High-brightness LED driver controller based on MAX16801A/B and its application

1 Introduction

MAX16801A/B and MAX16802A/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 drivers , while MAX16802A/B is specified for low-voltage (10.8~24V) DC input LED drivers. HB LED drive circuit control ICs composed of MAX16801A/B and MAX16802A/B are mainly used in LCD TV and LCD monitor RGB backlighting, commercial and industrial lighting, decorative and architectural landscape lighting, etc.

2 Main features

Key features of the MAX16801A/8 and MAX16802A/8 are as follows:

Suitable for use in buck, boost, flyback, SEPIC and other circuit topologies; up to 50 W or greater output power ; internal error amplifier with 1% accuracy; PWM or linear dimming; 262 kHz (1±12%) fixed switching frequency operation; thermal shutdown and digital soft-start with programmable input startup voltage; startup power current is typically 45 μA, and operating power current is typically 1.4 mA; thin 8-pin μMAX package.

3 Pin Function and Internal Structure

The pinout for the MAX16801A/B and MAX16802A/B is shown in Figure 1.

Figure 1: Pinout

Table 1 lists the functions of its various pins.

Table 1: Pin function table

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 driver circuits, 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 MAX16801 is shown in Figure 3.

Figure 3: Offline, non-isolated, flyback LED driver circuit with programmable input start-up voltage

4.1.1 Input startup circuit

The wake-up threshold voltage of the UVLO/EN pin of the MAX16801 is 1.28 V. The circuit can only start to work if the voltage of this pin exceeds 1.28 V. The resistor divider (R2 and R3) can be used as the programming input startup voltage.

The MAX16801 includes a bootstrap UVLO circuit with large hysteresis, which is very useful for designing high-voltage HB LED drivers. When VIN exceeds the 23.6 V threshold level, 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 oscillator are turned on and the output driver is allowed to switch. If VIN drops below 9.7 V, the comparator, oscillator, and output driver are turned off.

4.1.2 Startup Operation

At the beginning of the circuit startup, the tertiary winding (P3) of transformer T1 has no energy delivered to the IN terminal of the control IC. After power is applied, the current through R1 charges C1, and the internal regulator charges C2 at the same time. 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 transformer's tertiary winding determine the startup time. Use the following formula to approximate the required capacitor value:

where IIN is the internal supply current of the MAX16801 after startup (1.4mA), 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), and tSS is the 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 (+23.6V maximum), and ISTART is the supply current on IN during startup (90μA maximum).

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, 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 tertiary winding of the transformer also provides an output short-circuit holding function. As long as the voltage on the tertiary winding drops below 10 V, the gate drive of the external MOSFET will be turned off.

4.2 HB LED Driver Circuit

Figure 4 shows a universal AC input isolated flyback HB LED driver circuit with low-frequency PWM dimming function. In which, an inverted logic PWM signal is directly added to the DIM/FB terminal of the MAX16801B to achieve a wide range of low-frequency PWM dimming for HB LEDs. Transformer T1 provides safety isolation, and the lower end of T1's secondary winding is grounded through capacitor C5.

Figure 4: Universal AC input, offline, isolated flyback HB LED driver circuit using low-frequency PWM dimming

4.3 Typical Application Circuit of MAX16802

Figure 5 shows a buck HB LED driver circuit with dimming capability using the MAX16802B. The circuit operates in continuous conduction mode (CCM), and a linear dimming signal is applied to the CS pin of the MAX16802B. R5 is a current sensing resistor, and R4 and C1 form a low-pass filter. The DC input voltage range of the driver circuit is 10.8 to 24 V.

Figure 5: MAX16802 step-down HB LED driver circuit diagram with brightness adjustment

The MAX16802B can also form 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 flyback HB LED driver with brightness adjustment

5 Conclusion

The MAX16081A/B and MAX16802A/B are offline and DE-DE controllers for HB LED drivers for general lighting and backlighting applications . They can build circuit topologies such as buck, boost, flyback, and SEPIC, with programmable startup voltage, PWM or linear dimming, digital soft-start, and thermal shutdown protection, with output power up to more than 50 W.