LT3743 — High-Current Synchronous Step-Down LED Driver Controller (Up to 20A)

Overview

The LT3743 is a fixed-frequency, synchronous step-down DC/DC LED driver controller designed by Linear Technology (now Analog Devices). Tailored specifically for high-power illumination, DLP projectors, architectural stage lighting, and laser diodes, it uses average current mode control to drive high-brightness LEDs at continuous currents up to $20.0\text{ A}$ with conversion efficiencies reaching $94\%$ .

Operating across a wide $6.0\text{ V}$ to $36.0\text{ V}$ input supply, the LT3743 features revolutionary 3-state current control and ultrafast PWM dimming (achieving up to $3000:1$ true color PWM dimming ratios without LED color shift). By driving external high-side and low-side N-channel power MOSFETs and an external PWM disconnect switch, the LT3743 can transition between different current levels in less than $2\ \mu\text{s}$ .

Quick reference


Driver Type	Synchronous Step-Down DC/DC LED Driver Controller
Package	28-lead QFN (4mm × 5mm) / 28-lead TSSOP Exposed Pad (FE-28)
Input Supply Voltage Range ( $V_{IN}$ )	$6.0\text{ V}$ to $36.0\text{ V}$ DC
Maximum LED Current	Up to $20.0\text{ A}$ continuous (set by external sense resistor)
PWM Dimming Ratio	Up to $3000:1$ at $100\text{ Hz}$
Switching Frequency Range	$200\text{ kHz}$ to $1.0\text{ MHz}$ (programmable with resistor)
Peak Conversion Efficiency	Up to $94\%$
LED Current Accuracy	$\pm 6\%$ typical over temperature
Protection Features	Open LED overvoltage clamping, Short-circuit protection, Thermal shutdown

Pinout (28-Lead TSSOP FE-28 Package)

text

            ┌──────────────┐
      EN/UVLO ─│ 1          28 │─ CTRL_SEL
      CTRL_H  ─│ 2          27 │─ CTRL_L
      VREF    ─│ 3          26 │─ PWM
      SS      ─│ 4          25 │─ PWMOUT
      RT      ─│ 5          24 │─ HG (High Gate)
      SYNC    ─│ 6    EP    23 │─ BST (Bootstrap)
      VCC     ─│ 7   (GND)  22 │─ SW (Switch Node)
      INTVCC  ─│ 8          21 │─ LG (Low Gate)
      VIN     ─│ 9          20 │─ PGND
      SENSEP  ─│ 10         19 │─ VOUT
      SENSEN  ─│ 11         18 │─ FB
      SGND    ─│ 12         17 │─ VFB
      COMP    ─│ 13         16 │─ CLKOUT
      STATUS  ─│ 14         15 │─ TSET
            └──────────────┘

Pin	Name	Type	Description
1	`EN/UVLO`	Input	Precision enable and programmable undervoltage lockout pin
2	`CTRL_H`	Input	High-level LED current analog control reference input
3	`VREF`	Output	2.0V reference output for resistor divider networks
5	`RT`	Input	Timing resistor connection to set switching frequency ( $200\text{ kHz} \dots 1\text{ MHz}$ )
7	`VCC`	Power	Input supply voltage for internal control circuitry
8	`INTVCC`	Output	5.2V internal LDO output powering the external MOSFET gate drivers
9	`VIN`	Power	Main high-voltage power input (6V to 36V)
10	`SENSEP`	Input	Positive input to the average current sense amplifier
11	`SENSEN`	Input	Negative input to the average current sense amplifier
18	`FB`	Input	Output voltage feedback divider pin for open-LED overvoltage protection
21	`LG`	Output	Low-side synchronous N-MOSFET gate drive output
22	`SW`	Power	Switching node connecting inductor, high-side source, and low-side drain
23	`BST`	Power	High-side gate driver bootstrap supply pin
24	`HG`	Output	High-side N-MOSFET gate drive output
25	`PWMOUT`	Output	Fast gate drive for external series LED PWM disconnect switch
26	`PWM`	Input	Logic PWM dimming input signal
27	`CTRL_L`	Input	Low-level LED current analog control reference input
28	`CTRL_SEL`	Input	Logic input to select between `CTRL_H` and `CTRL_L` analog setpoints

LED Current Programming Formula

The maximum regulated LED current ( $I_{LED}$ ) is determined by the external current sense resistor ( $R_{SENSE}$ ) connected across SENSEP and SENSEN:

$I_{LED} = \frac{50\text{ mV}}{R_{SENSE}}$

For a $20\text{ A}$ LED driver, using a $2.5\text{ m}\Omega$ sense resistor yields:

$I_{LED} = \frac{0.050\text{ V}}{0.0025\ \Omega} = 20.0\text{ A}$

Analog dimming can scale this threshold continuously between $0\text{ mV}$ and $50\text{ mV}$ via the CTRL_H / CTRL_L control pins.

Typical Application Circuit Architecture

text

        +V_IN (6V - 36V)
           │
           ├───────────────────────────────┐
           │                               │
        [VIN / VCC]                     [Q_TOP High-Side N-MOS]
         LT3743                            │
        [Pin 24: HG]  ──────── Gate ───────┤
        [Pin 22: SW]  ──────── Source ─────┴──────┬─────[ Inductor L ]─────┬──[ R_SENSE ]──┬───[ Q_PWM Disconnect ]─── +LED (Up to 20A)
        [Pin 21: LG]  ──────── Gate ───────┐      │                        │               │
           │                               │      │                    [SENSEP]         [SENSEN]                      -LED
        [PGND / SGND]                   [Q_BOT Low-Side N-MOS]             │               │                            │
           │                               │      │                        └───────┬───────┘                           GND
          GND                             GND    GND                            LT3743

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Input Voltage Range	$V_{IN}$	6.0	—	36.0	V	Operating range
INTVCC Regulated Output	$V_{INTVCC}$	4.95	5.2	5.45	V	$V_{IN} \ge 7.0\text{V}$
Full-Scale Current Sense Threshold	$V_{SENSE(MAX)}$	47.0	50.0	53.0	mV	$CTRL\_H = 2.0\text{V}$
Switching Frequency	$f_{SW}$	200	500	1000	kHz	Set by $R_T$ resistor
High-Side Gate Rise Time	$t_{r(HG)}$	—	20	—	ns	$C_L = 3.3\text{ nF}$
Low-Side Gate Rise Time	$t_{r(LG)}$	—	20	—	ns	$C_L = 3.3\text{ nF}$
PWMOUT Rise / Fall Time	$t_{r(PWM)}$	—	15	—	ns	$C_L = 1.0\text{ nF}$
Minimum PWM Pulse Width	$t_{PWM(MIN)}$	—	300	—	ns	Fast current recovery

Common mistakes

Omitting the external series PWM MOSFET: To achieve the full $3000:1$ dimming ratio with microsecond response times, an external N-channel MOSFET must be placed in series with the LED cathode/anode driven by PWMOUT. Turning off the main inductor converter alone results in slow decay times limited by output capacitance.
Inadequate Kelvin sense connections on R_SENSE: At $20\text{A}$ , copper trace resistance between the sense resistor solder pads and the IC pins will inject milliohms of error into the $50\text{mV}$ full-scale threshold. Use a true 4-terminal Kelvin connection from SENSEP and SENSEN directly to the resistor pads.
Insufficient INTVCC capacitor: Pin 8 (INTVCC) powers both gate drivers. High-gate-charge ( $Q_g$ ) MOSFETs require a $4.7\ \mu\text{F} \dots 10\ \mu\text{F}$ low-ESR ceramic capacitor directly adjacent to the INTVCC and PGND pins.

Notes

Three-State Color Mixing: The dual analog inputs (CTRL_H and CTRL_L) combined with digital CTRL_SEL allow projecting systems to instantaneously switch LED drive current levels between white, red, green, and blue frames without loop settling lag.

Interface	PWM Dimming (PWM, CTRL_SEL, CTRL_L, CTRL_H), Gate Drive Outputs (TG, BG), Current Sense (SENSEP, SENSEN), Output Voltage Sense (VOUT)
Package	28-lead QFN (4mm × 5mm) / 28-lead TSSOP with Exposed Pad (FE-28)