Abstract: This application note presents a reference design for a 4S1P MR-16 LED driver that provides 750mA to a string of four white LEDs (WLEDs). The circuit operates from a 24V source and is based around the MAX16820 hysteretic LED driver. Also included is a MAX5033 24V-to-5V, 150mA switching power supply to power a Nuventix® pulsating LED cooler.

Overview
This reference design details a driver circuit using the MAX16820 hysteretic LED driver for a 4-LED MR-16 lamp with an associated power source (MAX5033) for a pulsed air cooler.

The following are the electrical input requirements and output capabilities.

V_IN: 24V_DC ±5%
Temperature: +80°C (max) ambient
V_LED configuration: 4 LEDs in series (16V_DC, max), 750mA
Auxiliary output power: 5V at 150mA (average), 300mA (peak)

The reference design is discussed in detail below, with analysis of the main circuit blocks and layout considerations.

Figure 1. Schematic of the driver design.

Figure 2. Photo of the PCB. The board was designed to fit directly into an MR-16 assembly.

Figure 3. Revised layout of the PCB (updated from the board in Figure 2).

Design Analysis
Designed to fit directly into an MR-16 assembly, this reference design delivers 750mA to a string of 4 LEDs from a 24V power source. The MAX16820 provides hysteretic control and regulates the current through the inductor and the LEDs. Meanwhile, the MAX5033 provides power to a pulsating cooler through the J2 connector. The cooler presents a sinusoidal load of 300mA peak and 150mA average to the power supply.

The R3 resistor is in the circuit to satisfy the stability requirements (with a ceramic output capacitor) of the MAX5033.

A provision was made for PWMing, but not tested. The center pin of J1 was routed directly to the DIM input of the MAX16820.

Power-Up Procedure

Connect 4 LEDs in series to the LED+ (anode) and LED- (cathode) pins.
Attach a 300mA load to J2.
Attach a 24V, 1A power supply to J1.
Turn on the power supply.
Table 1. Bill of Materials (BOM)

Designator Component Description Footprint Quantity

C1 Nonpolarized capacitor 10µF, 50V, X7R 2220 1

C2 Nonpolarized capacitor 470pF, 50V, COG 0402 1

C3 Nonpolarized capacitor 1µF, 16V, X7R 0603 1

C4, C5 Nonpolarized capacitors 0.1µF, 25V, X7R 0603 2

C6, C7 Nonpolarized capacitors 10µF, 10V, X7R 1206 2

D1 Schottky diode STPS2L60A SMA 1

D2 Schottky diode RB160M-60 SOD123F 1

J1 3 x 1 connector — 1.25mm 1

J2 2 x 1 connector — 100mil 1

L1 Inductor 82µH, MSS1038-823 10.2mm x 10mm 1

L2 Inductor 390µH, MSS1038-394 10.2mm x 10mm 1

Q1 n-channel MOSFET 2SK2857 SOT89 1

R1 Resistor 0.24Ω, 1% 1206 1

R2 Resistor 10kΩ, 1% 0603 1

R3 Resistor 0.27Ω, 5% 1206 1

R4 Resistor 100kΩ, 1% 0603 1

R5 Resistor 10kΩ, 1% 0603 1

U1 LED driver MAX16820ATT 6-TDFN 1

U2 Buck DC-DC converter MAX5033BUSA 8-SO 1

Nuventix is a registered trademark of Nuventix, Inc.

Related Parts APP 4381: Jun 29, 2009

MAX16820 2MHz High-Brightness LED Drivers with High-Side Current Sense and 5000:1 Dimming Full Data Sheet
(PDF, 220kB) Free Samples

MAX5033 500mA, 76V, High-Efficiency, MAXPower Step-Down DC-DC Converter Full Data Sheet
(PDF, 676kB) Free Samples

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AN4381, AN 4381, APP4381, Appnote4381, Appnote 4381