Liquid crystal display (LCD) indicators and displays using light emitting diodes (LED) can work from conventional power sources. However, this is not the best way to power. Below will be shown options include using specialized circuits, voltage regulators, which is manufactured by a company MAXIM.
The use of digital potentiometer to control the led illuminator
Produced 5-bit programmable potentiometer DS 1050 is used as a basic element of the pulse width modulator (PWM). The change in pulse width from 0 to 100% in steps of 3, 125%. The control potentiometer is a two-wire serial interface compatible with I2C addressable up to eight DS 1050 on a two-wire bus. Schematic control brightness led backlight LCD indicator is presented in Fig. 1.
Fig. 1.
This scheme is not designed to control the voltage contrast of the LCD display. Used in this example, the character display h type DMC him Optrex has a yellow - green led backlight. The direct voltage drop across the LEDs is 4.1 Volts, and a maximum forward current of 260 mA.
Modifying the duty cycle of the pulse width modulator, thereby modifying the input power to the LEDs. When a pulse is 100% cycle time mode - have a maximum power supply and, accordingly, the maximum brightness. Conversely, when the pulse cycle is 0%, the brightness is also zero.
Control PWM modulator is quite simple. The only requirement is that the LEDs are not blinking. Our eyes can't see the flashing at a frequency of 30 Hz and above. The "slow" DS1050 operates at a frequency of 1 kHz. This is enough for visual observation and minimize electromagnetic radiation. You must select MOS transistor Q1 so that it could be controlled directly from a 5 volt pulse width modulator, the voltage of which varies from ground to Vcc. The default at power-up PWM duty cycle is equal to 2. Transistor Q1, driven by a CHI-signal, can commutate a current of 260 mA, which is required for the led light. Threshold voltage of the gate of the transistor Q1 is 2-4 Volts. Diode D1 type part no 1n4001 is used to reduce Vcc to 4.3 volts, which is less than the maximum forward voltage drop of the LEDs. Resistor instead of the diode is not used because of the large power dissipation. For reliable closure of the MOSFET is put a resistor R3, which eliminates the "floating" shutter mode Q1.
Capacitor C1 is used as a power supply filter should work well at high frequency and is installed as close as possible to the terminals U1, with a minimum distance to a power source.
Digital potentiometer DS 1050 - 001 is set by hardware with address A=000. The program for the 8051 microcontroller type available in the app "App. note 163" on the website of the company MAXIM.
To control the LCD contrast indicators (LCD) instead of a conventional mechanical potentiometers are encouraged to use a digital potentiometer type DS1668/1669 Dallastats or DS 1803. Devices DS1668/1669 were chosen because they provide both keypad and microcontroller control current collector contact. It is also important that these devices have internal non-volatile memory, which allows you to save the position of the current collector without power. In Fig. 2. a diagram to control the contrast for the LCD using a digital potentiometer DS 1669.
Fig. 2. .
Of course, there can be applied a dual digital potentiometer type DS 1803.
The liquid crystal module (LCM) is powered by 5 Volts. This same voltage is supplied to DS 1669, the resistance of 10 ohms. The collector terminal is connected directly to the input supply Vo LCM driver.
The use of digital potentiometer allows you to reduce the size of the device, significantly increase the durability and to transfer control to the system microcontroller.
Well, now let's go back to driving LEDs. With the increasing popularity of color LCD displays in mobile phones, pocket computers, digital cameras, etc.) are becoming a popular source of illumination white LEDs.
White light can provide either a fluorescent lamp cold cathode (CCFLS), or white LEDs. Because of the size, complexity, high cost CCFLS, for a long time was the only source of white color. But now they are losing ground to the white LEDs. They do not require a high voltage (200 - 500 V AC) and a large transformer to obtain the strain. And although the forward voltage drop on a white led (from 3 to 4 V) higher than in the red (1.8 V) or green (2,2 - 2,4 In), still require a fairly simple power sources. The brightness of the white led is controlled by the change of passing current through it. Full brightness occurs at a current of 20 mA. With decreasing flowing through the led current, the brightness decreases. For digital cameras and mobile phones typically require 2 to 3 LEDs. Can be 2 ways a group of LEDs: parallel and serial.
In serial mode LEDs the current through each is guaranteed to be the same. But such inclusion requires a higher voltage than in parallel. In parallel, the voltage is approximately equal to the direct voltage drop across a single led instead of the voltage drop across the led row. However, the brightness of the diodes may be different due to the variation of the direct voltage drop across the LEDs, therefore, different currents, if they are not regulated. The battery voltage is in most cases insufficient to glow white led, so you need to use the Converter DC/DC. In this case, preferably parallel connection of the LEDs, since the DC/DC converters are the most effective at small high output voltage to the input.
Parallel connection of LEDs
There are three main ways parallel connection of LEDs, as shown in Fig. 3.
Fig. 3.
We consider in detail these options enable.
A simple method of controlling the current flowing through the LEDs is to use the chip, specifically designed for this purpose. The switching circuit is presented in Fig. 4. Shown here is a cheap chip MAX1916, which allows you to adjust the current through the 3 white LEDs. The absolute accuracy of the current is 10%, and the currents flowing through the LEDs, differ by no more than 0.3%. This is the most important feature, as light streams from each led must be the same. At full brightness the current through the led is 20 mA. In this case, it is sufficient 225 mV greater than the voltage drop across the LEDs, so that the chip is supported preset current. Installation of the current through the LEDs is performed using the Rset resistor. The equation for calculating the current has the following form.
where:
Iled is the current flowing through the led [mA]
230 - the conversion factor of the chip
Uout is the output voltage of the regulator
Uset = 1, 215 In
Rset is the resistor that is installed between the regulator output and input SET MAX1916 (IO).
Fig. 4.
Absolute current also has to be managed, but the brightness will change as a whole for the entire device (for example, the phone's display. The brightness change can be obtained by submitting to the enable input (EN) of the chip signal with pulse-width modulation. The maximum brightness will be at 100% pulse width, and at 0% - the led does not illuminate.
Using a power supply with adjustable output voltage.
This method of inclusion is less accurate, since it is not regulated individual currents through each led. How to increase the absolute accuracy of the leaked currents and matching them across each diode?
The current through the led is calculated by the formula:
Iled = (Vout - Vd)/R
Due to manufacturing variations even with the same direct currents the voltage drop across the led (Vd) may be different. You can write the ratio of the two currents through the diode 2
I1/I2 = R2/R1 [(Vout - Vd1)/(Vout - Vd2)]
Taking into account that the resistors have a high precision (that is legal), we have:
I1/I2 = (Vout - Vd1)/(Vout - Vd2)
It follows that the ratio (difference) of the currents through the diodes, the smaller the higher the output voltage of the power source. You need to keep in mind that the convergence of the values of the currents through the LEDs is paid a higher power. Therefore, it is possible to recommend to the output voltage of the regulator is equal to 5 Volts.
To obtain this voltage, you can use a simple type converters MAX 1595 (Uвых = 5V, IOUT = 125 mA), or use converters MAX1759 with adjustable output. Thus, by changing the output voltage of the regulator can adjust the currents in the LEDs to the desired level (for example, 20 mA). If it is not possible to adjust the adjustable current output of the power source voltage, in parallel with the ballast resistors R1a:R3a put the resistors and MOS transistors, as shown in Fig. 5. Toggling the logic level of the MOS transistors, it is possible to connect or disconnect additional resistors R1в:.R3в, effectively changing the value of the ballast resistor.
Fig. 5.
Using the Converter with adjustable output current. In Fig. 3C shows the principle of using Converter with adjustable output current. In this scenario, the current through one of the diodes (Fig. 3C - D1) is converted to the voltage drop across the resistor R1 and the voltage is supported by the Converter. The Converter may be a key type, the switched capacitor or a linear regulator.
The equation for the current through the led is the same as that presented above.
Ix = (Vout - Vdx)/Rx (1)
But in this case, Vout is not regulated, and I1 is regulated and its value is
I1 = Vo.c / R1 (2)
where: Vo.c - the feedback voltage taken from the resistor R1.
As regulated by the current of only one diode, different forward voltage drop of the LEDs may cause a difference of currents flowing through them. In this case, you can use the following. Divide the resistor into 2 parts: R1 = R1A + R1B and substitute in equation (1), and the value of R1 in equation (2) replace at R1В. For R2 and R3 not required split resistors. The values must be equal R1A + R1B. Now on the regulator output will be maintained at the voltage determined by the voltage drop across the resistor R1B, as shown in Fig. 6. If the setpoint from R1B is equal to the voltage of R1, the error amplifier will remain in the same state, the output voltage of the regulator will rise, which would align the current through each led.
Fig. 6.
Series connection of LEDs
The main advantage when you turn on the LEDs in a serial chain is that through all the diodes by the same current flows and the brightness turns out the same. The drawback with this inclusion: requires a higher voltage, since the voltage drop across each led is summarized. Even 3 white LEDs require a voltage of 9 to 12 volts. Usually for such inclusion are key regulators, as the most efficient converters for this purpose. Figure 7 presents the scheme of inclusion of key regulator MAX 1848 designed to control three white LEDs in series. The device can be powered from 2.6 to 5.5 volts when the output voltage to 13 volts. The input range is designed for one Li-ion battery or 3 NiCD/NiMH batteries. Operating frequency of the regulator is 1.2 MHz, which allows the use of external components with minimum dimensions. The output PWM signal. The excess voltage is rectified and supplied to the LEDs. The current through the LEDs, and thus, the brightness can be adjusted using a voltage taken from DAC or a filtered PWM signal applied to the input CTRL chips MAX 1848. Efficiency MAX 1848 when working with LEDs reaches 87%.
Fig. 7. .
For large displays that require a lot of LEDs, you can use a key regulator MAX 1698 (see Fig. 8). The chip can operate from an input voltage of 0.8 Volts, and the output voltage is limited by the operating voltage of the external n - channel MOS transistor. Low, up to 300 mB the feedback voltage (FB pin) helps to maximize the effectiveness of the scheme, which reaches 90%. The brightness of the led is controlled by the potentiometer, whose wiper connected to terminal ADJ of the chip. The potentiometer can be used both analog and digital.
Fig. 8.
Of course, the number of chips that are used to power and backlight in LCD and led displays, not limited in the article titles. If the reader will want to pick up necessary for the particular case of the chip, there is nothing easier, how to enter the site http://www.maxim-ic.com and there find the characteristics of the manufactured products.
Used information materials of the company MAXIM.
Author: AP Shitikov; Publication: www.radioradar.net