Linear LT3796 100V LED控制方案

Linear公司的LT3796用来驱动LED的可调整恒流或恒压的DC/DC控制器,3000:1 True Color PWM™调光,输入电压从6V到100V,电流监测高达100V,可调开关频率100kHz到1MHz,双电流检测放大器,可编线性电流检测阈值,短路保护,主要用在大功率LED和高压LED,电池和超级电容充电以及精密的限流电压稳压器.本文介绍了LT3796主要特性,方框图,多种应用电路以及100V评估板DC1706A主要特性,电路图,材料清单和PCB元件布局图.

The LT3796 is a DC/DC controller designed to regulate a constant-current or constant-voltage and is ideal for driving LEDs. It drives a low side external N-channel power MOSFET from an internal regulated 7.7V supply. The fixed frequency and current mode architecture result in stable operation over a wide range of supply and output voltages.

Two ground referred voltage FB pins serve as the input for several LED protection features, and also allow the converter to operate as a constant-voltage source.

The LT3796 features a programmable threshold output sense amplifier with rail-to-rail common mode range. The LT3796 also includes a separate high side current sensing amplifier that is gain configurable with two resistors. The TG pin inverts and level shifts the PWM signal to drive the gate of the external PMOS. The PWM input provides LED dimming ratios of up to 3000:1, and the CTRL input provides additional analog dimming capability.

LT3796主要特性:

3000:1 True Color PWM™ Dimming

Wide Input Voltage Range: 6V to 100V

Current Monitoring Up to 100V

High Side PMOS Disconnect and PWM Switch Driver

Constant-Current and Constant-Voltage Regulation

Dual Current Sense Amplifiers with Reporting

C/10 Detection for Battery and SuperCap Charging

Linear Current Sense Threshold Programming

Short-Circuit Protection

Adjustable Frequency: 100kHz to 1MHz

Frequency Synchronization

Programmable Open LED Protection with VMODE Flag

Programmable Undervoltage Lockout with Hysteresis

Soft-Start with Programmable Fault Restart Timer

Low Shutdown Current: <1μA

Available in 28-Lead TSSOP Package

LT3796应用:

High Power LED, High Voltage LED

Battery and SuperCap Chargers

Accurate Current Limited Voltage Regulators

图2. LT3796带输入电流监视的升压LED驱动器电路图

图6. LT3796 SEPIC密封铅酸(SLA)电池充电器电路图

评估板DC1706A

DC1706A is a 100V LED controller with an input current monitor. It is a single switch boost LED driver with LED+ to GND short circuit protection. It accepts an input voltage from 8V to 100V (7.5V UVLO and 8.7V turn-on), and drives up to 85V of LEDs at 400mA (when PVIN is less than VLED). DC1706A features both PWM and analog dimming of the LED string. It has a VMODE flag that indicates when the LED string has been removed and it has a FAULT flag that indicates that the output has been shorted to GND. In both cases, the IC remains in control and well protected.

DC1706A features high efficiency at 300kHz switching frequency. At high LED string voltages up to 85V and 400mA of LED current, the single switch controller has 93% efficiency. There are input current (CSOUT) and output current (ISMON) monitor pins that deliver a voltage proportional to the current for quick diagnostics.

Small ceramic input and output capacitors are used to save space and cost. The open LED overvoltage protection uses the IC’s constant voltage regulation loop to regulate the output to approximately 92.5V if the LED string is opened although it may reach 96.5V peak during transient from running LEDs to open.

For low input voltage operation, the CTRL pin voltage is reduced as the input voltage drops below 9V, reducing LED brightness and restraining the peak switch currents in order to limit inductor and switch size. UVLO turns the LEDs off when VIN drops below 7.5V. 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  DC1706A PWM dimming is simple. The topside PWM dimming MOSFET (M2) turns the LED string on and off with an input to the PWM dimming terminal. For the highest PWM dimming ratio, it is recommended to use 100Hz as a PWM dimming frequency. Information regarding PWM dimming ratios and performance can be found in
the LT3796 data sheet in the applications section. Analog dimming is also simple to use with a single voltage source on the CTRL terminal.

Modifications can be made to DC1706A in order to convert the board to higher or lower power or from an LED driver to a constant voltage regulator or battery charger. It can easily be changed from a boost topology to a SEPIC, buck mode, or buck-boost mode LED driver. Please consult the factory or refer to the LT3796 data sheet for details.

The LT3796 data sheet gives a complete description of the part, operation and applications information. The data sheet must be read in conjunction with this demo manual for DC1706A. The LT3796 is assembled in a 28-lead plastic TSSOP FE package with a thermally enhanced ground pad. Proper board layout is essential for maximum thermal performance. See the data sheet section Layout Considerations.