feature

Voltage Range: 2.7V to 5.5V at; fully programmable single wire current; 16 current levels; four low current settings down to 50 microamps; low I (50 microamps) in low current mode; three modes 1x, 1.5x, 2x maximum charge pump efficiency and V coverage Type F; drives up to four LED channels; no inductor, low noise operation; 1MHz constant switching frequency; small application circuit; built-in thermal protection; automatic soft-start; i<1µA Q at shutdown; 2.85x3.0mm TSOPJW-12 package.

application

Color (RGB) lighting; programmable current sink; white LED backlight; white flash for digital cameras.

General Instructions

The AAT3155 is a low noise, constant frequency charge pump DC/DC converter that uses tri-mode load switching (1x), fractional (1.5x) and multiplying (2x) conversion to maximize efficiency for white LED applications. The AAT3155 is capable of driving up to four LED channels from a 2.7V to 5.5V input at 20mA per channel. Current sinks can be operated individually or in parallel to drive higher current LEDs The low external part count (two 1µf flying capacitors and two small 1µf capacitors at input and output CP) makes this part ideal for small batteries powered applications.

Skyworks' SCWire (Simple Serial Control) serial digital input is used to enable, disable and set the current of each LED, with 16 settings down to 50 microamps. Low current mode supply current can be as low as 50µA to save power. 2™

Each output of the AAT3155 is equipped with built-in output short-circuit protection and automatic disabling under load short-circuit conditions. Built-in soft-start circuit to prevent excessive inrush current during startup. A low-current shutdown feature disconnects the load from the input and reduces quiescent current to less than 1 µA.

The AAT3155 is available in a lead-free, space-saving 2.85x3.0mm 12-pin TSOPJW package.

Typical features

cin=cout=c1=c2=1.0µf; ta=25°C, vin=3.6V unless otherwise noted.

Function description

The AAT3155 is a three-mode load switch (1X) and high efficiency (1.5X or 2X) charge pump device for white LED backlight applications. To maximize power conversion efficiency, an internal sense circuit monitors each constant current receiver input for any required voltage and set the load switch and charge pump mode based on the input battery voltage and current sink input voltage. As the battery discharges over time, the AAT3155 charge pump will enable the charge pump to initially start in 1.5x mode when any of the four current sink inputs are near disconnect. If the charge pump output drops enough that any current source output is close to falling, the charge pump will automatically switch to 2X mode. The AAT3155 requires only four external components: two 1µF ceramic capacitors for the charge pump flight capacitors (C and C), a 1µF ceramic input capacitor (C), and a 0.33µF to 1µF ceramic charge pump output capacitor (C).

Four constant current receiver inputs (D1 to D4) can drive four independent LEDs, each with a maximum current of 20mA. Unused receiver inputs must be connected to OTCP, otherwise the part will only operate in 2X charge pump mode with the SCWIRE serial interface enabled AAT3155 and set the current receiver size.

Constant current output level setting

Set constant current receiver levels from D1 to D4 via serial interface based on logarithmic scale for first 12 codes and separate low current scale for last 4 codes. For first 12 codes, each code is approximately lower than the previous code 1.5 dB. In this way, the LED brightness scales linearly with each additional code. Since inputs D1 to D4 are truly independent constant current sinks, the voltage observed on any given input will be determined by the actual forward voltage (V) of the LED being driven. Type F Since the input current sink of the AAT3155 is programmable, no PWM (pulse width modulation) or additional control circuitry is required to control the LED brightness. This feature greatly relieves the microcontroller or system IC of managing LED or display brightness, allowing users to "set it and forget it." Through its high-speed serial interface (1MHz data rate), the AAT3155's input receive current can be changed continuously, brightening or dimming LEDs, with smooth transitions (e.g., fading out) or in abrupt steps, giving the user full programmability and real-time Control LED brightness.

Serial interface 2

The magnitude of the current level is recorded by Skyworks' Simple Serial Control (SCwire) serial interface control interface to record the rising edge of the en/set pin and decode it into 16 different states. Table 1 shows the 16 available current level settings.

Serial interface timing

The SCWORE serial interface has flexible timing. Data can be input at speeds greater than 1 MHz or slower, such as 15 kHz. After the data is committed, en/set is held high to lock the data. Once en/set remains at logic high for time t, the programming current becomes active and the internal data registers are reset to zero. For subsequent current level programming, the number of rising edges corresponding to the desired code must be entered on the en/set pin.

When EN/SET is held low for an amount of time greater than T, the AAT3155 enters shutdown mode and draws less than 1µA of current from V. During shutdown, the internal data registers are reset to zero.

Auto-disable feature

The AAT3155 is equipped with an auto-disable function for each LED channel. After the IC is enabled and booted, a test current of 100 microamps (typ) is forced through each receiver channel. If the voltage on a particular receiver pin does not drop below a certain threshold, the channel will be disabled. Very handy for channels used or during short events of LED failure.

Thermal Protection

The AAT3155 has a thermal protection circuit that shuts down the charge pump when the die temperature rises above the thermal limit, as if the OUTP pin was shorted.

application information

Although the AAT3155 is designed to drive white LEDs, the device can also be used to drive most types of LEDs with forward voltage specifications ranging from 2.0V to 4.7V. LED applications may include main LCD and sub-LCD display backlighting, camera photo flash applications, color (RGB) LEDs, infrared (IR) diodes for remote controls, and other loads that benefit from controlled output current generated by varying input voltage due to D1 The input current sink to D4 is matched with negligible voltage dependence, so the LED brightness will match regardless of the specific LED forward voltage (V) level ), may need to drive high-V LEDs. The low leakage current receiver in the AAT3155 enables it to drive LEDs with forward voltages up to 4.7V at full current from an input supply as low as 3.0V. The outputs can be paralleled to drive high current LEDs without complications.

Device Switching Noise Performance

The AAT3155 operates at a fixed frequency of approximately 1MHz to control noise and limit harmonics, which can interfere with the RF operation of cell phone handsets or other communications equipment. The reverse injection noise present at the input pin of the charge pump is 20 mV peak-to-peak, typically ten times less than an inductor-based DC/DC boost converter white LED backlight solution. The AAT3155 soft-start feature prevents the charge pump Noise transient effects associated with inrush current during circuit startup.

Capacitor selection

Careful selection of the four external capacitors, C, C, and C, is important because they affect turn-on time, output ripple, and transient performance. Best performance is obtained with low Equivalent Series Resistance (ESR) ceramic capacitors. In general, low ESR can be defined as less than 100mW. When choosing capacitors, a 1µf value for all four capacitors is a good starting point. If the LED current source is only programmed for the photocurrent level, the size of the capacitor may be reduced.

Capacitor Characteristics

Ceramic synthetic capacitors are highly recommended over all other types of capacitors used with the AAT3155. Ceramic capacitors have many advantages over tantalum and aluminum electrolytic capacitors. Ceramic capacitors generally have very low ESR, lowest cost, smaller PCB footprint and non-polar Low ESR ceramic capacitors help maximize charge pump transients response. Since ceramic capacitors are non-polarized, they are less prone to damage by wrong connections.

Equivalent series resistance

ESR is an important characteristic to consider when selecting a capacitor. ESR is the resistance inside the capacitor, caused by the leads, internal connections, size or area, material composition, and ambient temperature. For ceramic capacitors, capacitor ESR is usually measured in milliohms, for For tantalum or aluminum electrolytic capacitors, the ESR range can exceed a few ohms.

Ceramic Capacitor Materials

Ceramic capacitors less than 0.1µF are usually made of NPO or C0G materials. The npo and c0g materials usually have tight tolerances and are very stable over temperature. Larger capacitance values typically consist of X7R, X5R, Z5U or Y5V dielectric materials. Large ceramic capacitors (i.e. greater than 2.2µF) are typically available for low-cost Y5V and Z5U dielectrics, but AAT3155 applications typically do not require capacitors greater than 1µF.

Capacitive area is another contributor to ESR. Capacitors with larger physical dimensions have lower esr compared to smaller capacitors of the same material. These larger devices can improve the transient response of the circuit compared to equivalent capacitors in smaller package sizes.