MR-16 LED Driver with a 5V Auxiliary Output to Power a Pulsating LED Cooler

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_LEDconfiguration: 4 LEDs in series (16VDC, 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 1. Schematic of the driver design<./p>

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	2200	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	3x1 Connector	--	1.25mm	1
J2	2x1 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	MAX516820ATT	6-TDFN	1
U2	Buck DC-DC Converter	MAX533BUSA	8-SO	1