Portable Projector 6A RGB LED Driver Reference Design

Portable Projector 6A RGB LED Driver Reference Design

Abstract: This article describes a reference design for a 6A step-down LED driver based on the MAX16821. This circuit can drive an LED. Reference designs include circuit specifications, schematics, circuit descriptions, and performance introductions.


This reference design is for a 6A step-down LED driver for portable projectors. The reference design is based on the PWM HB LED driver MAX16821, which drives one LED; three LEDs are required to drive RGB tri-color 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 for analog control.

Analog control voltage: 1.1V to 2.8V, corresponding to 1.5A to 6A.

Maximum LED on-duty: 50%.

Maximum LED current rise/fall time: < 1us.

Maximum LED current ripple: <15% at 6A.


Vin (J1 and J2 are connected to VIN+, J3 and J4 are connected to GND): 10V to 15V input power.

On/Off Control (J8): Floating or connected to +5V to enable the driver; J8 disables the board when shorted.

PWM input (J7): PWM dimming input, connected to a PWM signal with an amplitude of 3V to 5V. In order to ensure that the PWM input can drive Q1 and Q7, the signal source should drive the 300pF load with a rise/fall time of less than 500ns. Since the rise/fall time of the output signal is controlled within 1?s, any PWM signal of 3 to 4 times with a period of 1?s can be used.

LED current control (J6): ​​LED current regulation input. Load 1.1V to 2.8V and adjust the LED current from 1.5A to 5A.


LED+ (J9, J10): Connected to the LED anode.

LED- (J11, J12): Connect to the LED cathode.

Inductor Current Output (J5): Provides a signal proportional to the LED current. The OUTV voltage is 135 times the R9 voltage.

MAX16821 LED Driver Board

Figure 1 MAX16821 LED Driver Board

LED driver circuit board schematic

Figure 2 LED driver circuit board schematic

Circuit description

The LED driver bucks the input supply voltage from 10V to 15V, and the constant current drives an LED with a forward voltage of 4.5V to 6V. Buck 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 constant current by controlling the average inductor current, and the switching frequency is set to 300 kHz through the resistor R6 (200 kΩ).

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

Analog LED current control

Operational amplifier U1 accepts an analog input from 1.1V to 2.8V, driving 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 connected to U1 and the resistor divider produce a voltage of approximately 20mV (higher than VOL) ​​at the U1 output. The 2.8V analog control input produces this output voltage. When the LED current rises to 6A, the resistor divider formed by R1 and R22 divides the current detection signal of OUTV to generate a small voltage superimposed on the output of U1; the voltage generated by R1 and R22 is equal to 100mV of SENSE+ input. Outer loop reference voltage. Note that the OUTV signal is the amplified voltage of the R9 and R18 current detection signals, and the amplification factor is 135V/V. As the analog control input voltage begins to drop from 2.8V, the output voltage of U1 increases linearly from 20mV. The U1 output voltage rises so that the SENSE+ input reaches 100mV at a lower LED current. When the analog control input drops to approximately 1.1V, the U1 output is increased to 80mV and the LED current is reduced to 1.5A.

PWM dimming

When the PWM is off, the MOSFET Q9 at the LED output is turned on and the LED is shorted. The LED current drops to zero, depending on the turn-on time of Q1 (far less than 1?s in this design). The inductor current is always maintained while the PWM is off. When the PWM starts to turn on, Q1 turns off and the inductor current charges the output capacitor. Once the output voltage reaches the initial turn-on voltage of the LED, the LED current begins to rise. The time it takes for the LED current to rise from 0A to full scale depends on several factors: the inductor current, the output capacitor, and the change in the forward voltage of the LED. This reference design meets the < 1?s LED turn-on time requirement only when the LED current is set to 6A. To get a fast LED turn-on time at reduced current, 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 secondary harmonic oscillations. Setting a zero point in the current loop transfer function far below the unity gain frequency ensures that there is sufficient gain in the low frequency region and that the error in the inductor current is very small. This zero is constructed using C1 and C19. When the PWM is turned off and turned on, Q1 and Q2 are alternately connected to the RC network to achieve compensation. This design maintains the power of C1 and C19, making the PWM response more rapid.

Since the inductor current is directly measured, there is no output pole in the transfer function of the drive circuit. The external voltage loop is reduced to a single-pole system, and the voltage error amplifier determines this unique pole over the set frequency range. To avoid interference between the two feedback loops, 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, ensuring that the output of the voltage error amplifier can be switched to the desired value as soon as the PWM pulse changes. Resistors R24 and R25 can avoid charge/discharge of C21 and C22 caused by charge injection caused by Q7 and Q10 state changes.

LED current rise/fall time

This design requires that the rise/fall time of the LED current be kept within 1?s when the PWM operation produces 6A LED current. This requires the use of smaller output filter capacitors and larger inductors to meet the above conditions while meeting the maximum ripple current requirements of the LED current. When the PWM is off, Q9 turns on and a programmable inductor current loop is established. 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, based on which the value of C8 is calculated. Because the discharge speed of Q9 is much faster than C8, the fall time of LED current is much less than 1?s.

Circuit waveform

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

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

Test data for LED voltage (CH1) and LED current (CH2) rise time

Test data for LED voltage (CH1) and LED current (CH2) fall time

Temperature measurement

VIN: 10V

Iout: 6A

TA: 25 ° C

Board temperature: +50 ° C

Q3, Q4 and Q9 temperature: +52 ° C

U1 surface temperature: +47.5C

L1 core temperature: +75 ° C (L1 temperature is 40 ° C higher than the ambient temperature when the current is 5.8 A)

Power-on sequence

Connect 0 to 20V, 5A power supply (PS1) between VIN+ and GND.

Connect 0 to 5V power (PS2) at J6 (V_CONTROL).

Connect LEDs with a rating greater than 6A to LED+ and LED- through the shortest possible connection to reduce lead inductance. If you need a longer connection, be sure to use a twisted pair connection.

J5 and J8 remain open.

Turn on the PS2 power supply and output 1.1V.

Gradually increase the PS1 power output to 10V. The LED is illuminated and operates at 1.5A continuous current.

Connect a signal with a 3V to 5V, 30% duty cycle to the PWM pin. The LED current will be controlled on and off by the PWM signal.

Adjust the PS2 output voltage from 1.1V to 2.8V. During the PWM is on, the LED current rises from 1.5A to 6A.

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