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In the single-channel buck converter MOSFET projectors RGB LED driver of customer

In Electronic Infomation Category: I | on April 25,2011

This application note provides a low-power projector RGB LED driver reference design. Construction of single-chip based MAX16821 high-current LED driver can drive a buck RGB LED delivers up to 10A of current on / off time of less than 1us. Only drive one at a time of color LED, RGB PWM cycle proportional share.

LED Driver Specifications

input supply voltage: 10V to 15V

LED drive current: 10A

LED forward voltage: 4.5V to 6V

LED current rise / fall time:

LED current ripple: 10% peak to peak, max


VIN (J4): Power Input

PWMR, PWMB, PWMG (J8 pin 1,3 and LM431ACZ datasheet and 4): RGB PWM input signal amplitude should be 3.3V to 5V . When the output rise / fall time remained at 1? S or less, any out of 2? S of the PWM cycle can be used. A time only one of these three signals is high.

PWMN (J8 pin 4): PWMR, PWMG and LM431ACZ price and PWMB logical NOR. Only if all three PWM signals are low, PWMN is high.

ON / OFF (J1): to keep open or drive to the +5 V to enable the drive, connected to GND to disable the circuit board work.


LEDR, LEDG, LEDB (J5, J6 and LM431ACZ suppliers and J7): 10A RGB LED output. Connected to the LED + pin 3,4 and 5; the LED-connected to pins 6,7 and 8.

OUTV (J2): provides LED current proportional to the signal, OUTV the voltage of R12 | | R16 voltage of 135 times.

VIN_OUT (J3): input voltage, for connection to other circuit boards. Pin 1 and 2 VIN +; pin 3 and 4 GND.

Figure 1. LED drive circuit board (top)

Figure 2. LED drive circuit board (bottom)

Figure 3. Constituted by the MAX16821 LED driver schematic.

Circuit description

Low-power single-chip DLP projectors handle RGB colors. Any given time, only the light of a color DLP. This method can be used all the way to high-current drive, with a few additional switch can switch between the LED, so as to constitute a compact and economical design of the projector.

This article provides LED driver reference design uses a buck converter followed by a single RGB LED current drive to provide 10A. MOSFETS Q8, Q9 and Q10 to select and switch the stability of the inductor current, based on the PWM signal provided to one of RGB LED driver.

MAX16821B buck converter circuit in the heart of the average current control mode, the 10V to 15V input supply voltage down to 4.5V to 6V LED forward bias. buck converter operating frequency set by external ground resistance. MODE pin is connected to GND, select the drive mode buck IC work. Converter switching frequency of 300kHz, the setting makes the device work in a smaller size optimized to provide very high efficiency inductor.

Design LED current rise / fall time of less than 1? S, to reach this target, you must select a very small output filter capacitor, thereby increasing the load ripple current. Also, if you choose higher than the general value of inductance, the inductor ripple current will remain within the load ripple current. Output by 1? F capacitor (C11) will limit the output current slew rate 10A /? S, to prevent any overshoot caused by parasitic components.

LED driver 10A through the inductor current is controlled and maintained in accordance with any moment of need-driven LED, respectively, on-Q8, Q9, or Q10, the inductor current to switch to the corresponding LED. All three LED is off, the inductor current through Q4 constitute a local loop.

MAX16821 device has two control loops: the internal loop controls the inductor current, the external loop to determine the required inductor current LED driver. In a buck converter, the inductor current and LED current is the same. Therefore, the control circuit is reduced to a single inductor current monitoring loop. This design, in order to prevent the sub-harmonic oscillation of the inductor current, R5 will limit the current error amplifier gain of 11.5V / V. Current loop compensation is not zero - pole, thereby increasing the low-frequency gain, the inductor current accurate and stable setting in the voltage loop. Voltage error amplifier R11 | | R17 at both ends of the LED current-voltage compared with the internal 100mV reference and provides 70dB gain error. After amplification of the output drive internal current loop. Even if the lower internal current loop gain, due to the high-gain error amplifier voltage can stabilize the LED current 10A.

RGB LED and the inductor current in Q4 to switch between the formation of the local loop, the voltage error amplifier output requires four different levels. 4 under which the output voltage is different, so you need 4 level. 4 different compensation capacitor (C7, C10, C13 and C14) used to store the voltage error amplifier output, corresponding to 4 different load conditions. Compensation capacitor through the analog switch (Q2, Q5, Q6 and Q7) connected to the circuit, each connected way. Once opened, LED, corresponding to the error amplifier compensation capacitor instantly adjust the output voltage to the last store, so that the LED current is quickly increased to 10A.

Internal current loop absorbs the inductor pole formed by the LED dynamic impedance and output capacitor C11 form the output pole, the pole generated by the frequency is much higher than the switching frequency. Circle only one polarity of the voltage, the voltage error amplifier pole. Compensation capacitor (C7, C10, C13 and C14) form a pole at the origin, so that the voltage loop in the 1 / 10 octave by 0dB.

MAX15025 Dual-Channel MOSFET driver (U2, U3) drives (Q2, Q5, Q6 and Q7), quickly switch between the LED load current slew rate of up to 10A / 1? S. C9 and R10 formed by the loss of the circuit slows down the switching node LX edge to help suppress any overshoot / under red ring. If the output voltage exceeds * V, R3 and R4 from the feedback provided by over-voltage protection will shut down U1. Once the output voltage down to 5.4V the following, U1 recovery switching operation. Filter capacitor C1 to prevent false triggering due to noise. RC networks from the beginning of the power conduction along the 3ms delay, so that stable U1 in the input power to work.

Circuit waveforms

Figure 4. Oscilloscope screenshot shows one of the LED (CH3) current

OUTV pin (CH1) voltage representative of the size of the inductor current; CLP pin (CH2) representative of the voltage on the PWM duty cycle. Inductor current is the same throughout the cycle. Blue, green LED of the PWM duty cycle is almost the same, but smaller red LED. In the 1? S and stabilize the LED current is increased to the final value.

Temperature measurement:

VIN: 12V

IOUT: 10A to the RGB LED, each color is 20% PWM

Cooling: forced air-cooled circuit board

circuit board temperature: +53 C

Q1, Q3 shell: +60 C

Q4, Q8, Q9 and Q10 shell: +58 C

U1 top: +53 C

L1 coil temperature: +70 C

Power Step

10A RGB LED will be connected to J5, J6 and J7.

to keep PWMR, PWMG, PWMB and PWMN signal is low.

the power supply voltage was gradually increased to 10V, observe the current, the current should be less than 0.3A.

applied PWM signal PWMR, PWMG and PWMB, PWM duty cycle is 15% to 20% (positioning of each PWM pulse, but should be only one PWM signal is high.) PWMN signal should be PWMR, PWMG and the number of non-PWMB. All three LED duty cycle according to the set driven by the alternating current of 10A.

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