LT3496 - Triple Output LED Driver

Welcome to the training module on LT3496 – Triple Output LED Driver. This training module introduces LT3496 triple output LED driver and its basic operations.
The LT3496 is a triple output DC/DC converter designed to operate as a constant-current source and is ideal for driving LEDs. Each of the LT3496's three channels can drive up to eight 500mA LEDs in series, enabling it to drive up to 24 x 500mA LEDs at efficiencies up to 96%. All three channels are operated by an independent True Color PWM™ signal, enabling each to be dimmed independently to ratios as high as 3,000:1. A fixed frequency, current mode architecture ensures stable operation over a wide range of supply and output voltages. The LT3496 works in buck, boost or buck boost mode. A frequency adjust pin allows the user to program switching frequency between 330kHz and 2.1MHz to optimize efficiency and external component size.
The LT3496 is a triple output DC/DC converter designed for high performance, True Color PWMTM dimming in multi-channel LED lighting applications. By integrating three independent driver channels, the LT3496 provides a space-saving solution to drive multiple LED strings. The flexibility to operate in buck, boost or buck-boost mode makes the LT3496 feasible in many rugged applications. It can go into RGB Lighting, billboards and large displays, automotive and avionic lighting, constant current sources.
The LT3496 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation The oscillator, ramp generator, reference, internal regulator and UVLO are shared among the three converters. The control circuitry, power switch etc., are replicated for each of the three converters. If the SHDN pin is tied to ground, the LT3496 is shut down and draws minimal current from VIN. If the SHDN pin exceeds 1.5V, the internal bias circuits turn on. The switching regulators start to operate when their respective PWM signal goes high. The start of each oscillator cycle sets the SR latch, A3, and turns on power switch Q1. The signal at the non-inverting input (SLOPE node) of the PWM comparator A2 is proportional to the sum of the switch current and oscillator ramp. When SLOPE exceeds VC (the output of the error amplifier A1), A2 resets the latch and turns off the power switch Q1 through A4 and A5. In this manner, A10 and A2 set the correct peak current level to keep the output in regulation.
The three LT3496 channels operate independently, but function in the same way. For simplicity, the PWM operation of channel 1 is introduced here. LED1 can be dimmed with pulse width modulation using the PWM1 pin and an external P-channel MOSFET, M1. In the configuration of the figure, if the PWM1 pin is pulled high, M1 is turned on by internal driver A7 and converter 1 operates nominally. If the PWM1 pin is pulled low, converter 1 stops operating and M1 is turned off, disconnecting the LED string of channel 1 and stops current draw from output capacitor C2. The VC1 pin is also disconnected from the internal circuitry and draws minimal current from the compensation capacitor C C . The VC1 pin and the output capacitor store the state of the LED1 current until PWM1 is pulled up again. This leads to a highly linear relationship between pulse width and output light, and allows for a large and accurate dimming range.
Traditional LED drivers employ a low side LED disconnect approach, in which both the high side and the low side of each LED string must connect to the LED driver. The figure is a simplified configuration of LT3496, where M1-M3 are LED-disconnect PMOS switches. Because the LED disconnect and current sensing are on the high side of each LED string, the low sides of the LED strings can be tied together in boost or buck-boost mode to reduce the number of wires returning to the LED driver. In a boost configuration, each of the low side connections can be returned to ground anywhere, allowing a simple 1-wire LED connection for each LED string.
This page gives you information about programming the LED current, which can be done by connecting a external sense resistor R SENSE in series with LED load, and setting the voltage regulation threshold across that sense resistor using CTRL input . The CTRL pins should not be left open. The CTRL pin can also be used in conjunction with a PTC thermistor to provide over-temperature protection for the LED load.
Other features of the LT3496 include open-LED protection and undervoltage lockout thermal limiting. Open-LED protection is provided for all the three converters through OVP1/2/3 pins and the internal power switch Q1/Q2/Q3. In the event the LED string is disconnected or fails open, the converter output voltage will increase, causing OVP voltage to increase. When OVP voltage exceeds 1V, the power switch Q will turn off, and cause the output voltage to decrease. Eventually, OVP will be regulated to 1V and the output voltage will be limited. In the event one of the converters has an open-LED protection, the other converters will continue functioning properly. The LT3496 has an undervoltage lockout circuit that shuts down all the three converters when the input voltage drops below 2.4V. This prevents the converter from switching in an erratic mode when powered from a low supply voltage.
The LT3496 can be configured as a buck mode LED driver for applications where the LED voltage is lower than the supply voltage. This figure shows a triple buck mode LED driver. Each channel can drive 500mA of current to its LEDs. Each LED string can have from eight to twelve LEDs, depending on type. The 2.1MHz switching frequency minimizes the solution size by allowing the use of low profile inductors and capacitors. Efficiency can be above 95% for a LT3496 buck mode driver. At 120Hz PWM frequency, the PWM control of this configuration allows 5000:1 dimming as shown in the Dimming Waveforms chart.
The LT3496 can be configured as a boost LED driver for the applications where the LED voltage is higher than the supply voltage. Here shows a triple boost mode driver that delivers 200mA to each LED string from a regulated 12V. This configuration can be used for automotive lighting. D4, Q1–Q3, and R1–R4 create the battery surge voltage protection circuits to protect the LED string from being damaged by a battery surge voltage. The waveform chart shows the 3000:1 PWM dimming waveforms at 120Hz PWM frequency. Unlike the buck mode driver, the boost mode drivers always require an OVP circuit at the output for open LED protection.
In some LED applications, the desired supply voltage range and LED voltage range overlap, thus requiring buck-boost mode configuration. This figure shows a buck-boost mode LED driver. Here the switch voltage is the sum of the input voltage and the LED voltage. Therefore, it is necessary to turn off the internal power switch before the input voltage gets too high. It drives four LEDs, at 200mA per channel. The circuit monitors the Schottky diodes’ cathode voltage (VSC). The OVP logic turns off the main switch when VSC is above 38V, preventing the switch voltage from rising further. Since no IC pin experiences absolute maximum voltage, the circuit survives the load dump event. The waveform chart shows the 3000:1 PWM dimming waveforms at 120Hz PWM frequency.
Thank you for taking the time to view this presentation on LT3496 – Triple Output LED Driver . 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 Corporation site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.

LT3496 - Triple Output LED Driver

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