Design of a Current Mode PFM LED Driver IC

LED is the fourth lighting method for mankind after incandescent lamps, fluorescent lamps and energy-saving lamps. It has the advantages of good color reproduction, fast response speed, energy saving, safety, environmental protection and long life, so it is widely used in fluorescent lamps, indoor lighting and decoration, street lamps, stage lamps, display screens, backlight sources, indicator lights, etc. With the continuous improvement of LED luminous efficiency, LED has shown multi-level changes in application and is now widely used in automotive electronics, mobile devices, LCD backlights and general lighting.

With the widespread application of LEDs, the demand for LED driver ICs has also grown rapidly. LED driver ICs are mainly used to provide efficient and long-lasting drive for LEDs. In addition to providing simple control and drive for LEDs, they generally also have intelligent management functions, thereby achieving high performance, high efficiency, and various management and protection functions. The demand for driver ICs is inseparable from the application of LEDs. The application and technological development of LEDs have also promoted the development of driver ICs. In turn, driver technology is the key to improving the application level of LED lighting. Therefore, how to design LED driver ICs with higher efficiency, stronger functions, and better structure will be a new round of challenges for integrated circuit design.

1 Chip structure and working principle

Based on the market demand for switching power supplies, this paper designs an LED lighting driver chip. Figure 1 shows a high power supply rejection ratio reference voltage source structure designed based on the 0.5μm CMOS standard process that does not change with temperature, power supply voltage and process. The chip uses pulse frequency modulation and realizes the fixed off-time function of the circuit by designing a resistor-capacitor network structure. This circuit structure avoids the use of slope compensation technology, simplifies the circuit structure while realizing the PFM modulation of the circuit, and improves the chip efficiency. The chip includes core modules such as oscillator, PFM control circuit, reference voltage source, bias circuit, RS trigger, overvoltage, undervoltage protection circuit and drive circuit.

Figure 1 Chip circuit structure

When working, the chip feeds back the load change to the chip through a power MOSFET and a sampling resistor. The feedback signal first passes through a low-pass filter to filter out the high-frequency switching noise, and then is input into the Current COMP module in the form of current for adjustment to generate a signal to control the R end of the RS trigger, and then controls the drive circuit together with the S end signal generated by the Oscillator PFM Regulator module, and finally drives the power MOFFET to realize the PFM adjustment of the circuit. In addition, the Error AMP module, AMP module and Voltage COMP three modules constitute an overcurrent protection circuit. When the current flowing through the external circuit is too large, the current detection through the external resistor is used, and the feedback voltage is compared with the constant reference voltage, and then the difference between the two is compared and amplified to generate a signal to control the CtR end of the RS latch and force the power tube to be turned off. This feedforward protection mode can effectively reduce the risk of burning the power tube and LED lamp when the current is too large, and can provide reliable protection for the circuit. The Low Voltage Lockout protection module can force the chip to shut down when the input voltage is 2.5V lower than the normal working voltage, thereby preventing the chip from working at low voltage and improving the utilization efficiency of the power supply.

2. Design of main circuit modules of the chip

2.1 Self-bias circuit

The circuit shown in Figure 2 is the self-bias circuit of the IC. When the chip starts up by itself, it will provide two biases, namely N-tube bias and P-tube bias, thereby providing bias voltage for each module.

Figure 2 Bias circuit structure

Since the potentials of the gates of M1 and M2 are the same, we have:

Since (W/L) 2=K (W/L) 1, then:

So we can get:

Therefore, the N tube bias voltage and the P tube bias voltage are obtained as follows:

At the same time, the module is also designed with an undervoltage protection circuit, which uses an R network structure and a pull-up resistor clamp to force the chip to shut down when the voltage is lower than 2.5V, thereby improving the utilization efficiency of the power supply.

2.2 High PSRR Reference Voltage Source

The circuit structure of a high power supply rejection ratio reference voltage source that does not change with temperature, power supply voltage and process is designed based on the 0.5μm CMOS standard process as shown in Figure 3. At room temperature, the reference source has a zero temperature coefficient, and the voltage changes very little in the range of -40℃ to 120℃, and its temperature coefficient can reach the order of 3ppm/℃.

Figure 3 Reference circuit structure

According to the small signal model of the reference voltage source, the power supply rejection PSR is analyzed and the following is obtained:

At the same time, the module is also designed with an undervoltage protection circuit, which uses an R network structure and a pull-up resistor clamp to force the chip to shut down when the voltage is lower than 2.5V, thereby improving the utilization efficiency of the power supply.

2.2 High PSRR Reference Voltage Source

The circuit structure of a high power supply rejection ratio reference voltage source that does not change with temperature, power supply voltage and process is designed based on the 0.5μm CMOS standard process as shown in Figure 3. At room temperature, the reference source has a zero temperature coefficient, and the voltage changes very little in the range of -40℃ to 120℃, and its temperature coefficient can reach the order of 3ppm/℃.

Figure 3 Reference circuit structure

According to the small signal model of the reference voltage source, the power supply rejection PSR is analyzed and the following is obtained:

Since Is is proportional to the emitter area, we have:

From this, the voltage Vout at the output point can be obtained:

By analyzing formula (10), we can find that: The PSR of the reference voltage source is related to the open-loop gain and power supply rejection PSR of the operational amplifier. Therefore, if the open-loop gain A of the operational amplifier is increased, the power supply rejection PSR of its reference voltage source can be improved; and if the op amp Add is close to 1, then the PSR of the reference voltage source will be greatly improved. This paper uses an operational amplifier with a high open-loop gain to make Add approximately equal to 1, so as to obtain a very high PSR.

The input of the op amp can be considered as a virtual short, that is, V+=V-, and because R1=R2, the current flowing through Q1 and Q2 is equal, so:

Since Is is proportional to the emitter area, we have:

From this, the voltage Vout at the output point can be obtained:

2.3 RS Flip-Flop

Figure 4 shows the circuit structure of the RS trigger, which can improve the chip's anti-interference ability to ensure that only one working pulse reaches the output stage in one cycle, thereby ensuring that the circuit will not malfunction in a harsh noise environment. Table 1 lists the functions of the RS trigger.

Figure 4 RS trigger circuit structure

Table 1 RS trigger function

2.4 Overcurrent protection module

Figure 5 shows the overcurrent protection circuit in this system. The circuit consists of three modules: Error AMP module, AMP module and Voltage COMP. Among them, the AMP module plays the role of amplifying the Vcs voltage ten times. The amplified Vcs voltage is compared with the output voltage of the Error AMP module, and the output voltage can control the Ctr end of the RS trigger. The circuit is connected in a boost structure. When the current at the feedback end is too large and VFB is higher than 1V, the output of the Error AMP module is 0, and the output of the VoltageCOMP module is 1, that is, the Ctr end of the RS trigger is set to 1, so the RS trigger output is 0 to force the chip to shut down. This protection mode effectively reduces the risk of burning the power tube and LED lamp when the current is too large, and can provide reliable protection for the circuit.

Figure 5 Overcurrent protection circuit structure

2.5 Pulse Frequency Modulation Control Circuit

The fixed off-time function of the circuit can be realized by designing a resistor-capacitor network structure. At the same time, the fixed off-time can be easily adjusted through the series-parallel combination of resistors and capacitors in the external circuit of the chip, avoiding the use of slope compensation technology. It can also simplify the circuit structure and improve chip efficiency while realizing PFM modulation of the circuit.

2.6 Current Detection Comparator Module

The module uses a sampling resistor in series with the switching power tube, and reflects the current in the branch through the voltage drop on the sampling resistor. In this way, the reference output voltage with a constant precise sampling value can be compared and amplified. This precise control mode greatly improves the chip accuracy.

3 Layout Design and Simulation Results

3.1 Layout Design

Figure 6 shows the layout of the chip, where the reference voltage source in the middle is used to reduce stress effects. The bias circuit is on the far left, the Oscillator PFM Regulator module is on the upper right, and the sampling circuit is directly below the module; the RS trigger and DRV circuit layout are at the bottom of the chip, and the trimming resistor is directly above.

Figure 6 Overall circuit layout

3.2 Simulation Results

The circuit proposed in this paper can be simulated by using candence. Figure 7 shows the chip sampling detection voltage, DRV output voltage, load current and power-on voltage simulation waveforms. From the simulation results, it can be seen that: when the feedback voltage CS reaches 250mV, the shutdown function of the chip output terminal (DRV) can be realized through peak detection, and then the PFM Regulator unit circuit controls the opening of the circuit to realize the fixed shutdown time function, and the fixed shutdown time is 520ns; the ripple current amplitude on the LED lamp is within 4%; and the power-on is fast. Through circuit simulation verification, the chip design is reasonable and the performance is good.

Figure 7 Overall circuit simulation waveform

4 Conclusion

This paper proposes a design method for a current detection type LED drive switching power supply circuit based on the driving characteristics requirements of LED.

The driving circuit adopts the PFM control method, thereby avoiding the use of harmonic compensation technology, improving chip efficiency and simplifying the circuit structure.

At the same time, the circuit achieves high efficiency, safety, and reliability through high power supply rejection ratio, low temperature coefficient reference voltage source design, self-starting circuit, and undervoltage protection circuit design. The drive circuit can be widely used in lighting drive of various LED products.