The topology of the RT4730 power solution for wearable AMOLED devices

Wearable devices that use AMOLED displays are often powered by lithium-ion or lithium-polymer batteries. The batteries operate in the range between 3.0V and 4.xV, as shown in the figure below:

The batteries operate in the range

The display driver circuit usually operates at 3.3V, which is in the middle of the battery operating voltage. In order to operate normally in the full battery range, it requires both step-up and step-down regulations. Additionally, to drive AMOLED displays for correct display information, it requires one positive and one negative voltage rails.

As its name implies, the active matrix organic light-emitting diode (AMOLED) operates like a diode. Any changes of its supply voltage result in different I-V characteristics. Therefore, it requires high-precision power supply and fast transient responses to avoid any voltage fluctuations for stable picture quality.

Based on the requirements above, Richtek developed the RT4730, AMOLED power solution. Please see below for its topology:

RT4730 topology

As shown in the diagram, AVDD is generally set at 3.3V, for the drive circuit of AMOLED displays; OVDD is pre-set at 3.3V, ranging from 2.8V to 4.2V in 100mV per step for the positive voltage rail; OVSS is pre-set at -3.3V, ranging from -0.6V to -4V in 100mV per step for the negative voltage rail.

To ensure the accuracy of each output rail as well as fast transient response, linear regulators (LDOs) are usually used as the final power stage. The charge pump circuit operating in switch mode converts the positive voltage to a negative voltage. The Buck-Boost circuit converts the input battery voltage to intermediate voltage rails which are normally set higher than the maximum value of AVDD, OVDD or -OVSS by 0.2V. Therefore, regulators in the next stage have enough head room for adjustment to achieve better efficiency and performance.

The RT4730 application circuit only uses one inductor for three adjustable output rails. All output voltage rails can be adjusted by SWIRE interface protocols, allowing users to switch on/off each output rail and to programme voltage levels via SWIRE pulses.

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