Adding Three Power MOSFETs Lets a Single Step-Down

Maxim > Design Support > Technical Documents > Reference Designs > Display Drivers > APP 4367
Maxim > Design Support > Technical Documents > Reference Designs > LED Lighting > APP 4367
Maxim > Design Support > Technical Documents > Reference Designs > Power-Supply Circuits > APP 4367
Keywords: HB, high brightness, LED, HB LED, RGB, projection, projector, high current, luminus, PhlatLight
REFERENCE DESIGN 4367 INCLUDES: Tested Circuit Schematic Description Test Data Layout
Adding Three Power MOSFETs Lets a Single StepDown Converter Drive RGB LEDs in Projectors
By: Anil Baby, Senior Member of the Technical Staff
Dec 06, 2010
Abstract: This application note is an RGB LED driver reference design for a low-power projector. The
design features a single MAX16821 HB LED driver to drive the RGB LEDs one at a time. This approach
reduces the components needed, resulting in an efficient, small, and economical design. Board layout and
test results are shown.
This application note presents an RGB LED driver reference design for a low-power projector. This highcurrent LED driver is based on a single MAX16821 device that drives 10A to a set of RGB LEDs in stepdown mode with less than 1µs turn-on/turn-off times. Only one color LED is driven at a time, with RGB
colors sharing the PWM period in required proportions.
LED Driver Specifications
Input supply voltage: 10V to 15V
LED drive current: 10A
LED forward voltage: 4.5V to 6V
LED current rise/fall times: < 1µs
LED current ripple: 10% peak to peak, max
VIN (J4): supply voltage input
PWMR, PWMB, PWMG (J8 pins 1, 3, and 4): RGB PWM input signals. Amplitude should be 3.3V to
5V. As output can rise/fall within 1µs, any PWM period higher than 2µs can be used. Only one of the
above three signals should be high at a time.
PWMN (J8 pin 4): digital NOR of PWMR, PWMG, and PWMB. PWMN will be high only when all three
PWM signals are low.
ON/OFF (J1): leave open or drive +5V to enable the driver. Connect to GND for disabling the board.
LEDR, LEDG, LEDB (J5, J6, and J7): 10A RGB LED outputs. Connect LED+ to pins 3, 4, and 5;
connect LED- to pins 6, 7, and 8.
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OUTV (J2): provides a signal proportional to the LED current. The voltage at OUTV will be 135 times
the voltage across R12||R16.
VIN_OUT (J3): input supply-voltage output for connecting to other boards. Pins 1 and 2 are VIN+ ; pins 3
and 4 are GND.
More detailed image (PDF, 1.25MB)
Figure 1. The LED driver board (top view).
More detailed image (PDF, 1.35MB)
Figure 2. The LED driver board (bottom view).
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More detailed image (PDF, 112kB)
Figure 3. Schematic of the LED driver which features the MAX16821.
Circuit Description
Low-power projectors use a single DLP chip to process RGB colors. Only one color illuminates the DLP at
any given time. This approach enables the use of a single high-current driver with additional switches to
switch between the LEDs. The result is a compact and cost-sensitive projector design.
This LED driver reference design uses a single step-down converter to drive a 10A current to RGB LEDs
one after the other. MOSFETS Q8, Q9, and Q10 select and route the regulated inductor current to one of
the RGB LEDs, based on the PWM signals.
The core buck-converter stage that steps down the 10V to 15V input supply to the 4.5V to 6V LED forward
voltage is based on an average current-mode controller, the MAX16821B. The buck converter is wired in
synchronous mode with the output referenced to ground. The Mode pin is connected to GND to select the
IC operation in ground-referenced buck driver mode. The converter works at a 300kHz switching frequency,
optimally selected so the design can use a small size inductor and be highly efficient.
This design requires the LED current rise/fall times to be less than 1µs. To accomplish this, the value of
output filter capacitor must be very small, thus increasing the load ripple current. Instead, a larger than
usual inductor is chosen, so that the inductor ripple current is kept below the limit on the load ripple current.
A 1µF capacitor is used at the output (C11) to limit the output-current slew rate close to 10A/µs and to
prevent any overshoots caused by parasitic components.
This LED driver controls and maintains a 10A current through the inductor. Depending on the LED to be
driven at any instant, Q8, Q9, or Q10 is turned on and the inductor current is routed through the
corresponding LED. When all three LEDs are off, the inductor current is circulated locally through Q4.
The MAX16821 device has two control loops; an internal loop controls the inductor current and the external
loop determines the inductor current needed to drive the LED current. In a step-down converter the inductor
current is the same as the LED current. Therefore, the control is effectively reduced to a single loop that
monitors the inductor current. To prevent subharmonic oscillations in the inductor current in this design, R5
limits the current error-amplifier gain to 11.5V/V. The current loop compensation does not have a pole-zero
pair to increase the gain at low frequencies that makes the inductor current settle accurately to the value
programmed by the voltage loop. The voltage-error amplifier compares the LED current-sense voltage
across R11||R17 to an internal 100mV reference, and amplifies the error with a 70dB gain. This amplified
output drives the internal current loop. Even if the internal current loop gain is less because of the high gain
of the voltage-error amplifier, the LED current settles to 10A.
When the inductor current is switched between RGB LEDs and the local loop made by Q4, the voltage-error
amplifier output needs to be at four different levels. These four levels are needed because the output
voltage in each of these four conditions differs. Four different compensation capacitors (C7, C10, C13, and
C14) store the voltage-error amplifier outputs, corresponding to four different load conditions. Compensation
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capacitors are connected to the circuit, one at a time using analog switches (Q2, Q5, Q6, and Q7). As soon
as an LED is turned on, the corresponding compensation capacitor immediately adjusts the error amplifier
output to the voltage stored in the previous cycle, thus enabling the LED current to rise to 10A quickly.
The internal current loop absorbs the inductor pole. The output pole, formed by the LED dynamic
impedance and the output capacitor C11, occurs much higher than the switching frequency. The voltage
loop will have only a single pole that is the voltage-error amplifier pole. Compensation capacitors (C7, C10,
C13, and C14) produce a pole at the origin and make the voltage loop cross 0dB at one-tenth the currentloop crossover frequency.
The MAX5054 dual MOSFET driver (U2, U3) drive (Q2, Q5, Q6, and Q7) to enable fast switching between
the LED loads and the high-current slew rates of 10A/1µs. A dissipative snubber, formed by C9 and R10,
slows the switching edges at the LX node and also damps any overshoot/undershoot and ringing.
Overvoltage protection feedback provided by R3 and R4 shuts down U1 if the output voltage exceeds 6.4V.
U1 resumes switching once the output voltage reduces below 5.4V. Filter capacitor C1 prevents false
triggering due to any noise. The RC network provides a 3ms delay from the supply turn-on edge, so that U1
can start after the input supply settles.
Circuit Waveforms
Figure 4. The scope shot shows the current though one of the LEDs (CH3); the voltage at the OUTV pin
(CH1) that represents the inductor current; and the voltage at the CLP pin (CH2) that represents the PWM
duty cycle. Inductor current is the same throughout the cycle. The PWM duty cycle is almost the same for
blue and green LEDs, but less for the red LED. The LED current rises and settle to the final value in 1µs.
Temperature Measurements
VIN: 12V
IOUT : 10A to RGB LEDs with 20% PWM to each color
Cooling: board is cooled by forced air
Board temperature: +53°C
Q1, Q3 case: +60°C
Q4, Q8, Q9, and Q10: +58°C
U1 top: +53°C
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L1 winding temperature: +70°C
Power-Up Procedure
Connect 10A RGB LEDs to J5, J6, and J7.
Keep PWMR, PWMG, PWMB, and PWMN signals low.
Increase the supply voltage gradually to 10V, observing the current which should be less than 0.3A.
Apply the PWM signals, PWMR, PWMG, and PWMB with 15% to 20% PWM duty cycle. (Each of the
PWM on pulses should be positioned so that only one PWM signal is high at a time.) The PWMN
signal should be the digital NOR of PWMR, PWMG, and PWMB. All three LEDs will be alternately
driven with 10A for the set duty cycles.
Related Parts
High-Power Synchronous HB LED Drivers with Rapid
Current Pulsing
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REFERENCE DESIGN 4367, AN4367, AN 4367, APP4367, Appnote4367, Appnote 4367
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