Design of RGB LED driver for low power projector based on single chip microcomputer

This application note describes a reference design for an RGB LED driver for a low-power projector. The design uses a monolithic MAX16821 HB LED driver to drive one RGB LED at a time. This approach reduces component count, resulting in an efficient, compact, and economical design. The board layout and test results are shown.

Introduction
This application note provides a reference design for a low-power projector RGB LED driver. Based on the single-chip MAX16821, a high-current LED driver is built, which can provide up to 10A of current to a group of buck-driven RGB LEDs with an on/off time of less than 1µs. Only one color LED is driven at a time, and RGB shares the PWM cycle proportionally.

LED Driver Specifications
Input Supply Voltage: 10V to 15V
LED Drive Current: 10A
LED Forward Bias: 4.5V to 6V
LED Current Rise/Fall Time: < 1µs
LED Current Ripple: 10% p-p, max
Input
VIN (J4): Power Input
PWMR, PWMB, PWMG (J8 Pins 1, 3, and 4): RGB PWM input signals, should be 3.3V to 5V. Any PWM period longer than 2µs can be used while output rise/fall times are kept within 1µs. Only one of the three signals is high at a time.
PWMN (J8 Pin 4): PWMR, PWMG, and PWMB are logically NORed. PWMN is high only when all three PWM signals are low.
ON/OFF (J1): Leave open or drive to +5V to enable the driver, connect to GND to disable the board.
Output
LEDR, LEDG, LEDB (J5, J6, and J7): 10A RGB LED output. Connect LED+ to pins 3, 4, and 5; connect LED- to pins 6, 7, and 8.
OUTV (J2): Provides a signal proportional to the LED current. The voltage on OUTV is 135 times the voltage across R12||R16.
VIN_OUT (J3): Input supply voltage for connection to other boards. Pins 1 and 2 are VIN+; pins 3 and 4 are GND.

Figure 1. LED driver circuit board (top layer)

Figure 2. LED driver circuit board (bottom layer)

Figure 3. Schematic diagram of an LED driver using the MAX16821. Low-power projectors use single-chip DLPs to process RGB colors. Only one color DLP is lit at any given time. This approach allows the use of a high-current driver and a few additional switches to switch between LEDs, resulting in a compact, economical projector design.

The LED driver reference design provided in this article uses a single buck converter to sequentially drive RGB LEDs with 10A current. MOSFETS Q8, Q9, and Q10 select and switch the regulated inductor current to drive one of the RGB LEDs based on the PWM signal.

The core circuit of the MAX16821B buck converter operates in average current mode control, stepping down the 10V to 15V input supply voltage to a 4.5V to 6V LED forward bias voltage. The buck converter operating frequency is set by an external resistor to ground. The MODE pin is connected to GND to select the IC to operate in buck driver mode. The converter switches at 300kHz, which optimizes the device to provide very high efficiency with a small inductor size.

The design requires the LED current rise/fall time to be less than 1µs. To achieve this, a very small output filter capacitor must be selected, which increases the load ripple current. In addition, if the inductor is larger than the conventional value, the inductor ripple current will remain within the load ripple current. The output current slew rate is limited to 10A/µs by a 1µF capacitor (C11) at the output to prevent any overshoot caused by parasitic components.

The LED driver controls and maintains a 10A current through the inductor. According to the LED that needs to be driven at any moment, Q8, Q9 or Q10 is turned on respectively to switch the inductor current to the corresponding LED. When all three LEDs are turned off, the inductor current forms a local loop through Q4.

The MAX16821 device has two control loops: an inner loop that controls the inductor current and an outer loop that determines the inductor current required to drive the LED. In a step-down converter, the inductor current is the same as the LED current. Therefore, the control circuit is simplified to a single inductor current monitoring loop. In this design, R5 limits the current error amplifier gain to 11.5V/V to prevent subharmonic oscillations of the inductor current. The current loop compensation has no zero-pole pairs, which increases the low-frequency gain and allows the inductor current to be accurately stabilized at the value set by the voltage loop. The voltage error amplifier compares the LED current-sense voltage across R11||R17 with the internal 100mV reference and provides a 70dB error gain. The amplified output drives the inner current loop. Even with the lower gain of the inner current loop, the high gain of the voltage error amplifier can stabilize the LED current at 10A.

The inductor current switches between the local loop formed by the RGB LED and Q4, and the voltage error amplifier output requires 4 different levels. Because the output voltage is different under these 4 conditions, 4 levels are required. 4 different compensation capacitors (C7, C10, C13 and C14) are used to store the voltage error amplifier output, corresponding to 4 different load conditions. The compensation capacitors are connected to the circuit through analog switches (Q2, Q5, Q6 and Q7), and one is turned on at a time. Once the LED is turned on, the corresponding compensation capacitor immediately adjusts the error amplifier output to the last stored voltage, so that the LED current quickly rises to 10A.

The pole formed by the internal current loop absorption inductance and the output pole formed by the LED dynamic impedance and output capacitor C11 generate a pole frequency much higher than the switching frequency. The voltage loop has only one polarity, which is the pole of the voltage error amplifier. The compensation capacitors (C7, C10, C13 and C14) form a pole at the origin, making the voltage loop pass through 0dB in the 1/10 frequency range.

The MAX15025 dual-channel MOSFET driver (U2, U3) drives (Q2, Q5, Q6, and Q7) to quickly switch between LED loads with current slew rates up to 10A/1µs. The loss-reduction circuit formed by C9 and R10 slows down the switching edge at the LX node, helping to suppress any overshoot/undershoot ringing. If the output voltage exceeds 6.4V, the overvoltage protection feedback provided by R3 and R4 will shut down U1. Once the output voltage drops below 5.4V, U1 resumes switching operation. Filter capacitor C1 prevents false triggering due to noise. The RC network provides a 3ms delay from the power-on edge, allowing U1 to start operating after the input power supply stabilizes.

Figure 4. Oscilloscope screenshot showing the current in one of the LEDs (CH3); the voltage at the OUTV pin (CH1) represents the magnitude of the inductor current; and the voltage at the CLP pin (CH2) represents the PWM duty cycle. The inductor current is the same throughout the cycle. The PWM duty cycle is almost the same for the blue and green LEDs, but smaller for the red LED. The LED current rises and settles to its final value within 1µs.

Temperature Measurement:
VIN: 12V
IOUT: 10A to RGB LED, 20% for each color PWM
Cooling: Board is forced air cooled
Board Temperature: +53°C
Q1, Q3 Case: +60°C
Q4, Q8, Q9, Q10 Case: +58°C
U1 Top: +53°C
L1 Coil Temperature: +70°C
Power-up Procedure
Connect 10A RGB LED to J5, J6, and J7.
Keep PWMR, PWMG, PWMB, and PWMN signals low.
Increase the supply voltage gradually to 10V and observe the current, which should be less than 0.3A.
Apply PWM signals PWMR, PWMG, and PWMB with a PWM duty cycle of 15% to 20% (position each PWM pulse, only one PWM signal should be high at the same time). PWMN signal should be the digital OR of PWMR, PWMG, and PWMB. All three LEDs are alternately driven with 10A current at the set duty cycle.