Portable Projector LED Driver Reference Design

Overview

This reference design is a 6A step-down LED driver for portable projectors. The reference design is based on the MAX16821 PWM HB LED driver. The circuit can drive one LED; three MAX16821s are required to drive RGB LEDs.

LED Driver Specifications

• Input voltage range (VIN): 10V to 15V
• Output voltage (VLED): 4.5V to 6V
• Output current (ILED): 1.5A to 6A, analog controllable
• Analog control voltage: 1.1V to 2.8V, corresponding to 1.5A to 6A
• Maximum LED on-duty cycle: 50%
• Maximum LED current rise/fall time: < 1µs
• Maximum LED current ripple: < 15% at 6A

Figure 1. MAX16821 LED driver circuit board.

Figure 2. LED driver circuit board schematic

Circuit Description

The LED driver steps down the 10V to 15V input supply voltage and drives a LED with a forward voltage of 4.5V to 6V with a constant current. The step-down conversion is achieved using the MAX16821 PWM HB LED driver. Since the average inductor current is equal to the LED current, the LED can be driven with a constant current by controlling the average inductor current. The switching frequency is set to 300kHz by resistor R6 (200kΩ).

The circuit contains two control loops: the inner current loop controls the inductor current according to the output of the outer voltage loop; the outer voltage loop sets the inner current loop, which ultimately controls the LED current. The outer voltage loop monitors the OUTV pin, and the output of U1 generates the EAOUT signal. The EAOUT signal controls the inner current loop, which controls the inductor current.

Analog LED current control

Operational amplifier U1 accepts an analog input from 1.1V to 2.8V and drives the SENSE+ input pin of the MAX16821 to regulate the LED current from 1.5A to 6A. When the LED current reaches 6A, the reference voltage and resistor divider connected to U1 produce a voltage of about 20mV (above the worst value of VOL) ​​at the output of U1. The analog control input of 2.8V produces this output voltage. When the LED current rises to 6A, the resistor divider formed by R1 and R22 divides the current sense signal of OUTV to produce a small voltage superimposed on the output of U1; the voltage generated by R1 and R22 is equal to the external loop reference voltage of 100mV at the SENSE+ input. Note that the OUTV signal is the voltage after the current sense signal of R9 and R18 is amplified by 135V/V. As the analog control input voltage decreases from 2.8V, the output voltage of U1 increases linearly from 20mV. The increase in U1 output voltage causes the SENSE+ input to reach 100mV at lower LED currents. When the analog control input drops to approximately 1.1V, U1 output increases to 80mV and the LED current drops to 1.5A.

PWM dimming

When the PWM is off, MOSFET Q9 at the LED output turns on, shorting the LED. The LED current drops to zero, which is determined by the on-time of Q1 (much less than 1µs in this design). The inductor current is maintained during the PWM off period. When the PWM starts turning on, Q1 turns off, and the inductor current charges the output capacitor. Once the output voltage reaches the LED start-on voltage, the LED current starts to rise. The time it takes for the LED current to rise from 0A to full value depends on several factors: the variation of the inductor current, the output capacitor, and the forward voltage of the LED. This reference design only meets the < 1µs LED turn-on time requirement when the LED current is set to 6A. For faster LED turn-on times at reduced currents, increase the inductor value and reduce the output capacitor.

Feedback compensation

Resistors R2 and R23 limit the gain of the high-frequency current loop and compensate for subharmonic oscillations. Setting a zero point in the current loop transfer function far below the unity gain frequency can ensure sufficient gain in the low-frequency region and very small errors in the inductor current. C1 and C19 are used to construct this zero point. When the PWM is turned off and on, Q1 and Q2 are alternately connected to the RC network to achieve compensation. This design can maintain the charge of C1 and C19, making the PWM response faster.

Since the inductor current is measured directly, there is no output pole in the transfer function of the driver circuit. The outer voltage loop is simplified to a single-pole system, and the voltage error amplifier determines this single pole within the set frequency range. To prevent the two feedback loops from interfering with each other, C21 and C22 reduce the unity gain frequency of the outer loop to one-tenth of the unity gain frequency of the current loop. Q7 and Q10 maintain the charge of the compensation capacitor to ensure that the output of the voltage error amplifier can instantly switch to the required value when the PWM pulse changes. Resistors R24 and R25 can prevent charge injection caused by changes in the state of Q7 and Q10 from causing charging/discharging of C21 and C22.

LED current rise/fall time

This design requires that the rise/fall time of the LED current be kept within 1µs when the PWM is operating to generate a 6A LED current. This requires the use of a smaller output filter capacitor and a larger inductor to meet the above conditions while meeting the maximum ripple requirement of the LED current. When the PWM is in the off state, Q9 is turned on, establishing a programmable inductor current loop. If the LED current is set to 6A, the inductor current will be adjusted to 6A by the MAX16821. When the output is turned on again, the inductor current charges the output capacitor C8. The charging rate of C8 determines the rise time of the LED current, and the value of C8 is calculated based on this. Because Q9 discharges much faster than C8, the fall time of the LED current is much less than 1µs.

Circuit waveform

Figure 3. Reference design test data: LED voltage (CH1), LED current (CH2), and OUTV voltage (CH3)

Figure 4. Reference design test data: LED voltage (CH1), LED current (CH2), and CLP voltage (CH3)

Figure 5. Test data of LED voltage (CH1) and LED current (CH2) rise time

Figure 6. Test data of LED voltage (CH1) and LED current (CH2) fall time

Test parameter Temperature measurement

• VIN: 10V
• IOUT: 6A
• TA: 25°C
• Circuit board temperature: +50°C
• Q3, Q4 and Q9 temperature: +52°C
• U1 surface temperature: +47.5°C
• L1 core temperature: +75°C (at 5.8A, L1 temperature is 40°C higher than ambient temperature)

Power-on sequence

• Connect a 0 to 20V, 5A power supply (PS1) between VIN+ and GND.
• Connect a 0 to 5V power supply (PS2) to J6 (V_CONTROL).
• Connect LEDs rated greater than 6A to LED+ and LED- with the shortest possible wires to reduce lead inductance. If longer wires are required, be sure to use twisted-pair wires.
• Keep J5 and J8 open.
• Turn on the PS2 power supply and output 1.1V.
• Gradually increase the PS1 power supply output to 10 V. The LED is lit and operates at 1.5 A continuous current.
• Connect a signal with an amplitude of 3V to 5V and a 30% duty cycle to the PWM pin. The LED current will be turned on and off by the PWM signal.
• The PS2 output voltage is adjusted from 1.1 V to 2.8 V. During the PWM on period, the LED current rises from 1.5 A to 6 A.