ISL97702,Dual output boost converter,Boost with Dual Reference Outputs
Power the passive OLED display of handheld devices
OLED or organic light-emitting diode display is a new technology that has revolutionized the field of displays. Based on the principle that organic materials emit light when an electric current passes through them, OLED has many advantages over the current LCD (liquid crystal display) technology. One of the advantages is the ease of manufacturing, which will result in lower manufacturing costs, and other performance benefits include faster response, wider viewing angles, lower power, brighter and higher contrast. Another advantage is that it saves power and makes the display 1mm thick!
Similar to LCD displays, OLED also has passive array and active array modes. In passive array mode, the display is like a grid of diodes. Each time an external drive circuit is used, a diode is lit on each line. Active array displays contain transistors inside, meaning every pixel is emitting light all the time. However, unlike LCDs, OLED is driven by electric current. This increases the complexity of active array design, which is now used in most products. These PMOLED are widely used in mobile phones, car stereos, MP3s, and other consumer products.
Power OLED displays
Because many OLEDs are now used in portable applications, power consumption is very important. Power ICs must be designed for maximum efficiency and save as much power as possible to extend battery life, especially when the display is not working.
The power requirements of OLED displays depend on many factors. Since the display is driven by electric current, the demand for peak current depends on the total number of pixels that need to be lit at the same time and the maximum current that can be driven. The additional current can also be drawn by the display's drive circuitry. The required voltage depends on the forward voltage drop of the diode, the voltage drop at the diode connection, and the voltage drop required by the display driver (Figure 1).
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Figure 1. OLED display driver circuit.
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In this application, the maximum voltage required is:
Vdiode + Idiode ×(Rcol + Rrow) + VCD + VRD （1）
Thereinto:
● VDIODE is the forward voltage drop of the diode
● Idiode is the current of the diode
● RCOL is the resistor of the column connection
● Rrow is a row metal resistor
● VCD is the overhead required for column drives
● VRD is the overhead required for the row drive
In a typical application, this voltage is about 20V.
The peak current is:
Idiode x Xpixels+ICD+IRD （2）
● Idiode is the current of the diode;
● Xpixels are the number of pixels lit at the same time.
In portable devices that use LCD displays, the background light is usually turned off after a certain period of inactivity, and it takes a few seconds for the display to completely power off. With OLED displays, there is no backlight. As a result, the display dims after a period of inactivity and then turns off. In Equation 1, it can be seen that if the display current decreases, the maximum voltage required also decreases. In a typical application where the supply voltage is constant, the column driver will drop this extra voltage, resulting in additional power consumption. This is a waste of energy. By reducing the supply voltage, the column driver will no longer waste energy and the system efficiency will increase.
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| Figure 2. Typical ISL97702 circuit. |
ISL97702 introduction
Intersil's ISL97702 is specifically designed to power PMOLED displays in portable applications. The device features a very efficient boost converter that saves power. It also offers a very small component solution and a small QFN package to reduce the size of portable devices. The complex control circuitry used ISL97702 is the most advanced power IC available in portable applications. The typical circuitry of this device is shown in Figure 2.
Boost converter
The boost converter integrated in the ISL97702 operates from 2.3V to 5.5V. This includes the full input range of lithium-ion and of course can be operated from other 3V to 5V power supplies. ISL97702 integrates a 1.2A boost FET and Schottky diode and requires very few external components. It supports output voltages up to 28V and 90% efficiency. Compared with similar solutions and even synchronous structures, it competes fiercely. ISL97702 choose an asynchronous topology because the power supply and circuitry required for a second integrated FET to operate in low-current applications often exceed the capacity of the synchronous structure. The efficiency of the ISL97702 is further improved using Intersil's proprietary power IC design process.
Dual output voltage selection
ISL97702 also includes a dual feedback design. By the state of the SEL pin, one of two feedback networks can be selected to control the level of the output voltage. This feature can support the choice of three modes: light->-dark-> in typical PMOLED applications. Both feedback circuits have better output accuracy than ±2.5% at all temperatures.
The input voltage is disconnected
ISL97702 integrates a disconnect switch at the input of the boost circuit. When the device is disabled, the switch turns on to disconnect the OLED display, driver, and feedback network without creating leakage current. This reduces battery consumption and extends the time it takes to charge the portable device. In power-off mode, the power consumption of the ISL97702 itself is less than 3 cents.
Clock synchronization
ISL97702 uses an internal 1MHz oscillator when working properly. However, the SYNC input synchronizes the ISL97702's switching frequency with an external clock source from 600kHz to 1.4MHz. This is very useful in portable applications where some frequencies interfere with other devices, or when two conversion devices produce out-of-sync beats.
Soft start control
When ISL97702 is enabled, or when it recovers from a detected fault (see Fault Detection), the device enters soft-boot mode. This mode eliminates the starting currents that can cause battery supply voltage deviations that affect the operation of other components of the system. In this mode, when the output capacitor is first connected, the current that disconnects the switch by the input voltage is controlled, so there is no current spike pulse. When the boost converter starts working, the current limit is gradually increased, which also ensures that there are no spike pulses of the input current.
Fault detection
ISL97702 also integrates several protection circuits to ensure that the IC and external components are protected.
Low voltage lockout ensures that the device operates with the input voltage above the minimum required for normal operation.
Overcurrent protection monitors the output current and restarts the ISL97702 when the overload current is limited for a period of time.
Overvoltage lockout stops the ISL97702 when the output voltage exceeds the maximum allowable value. Once cleared, ISL97702 enter boot mode.
Overheat protection shuts off ISL97702 when the die temperature exceeds the maximum allowable value.