Application of high-brightness LED drive controller

1 Introduction MAX16801A/B and MAXl6802A/B use constant current to drive HB LEDs in general lighting and display applications to provide the required control functions. MAX16801A/B is suitable for universal AC input LED driver, while MAX16802A/B is specified for low voltage (10.8～24 V) DC input LED driver. The HB LED drive circuit control IC composed of MAX16801A/B and MAX16802A/B is mainly used in LCD TV and mobile phone LCD monitor RGB backlighting, commercial and industrial lighting, decoration and architectural landscape lighting and other fields. 2 Main features The main features of MAX16801A/8 and MAX16802A/8 are as follows: suitable for forming buck, boost, flyback, SEPIC and other circuit topologies; direct output power of 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 start voltage; typical start-up power current is 45μA , The typical operating power supply current is 1.4 mA; thin 8-pin μMAX package. 3 pin function and internal structure The pin arrangement of MAX16801A/B and MAX16802A/B is shown as in Fig. 1. Table 1 lists the functions of its various 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 The unit circuit such as the driver is shown as in Fig. 2. Pin function and internal structure MAX16801A/B and MAX16802A/B internal structure 4 Application circuit and working principle 4.1 Offline non-isolated flyback LED driver using MAX16801 Offline non-isolated flyback HB LED driver circuit using MAX16801 as shown in Figure 3 Show. Offline isolated regression LED driver circuit with programmable input startup voltage 4.1.1 Input startup circuit MAX16801 pin UVLO/EN wake-up threshold voltage is 1.28 V, this pin voltage only exceeds 1.28 V, the circuit can start to work. The resistor divider (R2 and R3) can be used as a programming input start voltage. The MAX16801 has a built-in bootstrap UVLO circuit with a large hysteresis, which is very 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 is allowed to switch. If VIN drops below 9.7 V, the comparator, oscillator, and output driver will be turned off. 4.1.2 Start-up operation When the circuit starts to start, no energy is delivered to the IN terminal of the control IC from the tertiary winding (P3) of the transformer T1. After power-up, the current through R1 charges C1, while the internal regulator charges C2, 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 starts to act, and the transformer transfers energy to the secondary and tertiary windings. The capacity of C1 and the connection configuration of the tertiary winding of the transformer determine the start-up time. The gate current of the MOSFET (VQl) is: If the off-line LED driver startup time is not more than 500 ms, and C1>C2, R1 can be calculated by equations (3) and (4). 4.1.3 Determination of LED current The current ILED through the LED is mainly determined by R7. The non-inverting input of the error amplifier in the IC is connected to a 1.23 V reference voltage VREF through a digital soft-start circuit. Therefore, ILEDu003dVREF/R7u003d1.23 V/R7. If the forward current rating of the HB LED is 750 mA, then R7u003d1.23 V/0.75 Au003d1.64 Ω 4.1.4 The tertiary winding of the output short-circuit protection transformer also provides the output short-circuit retention function. As long as the voltage on the tertiary winding drops below 10 V. The gate drive of the external MOSFET is turned off. 4.2 HB LED driver circuit Figure 4 shows the universal AC input isolated flyback HB LED driver circuit with low frequency PWM dimming function. Among them, an inverted logic PWM signal is directly added to the DIM/FB end of the MAX16801B, which can realize wide-range low-frequency PWM dimming for HB LEDs. The transformer T1 provides safety isolation, and the lower end of the secondary winding of T1 is grounded through a capacitor C5. Universal AC input isolated flyback HB LED driver circuit with low frequency PWM dimming 4.3 Typical application circuit of MAX16802 Figure 5 shows a step-down (Buck) HB LED driver circuit with dimming capability using MAX16802B. The circuit works in continuous conduction mode (CCM), and a linear dimming signal is applied to the pin CS of MAX16802B. R5 is a current sensing resistor, and R4 and C1 form a low-pass filter. The DC input voltage range of the drive circuit is 10.8～24 V. MAX16802B can also form a flyback HB LED drive 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. Using MAX16802B buck (Buck) HB LED driver circuit with dimming capability, flyback HB LED driver circuit topology 5 Conclusion MAX16081A/B and MAX16802A/B are HB LED drivers offline and DE-DE control for general lighting and backlighting applications They can build Buck, boost, flyback, SEPIC and other circuit topologies, programmable startup voltage, provide PWM or linear dimming, digital soft-start and thermal shutdown protection, and output power directly above 50 W.
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