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LT3755 - High Side Sense LED Controller for  High Current LED Applications
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LT3755 - High Side Sense LED Controller for High Current LED Applications

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An Overview Study on 40Vin, 75Vout LED Controller: LT3755

An Overview Study on 40Vin, 75Vout LED Controller: LT3755

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  • Welcome to the training module on LT3755
  • The LT3755 and LT3755-1 are DC/DC controllers designed to operate as a constant-current source for driving high current LEDs. The LT3755 uses an external N-channel MOSFET and can drive up to 14 1A white LEDs from a nominal 12V input, delivering in excess of 50 watts. It incorporates a high-side current sense, enabling it to be used in boost, buck, buck-boost or SEPIC and flyback topologies. The LT3755 can deliver efficiencies of over 94% in boost mode, eliminating any need for external heat sinking. The LT3755 uses True Color PWM™ dimming, which delivers constant LED color with dimming ranges of up to 3,000:1. Two versions of the LT3755 are available -- the standard LT3755, which offers an Open LED Status pin, and the LT3755-1, which replaces the Open LED Status pin with a frequency synchronization pin.
  • The device’s 4.5V to 40V input voltage range makes it ideal for a wide variety of applications, including automotive, industrial and architectural lighting.
  • The LT3755 is a constant-frequency, current mode controller with a low side n-type metal-oxide-semiconductor gate driver. The GATE pin and Pulse width modulation OUT pin drivers and other chip loads are powered from INTVCC, which is an internally regulated supply. In normal operation with the Pulse Width Modulation pin low, the GATE and Pulse Width Modulation OUT pins are driven to GND, the VC pin is high impedance to store the previous switching state on the external compensation capacitor, and the ISP and ISN pin bias currents are reduced to leakage levels. When the PWM pin transitions high, the PWMOUT pin transitions high after a short delay. At the same time, the internal oscillator wakes up and generates a pulse to set the PWM latch, turning on the external power MOSFET switch (GATE goes high).
  • The LED current is programmed by placing an appropriate value current sense resistor between the ISP and ISN pins. Typically, sensing of the current should be done at the top of the LED string. The CTRL pin should be tied to a voltage higher than 1.1V to get the full-scale 100mV (typical) threshold across the sense resistor. The CTRL pin can also be used to dim the LED current to zero, although relative accuracy decreases with the decreasing voltage sense threshold. The LED current programming feature can increase total dimming range by a factor of 10. The CTRL pin should not be left open.
  • FB is intended for constant-voltage regulation or for LED protection/open LED detection. In normal condition, the current loop dominates and the exact output voltage is less than the setting through FB. Do not leave the FB pin open. If not used, connect to GND. For a boost application, the output voltage can be set by selecting the values of R1 and R2 as shown in the figure. For a boost type LED driver, set the resistor from the output to the FB pin such that the expected VFB during normal operation will not exceed 1.1V. For an LED driver of buck or a buck-boost configuration, the output voltage is typically level-shifted to a signal with respect to GND as illustrated in the bottom figure.
  • There are two methods to control the current source for dimming using the LT3755. One can control by using CTRL pin to adjust the current regulated in the LED’s. A second method uses the PWM pin to modulate the current source. One area where these LED drivers especially excel is in the performance and flexibility of their PWM dimming capabilities. The PWM dimming method is straightforward; the LED is driven by a tightly regulated current for a fixed interval in every PWM period. During the off-phase, the current in the LED is zero. During the on-phase, the current is carefully regulated. The minimum PWM on or off time will depend on the choice of operating frequency through the RT input. For best current accuracy, the minimum PWM low or high time should be at least six switching cycles. Maximum PWM period is determined by the system and is unlikely to be longer than 12ms.The maximum PWM dimming ratio (PWM(RATIO)) can be calculated from the maximum PWM period (t MAX ) and the minimum PWM pulse width (t MIN ).
  • Here gives you application information of LT3755 like selection of input capacitor, output capacitor, and soft-start capacitor. An X7R type ceramic capacitor used as the input capacitor is usually recommended since it has the least variation with temperature and DC bias. The value can be calculated based on the given equation. The selection of the output capacitor depends on the load and converter configuration. For LED applications, the equivalent resistance of the LED is typically low and the output filter capacitor should be sized to attenuate the current ripple. Use of X7R type ceramic capacitors is recommended. The LT3755 has soft-start option for which a capacitor need to be connected. For many applications, it is important to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. The soft-start interval is set by the soft-start capacitor selection according to the equation as shown. Typical value of 0.01uF is used.
  • This is in continuation to earlier page which gives you application information like inductor selection and sense resistor selection. The inductor used for LT3755 i.e L (buck, boost, buck-boost) is given here and it should be chosen based on operating frequency, input, and output voltage which will provide current mode ramp sense. Sense resistor is connected between the source of external N-type MOSFET and GND to provide adequate switch current to drive the application without exceeding the current limit threshold on the SENSE pin of the LT3755. In buck mode configuration, the sense resistor should give a switch current at least 30% greater that the required LED current. For Buck-boost and boost configuration, the resistor can be estimated using the equations as shown.
  • This is a typical application of LT3755 which is an 8W buck-mode LED driver with 50:1 PWM dimming at 20kHz and 90% efficiency. This circuit shows a buck-mode converter that regulates a 16V LED string at 500mA from a 22V to 36V supply. It has an external charge pump and level shift to drive the gate of an LED disconnect n-type metal-oxide-semiconductor . This level shift provides much faster rise and fall times than the familiar resistor level shift driving a p-type metal-oxide-semiconductor , and uses much less current, it is clear that the output LED current has no perceptible variation as pulse width is smoothly adjusted between the minimum on-time and the minimum off-time. The efficiency of this 8W circuit exceeds 90%.
  • This typical application circuit shows a SEPIC converter driving a 1A, 20V LED string from a 10V-to-36V supply. In addition to providing step-up and step-down capability, this circuit is handy because it provides input-output isolation and built in protection from a short to GND on the output. The efficiency of this circuit exceeds 87%. The minimum efficiency combined with its excellent thermal management capability enables it to deliver up to 240W output power without a heat sink or forced airflow.
  • This is a typical application of LT3755 which is 21W Buck-Boost Mode with 250:1 PWM Dimming and Open LED Protection.
  • Thank you for taking the time to view this presentation on LT3755 – High Side Sense LED Controller for High Current LED Applications . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simple call our sales hotline. For more technical information you may either visit the Linear Technology site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.

LT3755 - High Side Sense LED Controller for  High Current LED Applications LT3755 - High Side Sense LED Controller for High Current LED Applications Presentation Transcript

  • LT3755 – High Side Sense LED Controller for High Current LED Applications
    • Source: Linear Technology
  • Introduction
    • Purpose
      • An Overview Study on 40Vin, 75Vout LED Controller: LT3755
    • Outline
      • Features and Application
      • Block Diagram
      • Application Information, Dimming control
      • Application circuit
    • Content
      • 14 pages
  • Key Features
    • 3000:1 True Color PWM Dimming
    • Wide Input Voltage Range: 4.5V to 40V
    • Output Voltage Up to 75V
    • Constant-Current and Constant-Voltage Regulation
    • 100mV High Side Current Sense
    • Drives LEDs in Boost, Buck Mode, Buck-Boost Mode, SEPIC or Fly-back Topology
    • Adjustable Frequency: 100kHz to 1MHz
    • Open LED Protection
    • Programmable Undervoltage Lockout with Hysteresis
    • Open LED Status Pin (LT3755)
    • Frequency Synchronization (LT3755-1)
    • Low Shutdown Current: <1μA
    • Programmable Soft-Start
  • Target Applications
    • High Power LED Applications
    • Industrial
    • Automotive
  • Block Diagram
  • LED Current Programming
    • The LED current is programmed by placing an appropriate value current sense resistor between the ISP and ISN pins.
    • The LED current programming feature can increase total dimming range by a factor of 10.
    ISP ISN Current Feedback Resistor V IN LED String (V CTRL > 1.1V) (V CTRL < 1.1V)
  • Programming Output Voltage Feedback Resistor Connection for Boost or SEPIC LED Driver Feedback Resistor Connection for Buck Mode or Buck-Boost Mode LED Driver
  • Dimming Control
    • There are two methods to control the current source for dimming
      • One method uses the CTRL pin to adjust the current regulated in the LED’s.
      • A second method uses the PWM pin to modulate the current source between zero and full current to achieve a precisely programmed average current.
    • The maximum PWM dimming ratio (PWM(RATIO)) can be calculated from the maximum PWM period (t MAX ) and the minimum PWM pulse width (t MIN )
  • Application Information
    • Input Capacitor Selection:
    • The input capacitor supplies the transient input current for the power inductor of the converter and must be placed and sized according to the transient current requirements.
    • An X7R type ceramic capacitor is usually the best choice since it has the least variation with temperature and DC bias.
    • Output Capacitor Selection:
    • The selection of the output capacitor depends on the load and converter configuration, i.e., step-up or step-down and the operating frequency.
    • Soft-Start Capacitor Selection:
    • The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot.
    • The soft-start interval is set by the soft start capacitor selection according to the equation.
    • A typical value for the soft-start capacitor is 0.01μF.
  • Application Information
    • Inductor Selection:
    • The inductor should have a saturation current rating appropriate to the maximum switch current selected with the R SENSE resistor.
    • Choose an inductor value based on operating frequency, input and output voltage to provide a current mode ramp on SENSE during the switch on-time of approximately 20mV magnitude.
    • Sense Resistor Selection:
    • The resistor, R SENSE , between the source of the external NMOS FET and GND should be selected to provide adequate switch current to drive the application without exceeding the 108mV (typical) current limit threshold on the SENSE pin of LT3755
  • Application Circuit – A n 8W Buck Mode LED Driver 50W White Automotive LED Headlamp Driver
  • Application Circuit – 20W SEPIC LED Driver 20W SEPIC LED Driver
  • 21W Buck-Boost Mode with 250:1 PWM Dimming and Open LED Protection
  • Additional Resource
    • For ordering the LT3755, please click the part list or
    • Call our sales hotline
    • For additional inquires contact our technical service hotline
    • For more product information go to
      • http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1094,C1769,P81080