The high output from the Luxeon and Nichia 1W white LED 12W incandescent lamps of similar brightness level, while the power consumption of only 1W, and the service life of up to 50,000 hours or more. For many light bulb applications, these devices is expected to achieve significant energy saving effect, and reduce maintenance costs. However, in order to maintain the correct brightness, must be a constant current to drive these LED lights. The range of process and temperature within the limits of the forward voltage drop of 2.8V to 4.0V. Circuit for driving the LED at the same time maintain a constant current drive to compensate for the forward voltage changes. The boost circuit is usually voltage feedback switching converter with additional circuitry to detect the output current (not voltage). This leads to the circuit structure is complicated and inefficient.
The LTC3490 provides a way for single or dual battery voltage is raised to the desired LED forward voltage and to adjust the current through the LED load a simple solution. The high frequency (1.3MHz) operation allows the use of small value inductors and capacitors. The current sense resistor and loop compensation components are built, thus reducing the number of components. The LTC3490 is a synchronous converter, eliminating the rectifier diode and the associated efficiency loss. Components needed for just the boost inductor and an output filter capacitor. Shutdown and dimming feature adds a small amount of resistance, and, in some cases also propose to introduce an input capacitor.
Circuit Description
The LTC3490 is a synchronous boost converter. Its box is shown in Figure 1. The use of a low-voltage startup circuit, it can start under normal conditions of input voltage down to 0.9V. When the output voltage exceeds 2.3V, the boost circuit will be connected to the start-up circuit is turned off. The boost converter is a fixed frequency current mode architecture.
the LTC3490 block diagram
LED current in the high pressure side of the internal 0.1Ω resistor to detect, which allows the LED tubes to negative ground. A sense amplifier is responsible for the voltage to be compared with the reference current flow through a ratiometric match 19.2Ω resistor. Detection voltage differential calculus, and for the setting of the PWM controller. Therefore, LED current is constant, this has nothing to do with the LED forward voltage.
Two battery applications, the LTC3490 efficiency up to 90%; in single cell applications, the LTC3490 efficiency higher than 70%.
the LTC3490’s efficiency
Overvoltage protection
Current sensing controller may drive the output voltage to the devastating levels (if there is no load), so, you need to output overvoltage protection. If the LED panel from the circuit is removed or fails, it will happen. As long as the output current is below 350mA, the output voltage will continue to climb and there may be damage to the LTC3490 in the case of the overvoltage protection circuit. When the output voltage higher than 4.5V, an overvoltage detector forces the LTC3490 into shutdown mode. Overvoltage detector to keep connected to the state, will resume normal operation when the output voltage drops below 4.5V.
Dimming function
The LTC3490 can use the CTRL / SHDN pin to gradually reduce the LED current. CTRL / SHDN input has three functions: stop, dimming control and constant current output. The pin with a proportional relationship exists between the VIN pin voltage, which makes using a simple resistor divider to set the current value. When the CTRL / SHDN pin voltage is below 0.2? The VIN, the device is in shutdown mode, while the absorption current is very small. When the CTRL / SHDN pin voltage is higher than 0.9? The VIN, the device is 350mA constant current mode. When the VIN between the CTRL / SHDN pin voltage is 0.2? The VIN and 0.9?, LED current from 0mA to 350mA linear change.
Low-battery detection
The LTC3490 provides two low-battery detection level. These level set by CELLS pin, for the number of the section indicates that the battery. CELLS pin is low, low battery detection level is set to 1.0V, and when connected to VIN CELLS, low battery detection level was set to 2.0V. Which correspond to single and double battery operation. When the battery voltage falls below the detection level / LOBAT pin on an open-drain output is pulled low. The output can be used to drive an indicator, or feedback to the CTRL / SHDN pin to reduce the LED current, thus extending the remaining running time of the battery.
Shutdown the LTC3490 when there is an undervoltage lockout circuit, the circuit in the battery voltage drops below 0.8V per section. This can prevent the battery current is too large (single cell), and uneven-discharge Ni-MH battery, reverse battery (two batteries).
Battery reality check
The battery there is a so-called “discharge recovery” phenomenon. When to move away load from a battery is almost exhausted battery, the battery terminal voltage will be restored to a surprisingly high voltage. LTC3490 when a nearly depleted battery dead battery shutdown transition occurs, the decrease of the absorption current makes the battery voltage can be restored. This will be re-connected to the LTC3490, and load again added to the battery. At this point, the battery voltage will drop to once again trigger the shutdown operation. This phenomenon has led to the LTC3490 quickly switched on and off the LED current. Observations: When the battery charge is close to exhausted, the average LED current decreases slowly. The LTC3490 provides a simple solution using alkaline batteries or Ni-MH battery to drive the high-output white LED’s. It has the characteristics of high efficiency and less number of components.