1 Introduction
The MAX16801A/B and MAXl6802A/B use a constant current to drive the HB LEDs in general lighting and display applications to provide the required control. The MAX16801A/B is suitable for general purpose AC input LED drivers , while the MAX16802A/B is specified for low voltage (10.8 to 24 V) DC input LED drivers . The HB LED driver circuit control IC consisting of MAX16801A/B and MAX16802A/B is mainly used in LCD TV and LCD monitor RGB backlighting, commercial and industrial lighting , decorative and architectural landscape lighting.
2 main features
The main features of the MAX16801A/8 and MAX16802A/8 are as follows:
Suitable for use as a Buck, Boost, Flyback, SEPIC and other circuit topologies; output power up to 50 W or more; internal error amplifier with 1% accuracy; PWM or linear dimming; 262 kHz (1±12%) fixed switching frequency operation; provides thermal shutdown and digital soft-start, programmable input startup voltage; startup supply current is typically 45μA, operating supply current is typically 1.4 mA; thin 8-pin μMAX Package.
3-pin function and internal structure
The pinouts of the MAX16801A/B and MAX16802A/B are shown in Figure 1.
Figure 1: Pinout
Table 1 lists the functions of their individual pins.
Table 1: Function Table of Pins
The MAX16801A/B and MAX16802A/B integrate a 1.23 V bandgap reference, digital soft-start, UVLO, voltage regulator, 262 kHz oscillator, error amplifier, CPWM comparator, current limit comparator, thermal shutdown, and MOS gate Unit circuit such as driver, as shown in Figure 2.
Figure 2: Functional Block Diagram
4 application circuit and working principle
4.1 Offline non-isolated flyback LED driver using MAX16801
The offline non-isolated flyback HB LED driver circuit using the MAX16801 is shown in Figure 3.
Figure 3: Offline, non-isolated, flyback LED driver circuit with programmable input startup voltage
4.1.1 Input Startup Circuit
The wake-up threshold of the MAX16801 pin UVLO/EN is 1.28 V. The voltage on this pin is only 1.28 V and the circuit can start working. Resistor dividers (R2 and R3) can be used as programming input startup voltages.
The MAX16801 has a built-in bootstrap UVLO circuit with large hysteresis that is useful for designing high-voltage HB LED drivers. When VIN exceeds the threshold level of 23.6 V, the circuit starts. During startup, the UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver off to reduce current consumption. Once VIN reaches 23.6 V, the CPWM and ILIM comparators and oscillators are turned on and the output driver switches are allowed. If VIN drops below 9.7 V, the comparator, oscillator, and output driver are turned off.
4.1.2 Startup operation
At the start of the circuit, the third stage winding (P3) of transformer T1 has no energy delivered to the IN terminal of the control IC. After power-up, C1 is charged through the current of R1, while the internal regulator charges C2, and the maximum current through R1 is 45μA. When the voltage on C2 reaches 9.5 V, C2 charging stops and the voltage on C1 continues to rise. When VIN exceeds the wake-up level of 23.6 V, VQ1 begins to operate and the transformer transfers energy to the secondary and tertiary windings. The capacity of C1 and the third stage winding connection configuration of the transformer determine the start-up time. Use the following formula to approximate the required capacitance value:
Where IIN is the internal supply current (1.4mA) of the MAX16801 after startup, Qgtot is the total gate charge of Q1, fSW is the switching frequency of the MAX16801 (262kHz), VHYST is the bootstrap UVLO hysteresis voltage (11.9V), tSS is internal Soft start time (60ms).
Assuming C1 > C2, calculate R1 as follows:
Where VIN(MIN) is the minimum input voltage in the application, VSUVR is the bootstrap UVLO wake-up level (maximum +23.6V), and ISTART is the supply current on IN at startup (maximum 90μA).
4.1.3 Determination of LED current
The current ILED through the LED is primarily determined by R7. The non-inverting input of the error amplifier in the IC is connected to a reference voltage VREF of 1.23 V through a digital soft-start circuit. Therefore, ILED=VREF/R7=1.23 V/R7. If the forward current rating of the HB LED is 750 mA, then R7 = 1.23 V / 0.75 A = 1.64 Ω
4.1.4 Output short circuit protection
The third stage winding of the transformer also provides an output short circuit hold function. As long as the voltage on the third stage winding drops below 10 V. The gate drive of the external MOSFET is turned off.
4.2 HB LED Driver Circuit
Figure 4 shows a general-purpose AC input isolated flyback HB LED driver circuit with low-frequency PWM dimming. Among them, an inverted logic PWM signal is directly added to the DIM/FB terminal of the MAX16801B to achieve wide-range low-frequency PWM dimming of the HB LED. Transformer T1 provides safe isolation, and the lower end of the T1 secondary winding is grounded through capacitor C5.
Figure 4: Low-frequency PWM dimming universal AC input, off-line, isolated retrace HB LED driver circuit
4.3 Typical Application Circuit of MAX16802
Figure 5 shows a Buck-type HB LED driver circuit with a MAX16802B dimming capability. The circuit operates in continuous conduction mode (CCM) and the linear dimming signal is applied to pin CS of the MAX16802B. R5 is a current sense resistor, and R4 and C1 form a low pass filter. The drive circuit has a DC input voltage range of 10.8 to 24 V.
Figure 5: Circuit diagram of the MAX16802 buck HB LED driver with brightness adjustment
The MAX16802B also forms a flyback HB LED driver circuit topology in discontinuous conduction mode (DCM), as shown in Figure 6. In Figure 6, the total voltage of the LED string can be lower or higher than the input voltage VIN.
Figure 6: MAX16802 Sweep HB LED Driver Circuit Diagram with Brightness Adjustment
5 Conclusion
The MAX16081A/B and MAX16802A/B are HB LED driver off-line and DE-DE controllers for general lighting and backlight applications that can be built into buck, boost, flyback, SEPIC, etc., start-up voltage programmable, PWM or Linear dimming, digital soft start and thermal shutdown protection, output power up to 50 W or more. (Edit: Technology)
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