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LT3743

3743fd

TYPICAL APPLICATION

DESCRIPTION

High Current Synchronous

Step-Down LED Driver with

Three-State Control

The LT

3743 is a fixed frequency synchronous step-down

DC/DC controller designed to drive high current LEDs. The

average current mode controller will maintain inductor

current regulation over a wide output voltage range of 0V

to (V

– 2V). LED dimming is achieved through analog

dimming on the CTRL_L, CTRL_H and CTRL

_T pins and

with PWM dimming on the PWM and CTRL_SEL pins.

Through the use of externally switched load capacitors,

the LT3743 is capable of changing regulated LED current

levels within several µs, providing accurate, high speed

PWM dimming between two current levels. The switching

frequency is programmable from 200kHz to 1MHz through

an external resistor on the RT pin.

Additional features include voltage regulation

and

overvoltage protection set with a voltage divider from the

output to the FB pin. Overcurrent protection is provided

and set by the CTRL_H pin.

92% Efficient 20A LED Driver

FEATURES

APPLICATIONS

PWM Dimming Provides Up to 3000:1 Dimming Ratio

CTRL_SEL Dimming Provides Up to 3000:1 Dimming

Ratio Between Any Current

Three-State Current Control for Color Mixing

±6% Current Regulation Accuracy

6V to 36V Input Voltage Range

Average Current Mode Control

2µs Maximum Recovery Time Between Any Current

Regulation State

<1µA Shutdown Current

Output Voltage Regulation and Open-LED Protection

Thermally Enhanced 4mm × 5mm QFN and

28-Pin FE Package

DLP Projectors

High Power Architectural Lighting

Laser Diodes

CTRL_SEL

5V/DIV

PWM

5V/DIV

20V/DIV

LED

10A/DIV

20µs/DIV

3743 TA01b

= 24V

0A TO 2A TO 20A LED CURRENT STEP

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

True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners. Protected by U.S. Patents including 7199560, 7321203,

8120335 and others pending.

EN/UVLO

PWM

CTRL_SEL

EN/UVLO

PWM

CTRL_SEL

CBOOT

REF

CTRL_L

CTRL_H

CTRL_T

LT3743

SYNC

GND

SENSE

–

PWMGH

PWMGL

CC_INT

220nF

1.1µH

22µF

330µF

×3

2.5mΩ

10nF

34k

8.2nF

34k

8.2nF

82.5k

100k

330µF

×3

51.1k

3743 TA01a

10.0k

4.7µF

330µF

×3

2.2nF

NTC

10k

4.7µF

×4

10V TO 30V

OUTPUT

20A MAXIMUM

1µF

HOT

499Ω

M1, M2: SiR462DP

M3, M4: Si7234DP

Summary of content (28 pages)

PAGE 1
LT3743 High Current Synchronous Step-Down LED Driver with Three-State Control DESCRIPTION FEATURES n n n n n n n n n n PWM Dimming Provides Up to 3000:1 Dimming Ratio CTRL_SEL Dimming Provides Up to 3000:1 Dimming Ratio Between Any Current Three-State Current Control for Color Mixing ±6% Current Regulation Accuracy 6V to 36V Input Voltage Range Average Current Mode Control 2µs Maximum Recovery Time Between Any Current Regulation State <1µA Shutdown Current Output Voltage Regulation and Open-LED Protecti
PAGE 2
LT3743 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN Voltage.................................................................40V EN/UVLO Voltage.........................................................6V VREF Voltage................................................................3V CTRL_L, CTRL_H, CTRL_T Voltage.............................3V PWM, CTRL_SEL Voltage............................................6V SENSE+ Voltage.........................................................40V SENSE– Voltage..................
PAGE 3
LT3743 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN/UVLO = 5V, VSYNC = 0V, VCTRL_SEL = 0V, VPWM = 2V, unless otherwise noted. PARAMETER CONDITIONS MIN Input Voltage Range VIN Pin Quiescent Current (Note 3) Non-Switching Operation Shutdown Mode TYP 6 VPWM = VCTRL_SEL = 0V, Not Switching, RT = 40k VEN/UVLO = 0V l EN/UVLO Pin Falling Threshold 1.
PAGE 4
LT3743 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN/UVLO = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS PWMG Control Signals CTRL_SEL High to PWMGL Low Delay 10 40 ns CTRL_SEL High to PWMGH High Delay 150 200 ns CTRL_SEL Low to PWMGH Low Delay 30 60 ns 220 ns CTRL_SEL Low to PWMGL High Delay 170 PWMGH and PWMGL Pull-up Impedance 3.
PAGE 5
LT3743 TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Pin Current 0.5 1.64 8 0.4 6 0.3 1.58 –50°C 1.52 130°C 1.46 1.40 6 12 18 24 VIN (V) 30 IQ (µA) 10 EN/UVLO PIN CURRENT (µA) EN/UVLO THRESHOLD (V) EN/UVLO Threshold (Falling) 1.70 4 2 0 36 12 18 24 VIN (V) 30 3743 G01 2.01 0 6 12 18 24 VIN (V) 30 8 16 24 VIN (V) 32 VREF Current Limit 1.4 2.00 VIN = 36V 1.99 VIN = 6V TA = 25°C TA = 130°C 1.2 TA = –50°C 1.0 1.
PAGE 6
LT3743 TYPICAL PERFORMANCE CHARACTERISTICS Internal UVLO VCC_INT UVLO CBOOT-SW UVLO Voltage 5.0 3.00 4.00 2.75 3.75 2.50 3.50 4.0 UVLO (V) VOLTAGE (V) VIN (V) 4.5 2.25 3.25 2.00 3.00 1.75 2.75 3.5 3.0 –50 –15 55 20 TEMPERATURE (°C) 90 1.50 –50 125 –15 20 55 TEMPERATURE (°C) 90 VCC_INT Load Reg at 12V OPEN-LED THRESHOLD (V) REGULATED CURRENT (%) VCC_INT (V) 4.
PAGE 7
LT3743 TYPICAL PERFORMANCE CHARACTERISTICS 4 4 PMOS 3 2 NMOS HG Driver RDS(ON) 5 3 PMOS 2 1 –15 55 20 TEMPERATURE (°C) 90 0 –50 125 –15 55 20 TEMPERATURE (°C) 90 120 PWMGH TO PWMGL 110 –15 55 20 TEMPERATURE (°C) 90 LG TO HG 60 30 90 HG 60 LG 30 –15 55 20 TEMPERATURE (°C) 90 0 –50 125 –15 55 20 TEMPERATURE (°C) 90 3743 G23 3 125 120 HG TO LG 0 –50 125 300 125 3743 G24 Regulation Accuracy CTRL_H = 1.5V, VIN = 12V 6 2 4 1 2 Regulation Accuracy CTRL_H = 0.
PAGE 8
LT3743 TYPICAL PERFORMANCE CHARACTERISTICS LED Current Waveforms (90% PWM) 0.5A to 5A Overcurrent Threshold LED Current Waveforms (2000:1) 3A to 10A 120 VSENSE+ – VSENSE– (mV) 80 60 ILED 5A/DIV 40 IL 10A/DIV 20 0 CTRL_SEL 5V/DIV CTRL_SEL 5V/DIV SW 20V/DIV 100 ILED 5A/DIV IL 10A/DIV 40µs/DIV 0 0.75 1.5 CTRL_H (V) 2.25 3743 G29 5µs/DIV 3743 G30 3.0 3743 G28 CTRL_SEL 5V/DIV SW 10V/DIV ILED 11.1A/DIV 20µs/DIV PWM 5V/DIV CTRL_SEL 5V/DIV PWM 5V/DIV CTRL_L 0.
PAGE 9
LT3743 PIN FUNCTIONS (QFN/TSSOP) GND (Pins 1, 5, 9, 20, 21, Exposed Pad Pin 29/Pins 2, 7, 11, 22, 27, Exposed Pad Pin 29): Ground. The exposed pad must be soldered to the PCB. EN/UVLO (Pin 2/Pin 4): Enable Pin. The EN/UVLO pin acts as an enable pin and turns on the internal current bias core and subregulators at 1.55V. The pin does not have any pull-up or pull-down, requiring a voltage bias for normal part operation. Full shutdown occurs at approximately 0.5V.
PAGE 10
LT3743 PIN FUNCTIONS (QFN/TSSOP) PWMGL (Pin 22/Pin 23): The PWMGL output pin drives the gate of an external FET to connect one of the switching regulator output capacitors to the load. The driver pull-up impedance is 3.2Ω and pull-down impedance is 1.75Ω. LG (Pin 26/Pin 28): LG is the bottom FET gate drive signal that controls the state of the low side external power FET. The driver pull-up impedance is 2.5Ω and pull-down impedance is 1.3Ω.
PAGE 11
LT3743 BLOCK DIAGRAM (QFN Package) VIN VIN 402k 2 INTERNAL REGUALTOR AND UVLO 133k 3 2.2nF SYNC 16 15 28 47µF EN/UVLO VREF 2V REFERENCE SYNC OSCILLATOR RT 82.5k – R + S Q PWM COMPARATOR VCC_INT 27 10µF HIGH SIDE CBOOT DRIVER 25 HG 23 SW SYNCRONOUS 24 CONTROLLER LG 26 LOW SIDE DRIVER SENSE+ 0.1µF 2.2µH 10Ω 11 – 33nF CTRL_L + + 5.5µA + – CTRL BUFFER 8 100nF 4 13 34k 8.2nF 14 SENSE– FB CTRL_T VCL 10Ω 12 RS 5mΩ 330µF ×3 10A LED OUTPUT 330µF ×3 40.
PAGE 12
LT3743 OPERATION The LT3743 utilizes fixed frequency, average current mode control to accurately regulate the inductor current, independently from the output voltage. This is an ideal solution for applications requiring a regulated current source including driving high current LEDs where the forward junction voltage can range from 2V to 6V with a dynamic resistance of 20mΩ to 40mΩ. The control loop will regulate the current in the inductor at an accuracy of 6%.
PAGE 13
LT3743 APPLICATIONS INFORMATION Programming Inductor Current Inductor Selection The analog voltage at the CTRL_L and CTRL_H pins is buffered and produces a reference voltage, VCTRL, across an internal resistor. The regulated average inductor current is determined by: The recovery time between regulated states is critical to maintaining accurate control of the LED current.
PAGE 14
LT3743 APPLICATIONS INFORMATION Switching MOSFET Selection When selecting switching MOSFETs, the following parameters are critical in determining the best devices for a given application: total gate charge (QG), on-resistance (RDS(ON)), gate to drain charge (QGD), gate-to-source charge (QGS), gate resistance (RG), breakdown voltages (maximum VGS and VDS) and drain current (maximum ID). The following guidelines provide information to make the selection process easier.
PAGE 15
LT3743 APPLICATIONS INFORMATION 7 2.5 MOSFET POWER LOSS (W) MOSFET POWER LOSS (W) 6 5 TOTAL 4 TRANSITIONAL 3 2 1 0 2.0 1.5 TOTAL 1.0 TRANSITIONAL 0.5 OHMIC OHMIC 0 10 20 30 0 40 INPUT VOLTAGE (V) 0 20 10 30 40 INPUT VOLTAGE (V) 3743 F04a 3743 F04b Figure 4a. Power Loss Example for M1 Figure 4b. Power Loss Example for M2 Figure 4 Whenever possible, utilize a switching MOSFET that minimizes the total gate charge to limit the internal power dissipation of the LT3743. Table 3.
PAGE 16
LT3743 APPLICATIONS INFORMATION VCC_INT Capacitor Selection 60 The bypass capacitor for the VCC_INT pin should be larger than 5µF for stability and has no ESR requirement. It is recommended that the ESR be lower than 50mΩ to reduce noise within the LT3743. For driving MOSFETs with gate charges larger than 10nC, use 0.5µF/nC of total gate charge.
PAGE 17
LT3743 APPLICATIONS INFORMATION MOSFET Selection for the Switched Output Capacitors The MOSFETs used for the switched-output capacitor need to also handle the maximum regulated current while the capacitor is charged. The output drivers on the PWMGH and PWMGL pins have a pull-up impedance of 3.2Ω and a pull-down impedance of 1.75Ω. This provides adequate gate drive for the PWM MOSFETs without the need for an additional gate driver.
PAGE 18
LT3743 APPLICATIONS INFORMATION Voltage Regulation and Overvoltage Protection The LT3743 uses the FB pin to regulate the output to a maximum voltage and to provide a high speed overvoltage lockout to avoid high voltage conditions that may damage expensive high current LEDs. The regulated output voltage is programmed using a resistor divider from the output and ground (Figure 12). When the output voltage exceeds 130% of the regulated voltage level (1.
PAGE 19
LT3743 APPLICATIONS INFORMATION Thermal Shutdown The internal thermal shutdown within the LT3743 engages at 163°C and terminates switching, resets soft-start and shuts down the PWMGL and PWMGH drivers. When the part has cooled to 155°C, the internal reset is cleared and softstart is allowed to charge once the PWM signal is asserted. The EN/UVLO pin as an absolute maximum voltage of 6V. To accommodate the largest range of applications, there is an internal Zener diode that clamps this pin.
PAGE 20
LT3743 APPLICATIONS INFORMATION Average Current Mode Control Compensation The use of average current mode control allows for precise regulation of the inductor and LED currents. Figure 16 shows the average current mode control loop used in the LT3743, where the regulation current is programmed by a current source and a 3k resistor. To design the compensation network, the maximum compensation resistor needs to be calculated.
PAGE 21
LT3743 TYPICAL APPLICATIONS 12V, 20A LED Driver EN/UVLO PWM CTRL_SEL 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC 1µF HG 100nF CBOOT VREF LT3743 2.2nF LG 22µF VIN 12V M1 L1 1.0µH SW VCC_INT CTRL_H 50k 10Ω M2 OUTPUT 20A MAXIMUM 2.5mΩ 10Ω C1 330µF ×3 33nF GND RNTC 10k 10nF M3 SENSE+ SENSE– PWMGH CTRL_T SS 1µF D1 C3 330µF ×3 M4 PWMGL 60.4k FB VCL VCH 34k 34k 4.7nF 4.
PAGE 22
LT3743 TYPICAL APPLICATIONS 6V to 36V, 2A LED Driver With Shunted Output EN/UVLO INTVCC CTRL_SEL 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC 1µF HG 100nF CBOOT VREF LT3743 2.2nF RHOT 499Ω CTRL_L CONTROL INPUT SW VCC_INT LG CTRL_H 8.2µF VIN 6V TO 36V Shunted Output with CTRL_H Equal to CTRL_L M1 L1 10µH OUTPUT 2A MAXIMUM 25mΩ 2.2µF 22µF CTRL_SEL 5V/DIV IL D1 2A/DIV M2 ILED 1A/DIV GND CTRL_T RNTC 10k SENSE+ SENSE– SW 2V/DIV PWMGH 10nF SS PWMGL M3 20µs/DIV 3743 TA03b 40.
PAGE 23
LT3743 TYPICAL APPLICATIONS 6V to 30V, 20A LED Driver with Switched Cathode EN/UVLO PWM VCC_INT 82.5k 1µF HG 150nF CBOOT VREF LT3743 2.2nF CTRL_L RHOT CONTROL 499Ω INPUT 10nF 82µF M1 L1 1.1µH 2.5mΩ SW VCC_INT LG CTRL_H 22µF 10Ω M2 CTRL_T SENSE+ SENSE– PWMGL SS PWMGH OUTPUT 20A MAXIMUM 10Ω C1 330µF ×3 D1 33nF GND RNTC 10k VIN 6V TO 30V VIN EN/UVLO PWM CTRL_SEL RT SYNC M3 60.4k FB VCL VCH 34k 4.
PAGE 24
LT3743 TYPICAL APPLICATIONS 24V, 20A 3-LED Driver 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC 1µF HG 100nF CBOOT VREF LT3743 2.2nF 20k CTRL_H 60.4k RHOT 499Ω SW VCC_INT LG GND CTRL_L 82µF RNTC 10k 10nF 95 L1 1.2µH VCL 90 OUTPUT 20A MAXIMUM 2.
PAGE 25
LT3743 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UFD Package 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.50 REF 2.65 ± 0.05 3.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (2 SIDES) 0.75 ± 0.05 R = 0.
PAGE 26
LT3743 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev I) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 27 26 2524 23 22 21 20 1918 17 16 15 6.60 ±0.10 4.50 ±0.10 2.74 (.108) SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.
PAGE 27
LT3743 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 2/10 Revised Features and Typical Application 1 Updated Electrical Characteristics values 3, 4 Revised values on curves G32 and G33 in the Typical Performance Characteristics section 8 Revised the Block Diagram 11 Changed value in equation and made minor text edit in the Inductor Selection section 13 Revised Table 4 values 18 Added text to Average Current Mode Control Compensation and Board Layout Considerations sections in the App
PAGE 28
LT3743 TYPICAL APPLICATION 12V, 40A Pulsed LED Driver EN/UVLO PWM CTRL_SEL EN/UVLO PWM CTRL_SEL 150k RT SYNC VIN HG 4.7µF ×8 1µF RHOT 499Ω 50k 1µF CTRL_L 220nF L1 1µH CTRL_H 50k 1µF SW VCC_INT LG CTRL_T RNTC 10k 22µF M2 ×2 GND SENSE+ SENSE– 220µF M1 ×2 CBOOT VREF LT3743 220µF OUTPUT 40A MAXIMUM 1.