The efficiency of an LED drive can be as high as 91% independent control of 18 LED

In addition to call function, the newest cellular phone function is increasing, can browse the web, with wireless e-mail, photos and even watch TV shows, which requires the ability to provide multiple display backlighting, driving all kinds of lights and lighting for the camera flash. For example, a cellular phone may have a main screen for viewing text and images, a secondary display of time and date, and a white LED for camera flash. Due to the limited space of the cellular phone, so the high integration multi display LED driver is a good choice.

Linear Technology Corporation (Linear), before the launch of inductorless, low noise, high efficiency LED driver LTC3220/LTC3220-1, can be used for cellular phone screen and other programmable lighting applications. The device provides 18 separate configurable LED current sources with full flexibility when programming multiple screens. The LED current is set by an internal precision current reference. All the current source of the independent dimming, turn on / off, flashing and gray level control are achieved through the I2C serial interface. 6 bit linear DAC can be used to independently adjust the brightness level of each common LED current source. The 2.9~5.5V input voltage range of the LTC3220/LTC3220-1 has been optimized for single-cell Li ion / polymer battery applications. Lithium ion battery (nominal value of 3.6V) drive, the efficiency of up to 91%, quiescent current as low as 500uA, to maximize the battery working hours. In addition, the slew rate limited switch reduces conduction and radiation noise (EMI).

The LTC3220/LTC3220-1 charge pump optimizes the efficiency according to the voltage of the LED current source. These devices are powered up in the 1x mode, and automatically switch to boost mode when the LED current source that is being started to enter the voltage drop. The first voltage drop will convert these devices to the 1.5x mode, and then the voltage drop will switch the LTC3220/LTC3220-1 to 2x mode. Specific operation is as follows: TC3220/LTC3220-1 uses a switched capacitor charge pump will boost CPO to 2 times the input voltage, up to 5.1V. The device begins to enter the 1x mode, in this mode, the VIN is directly connected to the CPO. This model can provide the highest efficiency and minimum noise. The LTC3220/LTC3220-1 will remain in the 1x mode until the LED current source voltage drops. A voltage drop occurs when the current source voltage is too low relative to the programmed voltage. After the pressure drop is detected, the LTC3220/LTC3220-1 will be converted to 1.5x mode. Then the CPO voltage will begin to increase, up to 1.5 times the input voltage up to 4.6V. Any subsequent pressure drop will cause the device to enter the 2x mode. CPO voltage will rise to 2 times the input voltage up to 5.1V. Activating a charge pump switch with a dual phase non overlapping clock. In the 2x mode, a fast capacitor (flying capacitor) is used to minimize the CPO voltage ripple in the alternating clock phase from the VIN. In the 1.5x mode, the two fast capacitors are connected in series during the first clock phase and are connected in parallel with the VIN in the second clock phases. Charging and discharging the fast capacitors at constant 850kHz frequency.

The current provided by each LED current source is controlled by an associated DAC, each of which is programmed by the I2C interface DAC. When a data bit is updated via the I2C port, these devices will be reset to 1x mode. The internal circuit prevents inrush current and excessive input noise when starting and mode switching.

Other features of the device include: multi mode (1x/1.5x/2x) low noise charge pump with high efficiency up to 91%, the total output current up to 360mA, a single reset pin for asynchronous shutdown and all data register reset, provides two I2C addresses (LTC3220:0011100; LTC3220-1:0011101), with automatic or forced mode switching and short circuit and overheating protection.

LTC3220/LTC3220-1 features an ultra thin (0.55mm) 28 lead QFN (4 x 4mm) package. Only 5 small ceramic capacitors LED power supply can be composed of tiny, complete and current controller solution. The device is suitable for the use of QVGA+ display video phone, keyboard lighting and general / all kinds of lighting.

Typical applications of LTC3220/LTC3220-1 multi output LED driver.

Source: electronic system design