ISL97702,Dual output boost converter,Boost with Dual Reference Outputs
Power passive OLED displays for handheld devices
OLED, or organic light-emitting diode display, is a new technology that revolutionizes the display field. Based on the principle that organic materials emit light when an electric current passes through, OLED has many advantages over the LCD (liquid crystal display) technology currently used. One of the advantages is simple manufacturing, which leads to lower manufacturing costs, and other performance advantages include faster response, wider viewing angles, lower power, brighter and higher contrast. Another advantage is that it saves power and makes the monitor 1mm thick!
Similar to LCD displays, OLEDs also have passive array and active array modes. In passive array mode, the display resembles a grid of diodes. Each time an external drive circuit is used, one diode is lit per row. Active array displays contain transistors inside, meaning that each pixel shines light all the time. However, unlike LCDs, OLEDs are driven by electric current. This adds complexity to active array designs, which are now used in most products. These PMOLED are widely used in mobile phones, car stereo units, 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 with maximum efficiency and save as much power as possible to extend battery life, especially when the display is not operating.
The power requirements of an OLED display depend on many factors. Since displays are driven by 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. Additional current can also be consumed by the display's drive circuitry. The voltage required 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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fig1. 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
● Ricol is the resistor connected by the column
● Rrow is a row metal resistor
● VCD is the overhead required for column drives
VRD is the overhead required for row drives
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 monitors, the backlight 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 does not change, the column driver causes this additional voltage to drop, resulting in additional power dissipation. 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. A typical ISL97702 circuit. |
Introduction to ISL97702
Intersil's ISL97702 is 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 in the ISL97702 is the most advanced power IC available in portable applications. A typical circuit for this device is shown in Figure 2.
Boost converter
The boost converter integrated in the ISL97702 operates from a 2.3V to 5.5V supply. This includes the full input range of lithium-ion and of course can operate from other 3V to 5V supplies. The ISL97702 integrates a 1.2A boost FET and Schottky diode, requiring very few external components. It supports output voltages up to 28V with 90% efficiency. Compared with similar solutions, even synchronous structure products, it is extremely competitive. The ISL97702 uses an asynchronous topology because the power and circuitry required for the operation of a second integrated FET in low-current applications typically exceeds the capacity of the synchronous architecture. The efficiency of the ISL97702 was further improved using Intersil's proprietary power IC design process.
Dual output voltage options
The ISL97702 also includes a dual feedback design. The status of the SEL pin allows you to select one of two feedback networks to control the level of the output voltage. This feature supports the selection of light->-dark-> modes in typical PMOLED applications. Both feedback circuits have better output accuracy than ±2.5% at all temperatures.
The input voltage is disconnected
The 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 generating leakage discharge current. This reduces battery consumption and extends the time it takes to charge portable devices. In power-down mode, the ISL97702 itself consumes less than 3 fundamentals.
Clock synchronization
The ISL97702 uses an internal 1MHz oscillator for normal operation. However, the SYNC input synchronizes the switching frequency of the ISL97702 with an external clock source from 600kHz to 1.4MHz. This is useful in portable applications where some frequencies interfere with other devices, or when two conversion devices produce out-of-sync beat frequencies.
Soft-start control
The device enters soft-start mode when the ISL97702 is enabled, or when it recovers from a fault detection (see Fault Detection). This mode eliminates the starting current 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 connected for the first time, the current through the input voltage disconnect switch is controlled, so there are no current spikes. When the boost converter starts operating, the current limit is gradually increased, also ensuring that there are no input current spikes.
Fault detection
The ISL97702 also integrates several protection circuits to ensure that the IC and external components are protected.
Low-voltage lockout ensures that the device operates at an input voltage higher than the minimum required for proper operation.
Overcurrent protection monitors the output current and restarts the ISL97702 after the overload current has been limited for a period of time.
Overvoltage lockout stops the ISL97702 when the output voltage exceeds the allowable maximum. Once cleared, the ISL97702 enters boot mode.
Overtemperature protection shuts down the ISL97702 when the die temperature exceeds the allowable maximum.