High-brightness LED driver microcircuits. Encyclopedia of radio electronics and electrical engineering

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It is not difficult to light an LED, for this it is enough to connect it in direct connection through a limiting resistor to a power source. But this method is extremely uneconomical, since a large voltage drop is created on the limiting resistor, and hence large losses. In addition, the current through the LED and the brightness of its glow with such an inclusion will be extremely unstable. To increase the efficiency and stability of the glow of LEDs, drivers on specialized microcircuits are used. Some of them will be discussed in this article. The author considers a number of driver chips from Monolithic Power Systems (MPS).

Classification of driver chips based on DC / DC converters

Driver chips for powering ultra-bright LEDs can be found in devices of varying complexity from LED flashlights to mobile phones, digital cameras, computers, etc. One of the most common uses for LEDs is in LED backlight circuits for LCD displays. Drivers for self-powered devices usually have high efficiency (over 90%). They are adjustable switching DC/DC boost or buck-boost converters. You can find so-called capacitive drivers with a voltage boost circuit and inductive drivers. They usually use stabilization of the output current (that is, the current of the LEDs), which ensures a stable brightness of the LEDs. Less often, voltage stabilization on LEDs is used for this.

Capacitive boost converters are also called charge pump converters. This is a literal translation of the English term Charge Pump, which refers to these schemes in foreign technical literature and documentation. They can work as buck-boost converters. The indisputable advantages of Charge Pump drivers are their simplicity and low cost.

Drivers also use inductive SEPIC-architecture converters (Single-ended primary-inductor converter - a single-ended primary converter on inductance) as boost-lower DC / DC converters, the advantages of which are somewhat higher output current and efficiency than converters with a circuit voltage boost. Boost converters have also found their main use in low-voltage powered applications. They have a high efficiency and a large output current with other average indicators. Features of drivers on DC/DC converters given in [1] are summarized in Table 1.

Table 1. Features of drivers based on DC/DC converters

Step-down converters in household appliances are rarely used as LED drivers. Therefore, we will consider the features of the circuitry of the drivers of the remaining three types on microcircuits from Monolithic Power Systems in more detail.

Drivers for powering ultra-bright LEDs with a voltage boost circuit (Charge Pump) from MPS

The MP1519 chip is a driver for powering four white LEDs with a voltage boost circuit (Charge Pump) powered by a 2,5 ... 5,5 V source (see Fig. 1).

Rice. 1. Functional diagram of the MP1519 chip (click to enlarge)

The microcircuit is manufactured in a miniature 16-pin QFN16 package with a size of 3x3 mm. The purpose of the pins of this microcircuit is shown in Table 2.

Table 2. Purpose of the pins of the MP1519 chip

The MP1519 IC contains a battery voltage sensor, a control controller, a current generator, a forbidden zone reference voltage source (ION), four LED current sources (stabilizers), and a voltage boost circuit.

In series with each LED inside the microcircuit, a current stabilizer (Current Source - current source) is turned on, and the current generator controls the mode of all four current sources. The control controller provides automatic selection of the boost mode, "soft" start, etc. The voltage boost circuit converts the supply voltage into 1,3 MHz pulses, which are rectified and charge the storage capacitors C1 and C2. When using a voltage boost circuit to power the LEDs, the battery voltage is added to the voltages on these capacitors. For the correct operation of the voltage boost circuit, capacitors C1 and C2 must have the same capacitance. One of the features of the MP1519 chip is automatic switching of the voltage boost ratio: 1x, 1,5x and 2x. This provides optimally effective stabilization of currents, and, hence, the brightness of the LEDs when the supply voltage changes (for example, during aging or battery replacement). To do this, during operation, the microcircuit continuously monitors the current of the LEDs and the battery voltage.

To prevent battery overload, the MP1519 chip uses a "soft" start and "soft" switching of boost modes.

The current of the LEDs is set by the resistor R1, the resistance of which can be calculated by the formula:

R1(kOhm) = 31,25/I_LED (mA)

In the presence of a supply voltage of 2,5 ... 5,5 V on the pin. 5 and 13 of the IC, the driver is turned on by applying a high voltage level to the EN permission input (pin 12) of this microcircuit. When turned on, the controller of the MP1519 microcircuit analyzes the magnitude of the supply voltage, the current of the LEDs, and turns on one or another mode of the voltage boost. The driver turns off (extinguishing the LEDs) with a low level on the pin. 12 with a delay of 30 µs.

The EN input can be used for both analog and PWM dimming of LEDs. It is for PWM dimming that the turn-off delay of the microcircuit is necessary. To do this, an external control PWM signal with a frequency of 50 Hz ... 50 kHz is applied to the enable input EN. When the control signal pulse ends, the current of the LEDs and their brightness gradually decrease to zero within 30 µs. The greater the duty cycle of the control pulses, the lower the average brightness of the LEDs. At a control signal frequency of more than 50 kHz, the brightness is regulated inefficiently, and at a frequency below 50 Hz, blinking of the LEDs becomes noticeable.

For analog dimming on pin. 11 The MP1519 is supplied with a constant adjustment voltage through the voltage divider R2 R1 (see Fig. 2). By changing this voltage from 0 to 3 V at the input of the divider R2 R1, you can change the LED current from 0 to 15 mA.

Rice. 2. Constant voltage dimming circuit

MPS produces two more microcircuits similar in circuitry and pinout to MP1519 - these are MP1519L and MP3011.

The MP1519L chip is designed to work with three white LEDs and differs from the MP1519 in that the MP1519L pin. 1 is not used. It is available in QFN16 (3x3mm) and TQFN16 (3x3mm) packages. The MP3011 chip is designed to work with only two white LEDs. This chip also does not use pin. 14. This chip is available in a QFN16 package (3x3mm).

Drivers for powering ultra-bright LEDs based on step-up (Boost, Step-Up) DC / DC converters from MPS

A detailed description of the MP2481 chip can be found in [2], so consider the following chips: MP3204, MP3205, MP1518, MP1523, MP1528, MP1521, MP1529 and MP1517.

The MP3204 microcircuit is a classic DC/DC boost converter, which, with an input voltage of 2,5 ... 6 V, allows you to get a constant voltage of up to 21 V on series-connected LEDs. Up to five LEDs can be connected to the MP3204 maximum, but for optimal control, the manufacturer recommends connecting three white LEDs to the output of the microcircuit (see Fig. 3).

Rice. 3. Scheme of switching on the MP3204 chip

The microcircuit contains a 1,3 MHz oscillator, PWM, a feedback signal amplifier, a signal amplifier from a current sensor, and a field-effect transistor output switch. It is manufactured in a miniature TSOT23-6 package. The purpose of the pins of this microcircuit is shown in Table 3.

Table 3. Purpose of the pins of the MP3204 chip

The driver for the MP3204 (Fig. 3) works as follows. The microcircuit is turned on by applying a high level to the EN enable input (pin 4). When the output key (pins 1 and 2) is closed, an increasing current flows from the power source through the L1 inductor and a magnetic field is created in the inductor core. When the output switch opens, a self-induction EMF appears in the inductor ("+" on the right in Fig. 4 and "-" on the left), which is added to the supply voltage of the circuit. With this total voltage, the storage capacitor C1 is charged through the diode D2. The voltage from this capacitor is used to power the series-connected LEDs.

Ceramic capacitors are usually used as the input filter capacitor C1 and the storage capacitor at the output C2. The 2 uF storage capacitor C0,22 is sufficient for most applications, but it can be increased to 1 uF. Choke L1 should have a small DC resistance. In position D1, a Schottky diode with a direct current of 100 ... 200 mA is installed. Resistor R1, connected in series with the LEDs, is used as a current sensor for the LEDs. To stabilize the current of the LEDs, the voltage from R1, proportional to this current, is fed to the feedback input FB of the microcircuit. The resistance of the resistor R1 sets the current of the LEDs.

The dependence of the LED current on the resistance of the resistor R1 is shown in Table 4.

Table 4. Dependence of LED current on R1

To protect the power supply from overload when turned on, the microcircuit has a built-in soft start circuit.

The chip provides analog and PWM dimming, and there are three different ways to adjust the brightness. For analog adjustment, the circuit shown in fig. 4.

Rice. 4. Analog dimming circuit

When the control voltage changes from 2 to 0 V, the LED current changes from 0 to 20 mA.

In addition to analog dimming, two PWM dimming methods can be used.

The essence of the first method is that a PWM signal with a frequency of up to 1 kHz is applied directly to the EN input (pin 4). The current and brightness of the LEDs are inversely proportional to the duty cycle of the control PWM pulses, that is, they are directly proportional to the duration of these pulses.

In the second method, a PWM signal with a frequency of more than 1 kHz is fed to the FB feedback input (pin 3) through an isolation filter (see Fig. 5).

Rice. 5. PWM dimming circuit at FB input

The microcircuit has overload protection when the input voltage decreases (Under Voltage Lockout) with a response threshold of 2,25 V and a hysteresis of 92 mV and overload protection for exceeding the output voltage, for example, if one of the LEDs breaks. To do this, the output voltage of the converter is applied to the input of the OV protection circuit (pin 5). This protection is activated when the output voltage is 28 V and turns off the inverter. To re-attempt to turn it on, you must turn off and then turn on the power supply of the circuit.

The MP3205 microcircuit, unlike the MP3204, does not have output voltage protection and the OV input. The MP3205 microcircuit is manufactured in a 5-pin TSOT23-5 package. Pin. 5 of the TSOT23-5 case of this microcircuit, in terms of location and purpose, corresponds to pin. 6 MP3204 chips in TSOT23-6 package.

Very close in parameters and circuitry to the MP3204 and MP3205 microcircuits are the MP1518 and MP1523 microcircuits, which are designed to control up to 6 LEDs. The MP1518 is available in TSOT23-6 and QFN-8 packages. The MP1518 chip in the TSOT23-6 package is completely identical in pins to the MP3204.

The MP1523 chip is manufactured only in the TSOT23-6 package and has a number of differences from the MP1518.

The pinout of the MP1523 chip is practically the same as the MP3205, but differs from it in that the pin. 5 (BIAS) MP1523 can be connected either to the plus of the power supply (2,7 ... 25 V) - almost like a pin. 5 (IN) of the MP3205 chip, or to the output of the circuit (to the cathode D1). In the latter case, the MP1523 microcircuit will have an overload protection circuit for exceeding the output voltage with a threshold of 28 V. The current sensor resistor connected in series with the LEDs must have a resistance of 20 ohms for this microcircuit. The MP1523 has no LED dimming circuitry.

Another step-up driver for powering 9 LEDs is performed on the MP1528 chip (6x3 mm QFN3 package or MSOP8, in which the chip is marked as MP1528DK). The pin assignments of the MP1528 are shown in Table 5.

Table 5. Purpose of the pins of the microcircuit

The typical switching circuit of the MP1528 microcircuit differs slightly from the other drivers discussed above (see Fig. 6).

Rice. 6. Scheme of switching on the MP1528DQ chip (in the QFN6 package)

To ensure the maximum brightness of the LEDs, a voltage of more than 1,2 V must be applied to the BRT input. The current of the LEDs at maximum brightness is determined by resistor R1, the resistance of which can be calculated by the formula:

R1(kOhm) = U_WATT/(3 I_LED (mA))

Analog dimming is done by changing the DC voltage at the BRT pin from 0,27V to 1,2V.

To provide PWM dimming, a PWM signal with a frequency of 100 to 400 Hz is applied to the BRT input, the low level of which should not exceed 0,18 V, and the high level should not be less than 1,2 V.

The microcircuit has protection against exceeding the output voltage, with a response threshold of 40 V, as well as protection against lowering the input voltage (operating threshold 2,1 ... 2,65 V) and temperature protection with a threshold of 160 ° C.

One of the most powerful drivers on DC-DC converters from MPS is the MP1529 chip (only the MP1517 is more powerful than the ICs under consideration). The MP1529 chip should be of particular interest to readers, as it is used in digital cameras, camcorders and mobile phones with a built-in digital camera. It can drive three chains (lines) of white ultra-bright LEDs connected in series.

Two of these lines (LED1 and LED2) of six LEDs each are used for backlighting liquid crystal (LCD) indicators, and the third (LED3) of four LEDs is used for flash and for illuminating objects in the dark (Preview mode).

The supply voltage of the MP1529 microcircuit is 2,7 ... 5,5 V, and the output voltage is 25 V. It has protection against exceeding the output voltage with a threshold of 28 V, as well as protection against undervoltage of the input voltage with a threshold of 2 ... 2,6 V and hysteresis 210 mV. The MP1529 also has temperature protection (160°C) and comes in a 16x4mm QFN4 package. The purpose of the MP1529 pins is shown in table 6, and a typical switching circuit is shown in fig. 7.

Table 6. Purpose of the pins of the MP1529 chip

Rice. 7. Scheme of switching on the MP1529 chip

Enable inputs EN1 and EN2 are used to enable various modes. If both inputs are low logic level L (0,3 V), then all 16 LEDs will be extinguished. If the EN2 input is kept low and EN1 is set to a high level H (1,4 V), the flash LEDs (LED3) will remain off, and the 12 backlight LEDs (LED1 and LED2 chains) will glow as brightly as possible.

The maximum brightness and current of the backlight LEDs are set by the resistance of the RS1 resistor (connected to pin 9). If, at the same time, a control PWM signal with a frequency of 1 ... 1 kHz is applied to the EN50 input, then, depending on the duty cycle of this signal, the brightness of the illumination of the backlight LEDs will change. If the enable input EN2 is set to a low logic level, a chain of four LEDs (LED3) will additionally turn on in the lighting mode (preview). In this case, the current of the LED3 LEDs will be determined by the resistance of the RS2 resistor (pin 10). If a low level is applied to the EN1 input, and a high level is applied to the EN2 input, then the backlight LEDs LED1 and LED2 will go out, and the LED3 LEDs will light up as brightly as possible (flash mode). In this mode, the current of the LED3 LEDs is set by the resistance of the RS3 resistor (pin 11).

The resistance of resistors RS1, RS2 and RS3 (in kΩ) is calculated by the formulas:

RS1 = (950 U_SET) / I_{LED_BL}

RS1 = (1100 U_SET) / I_{LED_PV}

RS1 = (1000 U_SET) / I_{LED_FL}

where U_SET - internal reference voltage 1,216 V, I_{LED_BL} - current (in mA) of one of the backlight LED circuits LED1 or LED2, I_{LED_PV} - current (in mA) of LED3 LEDs in lighting mode, I_{LED_FL}- current (in mA) of LED3 LEDs in flash mode.

Information about the operating modes of the MP1529 chip, depending on the logic levels at the enable inputs EN1 and EN2, is summarized in Table 7.

Table 7. Operating modes of the MP1529 chip depending on the signals at the EN1 and EN2 inputs

* L - low level, H - high level

Capacitors C1 and C2 are the storage capacitors of the filters at the input and output of the circuit, respectively, C3 is the storage capacitor of the control voltage filter at the input of the PWM stage (this PWM provides output voltage stabilization), C4 is the capacitor of the soft start circuit (PWM timer).

The MP1521 chip with a supply voltage of 2,7 V allows you to connect up to 9 super-bright LEDs to it, and with a supply voltage of 5 V - up to 15 super-bright LEDs. The maximum supply voltage of the IC is 25 V. The MP1521 is available in MSOP10 (MP1521EK) and QFN16 (MP1521EQ) packages. The purpose of the pins of this microcircuit is shown in table 8, and the switching circuit for powering 9 LEDs is shown in fig. 8.

Table 8. Pin assignment of the MP1521 chip in MSOP10, QFN16 (3x3 mm) packages

Rice. 8. Scheme of switching on the MP1521 chip in the MSOP10 package

Resistors R1, R2 and R3 (Fig. 8) are LED current sensors.

With analog dimming, voltage is applied to the EN input in the range of 0,3 ... 1,2 V, and with PWM dimming, a PWM signal with a frequency of 100 ... 400 Hz with a low level of no more than 0,18 V and a high level of no more than 1,2, XNUMX V.

Boost converter and SEPIC type converter on the MP1517 chip

The manufacturer recommends using the MP1517 chip not only as a DC/DC boost converter, but also as a SEPIC (Single-Ended Primary Inductance Converter) converter. The supply voltage of this microcircuit lies in the range of 2,6 ... 25 V. It is manufactured in a QFN16 package 4x4 mm in size. The pin assignment of the MP1517 chip is shown in table 9, and a typical switching circuit is shown in fig. 9.

Table 9. Purpose of the pins of the MP1517 chip

Rice. 9. Typical circuit for switching on the MP1517 chip to power 18 LEDs

This circuit differs from the previous ones (see Fig. 6 or 8) only in that the current sensor of one of the three LEDs in series is used to stabilize the LED current. Therefore, we will dwell in more detail only on the description of the circuit of the DC / DC converter of the SEPIC type on the MP1517 (see Fig. 10).

Rice. 10. DC / DC converter type SEPIC on the chip MP1517

A feature of the SEPIC converter is that the voltage at its output can be either higher or lower than the input, which is ensured by the presence of a coupling capacitor C8 (see [3, 4]). The scheme in fig. 10 produces a voltage of 3,3 V at the output when the input voltage changes from 3 to 4,2 V. Any SEPIC type converter is assembled on the basis of a switching boost converter, which is easy to see in the diagram below. In addition, this boost converter (on L1, D2) is used to power the microcircuit itself.

Let's see how the MP1517 SEPIC converter works in steady state.

As a result of the previous work, by the time the internal key of the MS on the field-effect transistor is unlocked, the capacitor C8 will be charged ("+" - on the left in Fig. 10, "-" - on the right). When this key is opened, C8 will be discharged through the inductor L2, in which the energy of the changing magnetic field will accumulate. In addition, the inductor L1 will also accumulate magnetic energy, through which an increasing current will flow from the power source through the same internal key of the microcircuit. When the key is locked in the inductor L1, an EMF appears ("+" - on the right, "-" - on the left), which adds up to the voltage of the power source and charges C8 ("+" - on the left, "-" - on the right) through D1 and capacitor C2. In addition, an EMF appears in L2 ("+" - at the top, "-" - at the bottom), charging C2 through D1. At the next unlocking of the internal key of the microcircuit, the process will be repeated.

The voltage value at the output of the converter (at C2) depends primarily on the duty cycle of the key control pulses and on the load current.

R1 R2 - feedback voltage divider, which provides output voltage stabilization, C6 - error voltage filter capacitor. C5 is the decoupling resistor and C4 is the soft start capacitor.

Literature

1. Deng K. "Comparison of capacitive and inductive DC converters". "Electronic components". No. 8. 2007.
2. Tsvetkov D. "A new adjustable DC/DC converter for powering high-power LEDs". "Modern Electronics". No. 9. 2008.
3. Ioffe D. "Development of impulse voltage converters with SEPIC topology". "Components and Technologies". No. 9. 2006.
4. Ridley R. "SEPIC Converter Analysis". "Components and Technologies". No. 5. 2008.

Author: I. Bezverkhny

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