OLED and led lights, also known as OEL (Organic Light-Emitting Devices). This device has a self-luminous, clear and bright, thin, fast response, wide viewing angle, low power, low cost, simple manufacturing process, characteristics of its inception, has been regarded as the most promising after the following LCD display. Currently, cellular phones, personal assistants (PDAs), digital cameras and other areas, OLED has been widely used, but very strict requirements for OLED power bias circuit, generally require high efficiency, small size, light weight the step-up transformer step-up transformer should also have good electromagnetic compatibility.
The MAX8570 is a MAXIM company launched the OLED step-up transformer chip. It is not only advanced design, perfect function, and the external circuit is simple, very flexible to use, design OLED power bias circuit is an ideal device.
Characteristics and working principle of the MAX8570 converter The MAX8570 converter characteristics
The MAX8570 converter main features are as follows:
(1) The chip power MOSFET, energy-saving circuit, control logic circuit and protection circuit integrated to simplify the peripheral circuit design, reduce costs, enhance system reliability;
(2) Working frequency up to 800kHz, allows the use of miniature table-mount components;
(3) ultra-low power consumption: quiescent current is 25mA, in True shutdownTM mode, current consumption as low as 0.05μA (typical);
(4) open-circuit protection to prevent the output capacitor and load damage;
(5) allows the use of the battery, the output voltage up to 20V, to meet the high voltage requirements of the OLED power bias circuit. working principle
(1) control mode
Figure 1 shows the internal block diagram of the MAX8570. When the MAX8570 power its chips inside the P-channel MOSFET on, the power supply voltage VCC through the inductor L1 is divided into two: one sent to the drain of the internal N-channel MOSFET, another path VD1 to C4 charge. When the N-channel MOSFET on, the current flowing through L1 gradually increases from zero, while the electrical energy stored in L1, once the current limit is reached, the N channel MOSFET turns off, L1 current by VD1 the load. In the way of the switching frequency will vary with the size of the load and supply voltage of up to 800kHz.
(2) shutdown (SHDN)
SHDN pin is pulled low, the device will enter the True shutdownTM Mode (off state), in True shutdownTM mode, the circuit’s current consumption as low as 0.05μA output and the input is disconnected, the LX pin is in high resistance state in the conventional boost converter circuit, the converter shutdown, the output and input will always be through the inductor and freewheeling diode connected to the load to draw some power from the input. The MAX8570 converter with True Shutdown, in True shutdownTM mode, it will make use of a P-channel MOSFET switch to disconnect the input and output, thus eliminating the shutdown, the power drawn from the input side.
(3) separate to the inductance power supply
Can be a separate supply of the IC and the inductor, the inductor supply voltage range of 0.8-28V, as shown in Figure 2 is a separate application to the inductor supply circuit, this circuit structure of the SW pin should be vacant, so that the power supply directly connected inductance, then output and input is disconnected, but no longer shutdown shutdown, the output voltage than the low inductance power supply voltage one diode drop.
(4) soft-start
When in True shutdownTM function, the circuit will slowly open the gate of the internal P-channel switch, to be fully open SW, the internal N-channel switch turns on, so to avoid inrush currents when VFB is lower than 0.5V , the internal N-channel switch off-time from 1μs to extend to 5μs, to control the inrush current.
(5) open-circuit protection
Open-circuit failure occurs when the upper feedback resistor or external diode The MAX8570 converter will immediately stop switching, in order to avoid too much damage to the output filter capacitor and load output voltage.
OLED power bias circuit design
The limit the current settings
Design, according to the following formula to estimate the value of the limiting current:
Where, POUT (MAX), maximum load power; VBATT (MIN) is the minimum supply voltage; POUT (MAX) for the maximum converter output voltage.
The output voltage setting
The MAX8570’s output to GND is connected between a resistor divider between the point then the FB voltage divider ratio determines the output voltage (the voltage adjustment range for the VCC-28V), the range of R2, 10-600KΩ So according to the following formula R1:
In order to improve the accuracy of the output voltage, VFB 1.266V; value of VOUT VCC to 28V, the design should be to ensure minimum bias current flowing through the feedback resistor 2μA.
The inductance of the Select
The inductor is a key component of the boost converter, its value directly affects the size of the normal work of the entire circuit. If you need to pursue a high-performance, the best choice for low inductance and inductance, MAX8570 work in the lower frequency, the inductor will produce magnetic saturation, causing ripple current increases or damage to the inductance, it is in the choice of inductance , according to the output current The MAX8570’s operating frequency of the inductor, the inductance of the rated current and ripple voltage conditions to the decision here Recommended inductor values range from 10-100μH.
The diode of choice
Diode switching losses to a fifth sixth of the total system loss is a major factor affecting the efficiency of the boost converter, including forward losses and reverse recovery loss, and recommend the use of a fast recovery time and forward voltage small drop of the Schottky diode, the rated current value should be greater than the peak switch current, reverse breakdown voltage greater than the output voltage. The Schottky diode MBR0520, MBR0530, ZHCS400.
The capacitance of the Select
The output capacitor can maintain the output voltage can also smooth the ripple voltage due to MOSFET switch (N-channel MOSFET on the output capacitor to the load). Therefore, under the premise of ensuring sufficient bandwidth, you should select ESR (equivalent series resistance) ESL (series equivalent inductance) is small, high pressure value of the output capacitor such as X5R or X7R dielectric material in ceramic capacitors, most applications, circuit design, its capacity is desirable 1μF.
The input capacitance is mainly used to filter out the ripple voltage in the power supply, it is recommended that ceramic capacitors with X5R or X7R dielectric materials, capacitance values between 1-6.8μF.
Increase a feedback capacitance, and improve the stability of the output voltage, it is recommended to use a 47pF ceramic capacitor in the the MX85701 output and FB.