) IVCC (mA) 3.
PAGE 9
LT3746 Typical Performance Characteristics (VIN - VCAP) UVLO Threshold vs Temperature 4.90 1000 TA = 25°C, unless otherwise noted. Oscillator Frequency fOSC vs Temperature Oscillator Frequency fOSC vs RT 520 800 4.70 600 4.60 510 fOSC (kHz) 4.80 fOSC (kHz) VIN - VCAP (V) UVLO+ 400 UVLO– 4.50 490 200 4.40 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 0 125 20 60 100 140 180 RT (kΩ) 220 1.208 –10.0 –10.2 0.51 1.202 –10.
PAGE 10
LT3746 Typical Performance Characteristics 1.208 16 1.206 12 1.204 LED Current ILED vs Dot Correction Nominal LED Current vs RISET 30 25 20 ILED (mA) 20 ILED (mA) VISET (V) 1.210 VISET Pin Voltage vs Temperature TA = 25°C, unless otherwise noted. 8 15 10 1.202 4 1.200 –50 –25 0 25 50 75 100 JUNCTION TEMPERATURE (°C) 0 125 5 40 80 120 160 200 RISET (kΩ) 240 280 3746 G28 23 9 DC = 0×20 20 15 DC = 0×00 VLED (V) 21 20 6 19 10 3 18 5 0 0.5 1.0 1.5 2.0 VLED (V) 2.
PAGE 11
LT3746 Pin Functions EN/UVLO (Pin 1): Enable and Undervoltage Lockout (UVLO) Pin. The pin can accept a digital input signal to enable or disable the chip. Tie to 0.35V or lower to shut down the chip or tie to 1.34V or higher for normal operation. This pin can also be connected to VIN through a resistor divider to program a power input UVLO threshold. If both the enable and UVLO functions are not used, tie this pin to VCC pin. LED00 to LED31 (Pins 2-17, 30-45): LED Driver Output Pins.
PAGE 12
CVCC VCC RFB1 RFB2 CSS For more information www.linear.com/3746 25 26 LDO SDO SCKO SCKI SDI 384 SCKO SDO SHIFT REGISTER C1 x STATUS INFO (FAULT LEDXX) C0 OT SCKI SDI 384 R S Q 32X 6 12 POR – – GM2 ... DELAY 6 12 FAULT LEDXX DC REGISTER FS GS REGISTER FS FS 0.5V LED00 LED01 EN + RAMP – R S OPEN/SHORT LED + – + – IREF DRIVER TSET ISET CAP GATE VIN VPTAT OT CONSTANT CURRENT SINK 1.205V 6.
PAGE 13
LT3746 OPERATION The LT3746 integrates a single constant-frequency currentmode nonsynchronous buck controller with thirty-two linear current sinks. The buck controller generates an adaptive output LED bus voltage to supply parallel LED strings and the thirty-two linear current sinks regulate and modulate individual LED strings. Its operation is best understood by referring to the Block Diagram.
PAGE 14
LT3746 OPERATION LT3746 6-WIRE TOPOLOGY CHIP 1 LDO LDI CHIP 2 LDO LDI CHIP N LDO LDO LDI SCKO SCKI SCKO SCKI SCKO SCKI SCKO SDO SDI SDO SDI SDO SDI SDO LDI CHIP N LDO CONTROLLER SDI SCKI LDI CONVENTIONAL 4-WIRE TOPOLOGY CHIP 1 LDO LDI CHIP 2 LDO LDO LDI SCKO SCKI SCKO SCKI SCKO SCKI SCKO SDO SDI SDO SDI SDO SDI SDO CONTROLLER SDI 3746 F01 Figure 1.
PAGE 15
LT3746 OPERATION tSU-LDI tWH-LDI tHD-LDI LDI tWH-CKI SCKI tSU-SDI tWL-CKI SDI SR[0] 385 378 1 DC 31 MSB SR[1] 384 1 385 386 1 tHD-SDI DC 0 LSB DC 31 MSB 386 C1 GS 31 MSB C0 = 1 GS 0 LSB C1 GS 31 MSB C0 = 0 DC 0 LSB C1 C0 = 1 F0 GS 31 MSB GS 0 LSB C1 C0 = 0 F0 DC 0 LSB + 1 x C1 0 F0 GS 0 LSB + 1 GS 0 LSB C1 0 GS 31 MSB F0 tPD-LD↓ tPD-LD↑ LDO tPD-SCK↑ SCKO 1 378 385 tPD-SCK↓ 386 384 385 386 tPD-SD DC 31 MSB SDO/ SR[385] 1 DC 31 MSB DC 31 MSB – 1 0 G
PAGE 16
LT3746 OPERATION Figure 2 shows two serial data input SDI frames (GS frame and DC frame) and one serial data output SDO frame (status frame). All the frames have the same 386-bit in length and are transmitted with the MSB first and the LSB last. The SDI frames are sent with the SCKI signal and the SDO frame is received with the SCKO signal.
PAGE 17
LT3746 OPERATION LED bus voltage VOUT will force a higher LED pin voltage across the current sink, thereby dissipating more power inside the part. See more details about the choice of the LED bus voltage and the power dissipation calculation in the Application Information section. Dot Correction and Grayscale Digital-to-Analog Conversion The resistor on the ISET pin programs the nominal LED current (4mA to 20mA) for all the channels.
PAGE 18
LT3746 OPERATION period) and goes high for the rest 512 clock cycles (1/8 of the grayscale PWM dimming period). In the event of all the LED channels being not active (i.e., either fault or off) before the 3585th PWMCK clock, the PRECHG signal will go high immediately. To better explain the operation of the adaptive-trackingplus-precharging technique, a simplified application system with 3-channel LED array is presented in Figure 5.
PAGE 19
LT3746 Applications Information Globally, the LT3746 converts a higher input voltage to a single lower LED bus voltage (VOUT ) supplying 32 parallel LED strings with the adaptive-tracking-plus-precharging technique. Locally, the part regulates and modulates the current of each string to an independent dot correction and grayscale PWM dimming setting sent by TTL/CMOS logic serial data interface.
PAGE 20
LT3746 Applications Information frequency is usually chosen to keep the switching noise out of a sensitive frequency band. Switching Frequency Setting and Synchronization The LT3746 uses a constant switching frequency that can be programmed from 200kHz to 1MHz with a resistor from the RT pin to ground. Table 2 shows RT values for common switching frequencies. Table 2. Switching Frequency fSW vs RT Value f SW (kHz) RT * (kΩ) 200 280 300 182 400 133 500 105 600 84.5 700 71.5 800 60.4 900 53.
PAGE 21
LT3746 Applications Information The largest inductor ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation: L≥ VIN(MAX) – VOUT VOUT + VD • VIN(MAX) + VD f SW • ΔI L The inductor DC or RMS current rating must be greater than the maximum output load current IOUT(MAX) and its saturation current should be higher than the maximum inductor current IL(MAX).
PAGE 22
LT3746 Applications Information The bulk capacitance is calculated based on maximum input ripple, ∆VIN: C IN = D MAX • I OUT(MAX) ΔVIN • f SW ∆VIN is typically chosen at a level acceptable to the user. 100mV is a good starting point. For ceramic capacitors, only X5R or X7R type should be used because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U.
PAGE 23
LT3746 Applications Information pin can also be connected to a resistor divider between VIN and ground to program a power input VIN UVLO threshold. After RUV1 is selected, RUV2 can be calculated by: Ê VIN(ON) ˆ R UV2 = R UV1 • Á – 1˜ Ë 1.3V ¯ FROM µCONTROLLER VIN VCC VIN RUV2 VCC EN/UVLO EN/UVLO RUV1 where VIN(ON) is the power input voltage above which the part goes into normal operation.
PAGE 24
LT3746 Applications Information Setting Grayscale Although adjusting the LED current changes its luminous intensity, or brightness, it will also affect the color matching between LED channels by shifting the chromaticity coordinate. The best way to adjust the brightness is to control the amount of LED on/off time by pulse width modulation (PWM). The LT3746 can adjust the brightness for each channel independently. The 12-bit grayscale PWM dimming results in 4096 linear brightness steps from 0% to 99.98%.
PAGE 25
LT3746 Applications Information where IVIN is the power input VIN quiescent current, IVCC is the VCC supply current, and VLEDn is the LED pin voltage for channel n. Keep TJ below the maximum operating junction temperature 125°C. From the total power dissipation PTOTAL, the junction temperature TJ can be calculated as: TJ = T A +PTOTAL • θ JA Typical Application VIN 10V TO 30V 0.47µF 16V VIN EN/UVLO EN VCC 3V TO 5.5V CAP GATE 100k VCC D1 SYNC RT 105k SS 409.
PAGE 26
LT3746 Package Description UHH Package UHH Package 56-Lead Plastic QFN (5mm × 9mm) 56-Lead Plastic QFN (5mm × 9mm) (Reference LTCLTC DWG # 05-08-1727 Rev Rev A) A) (Reference DWG # 05-08-1727 0.70 ± 0.05 5.50 ± 0.05 (2 SIDES) 4.10 ± 0.05 (2 SIDES) 3.60 REF (2 SIDES) 3.45 ±0.05 7.13 ±0.05 PACKAGE OUTLINE 0.20 ± 0.05 0.40 BSC 6.80 REF (2 SIDES) 8.10 ± 0.05 (2 SIDES) 9.50 ± 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 0.75 ± 0.05 5.00 ± 0.
PAGE 27
LT3746 Revision History REV DATE DESCRIPTION A 04/13 Clarified fOSC and IVCC specifications PAGE NUMBER 3 Clarified ILED, Line/Load delta specifications 4 Clarified Table 1 Equations 5 Clarified FB Pin description 11 3746fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representaForofmore information www.linear.
PAGE 28
LT3746 Typical Application VIN 13V TO 42V 0.47µF 16V VIN EN VCC 3V TO 5.5V CAP GATE EN/UVLO 100k D1 SYNC SS 80.6k GND LT3746 10nF ISP C1 2 × 47µF 10k ISN FB ISET LED00 TSET LED01 32.4k 2.048MHz CLOCK 11V MAXIMUM OUTPUT VOLTAGE 33mΩ . .. RT 60.4k M1 L1 22µH VCC 10µF 10V 46.4k 4.7µF 50V LED02 .. .