Reference Design for a Signal-Lamp LED Driver

Abstract:?This application note presents a reference design for a signal-lamp LED driver that consists of six strings of 4 LEDs per string and delivers 350mA per string. Because of its wide input-voltage swing (11V to 28V), parallel linear controllers (the MAX16836) are used for each string to distribute heat.

Circuit Description

This application note describes a linear LED driver for six strings of 4 LEDs to be used in a signal lamp. The circuit is designed to operate from an automotive battery with a minimum voltage of 10V and a maximum of 16V, while delivering 350mA to each string.

The LED driver's (MAX16836) current-sense inputs are limited to a 4V common-mode voltage; matched PNP transistor pairs are used to translate the LED current-sense resistor voltage to the input current-sense pins of the driver.

When the LED string is at its minimum voltage (7.6V) and the input is at its maximum (28V), the LED drivers will experience a dissipation of over 7W. Since this amount of dissipation is impossible, even with the best heatsinking methods, the duty cycle must be reduced to less than 25%.

This reference design allows both for a universal dimming input that will control all six drivers, as well as independent enables that allow each string to be controlled separately.

Figure 1 presents a schematic for the circuit, and Table 1 provides the Bill of Materials (BOM). The following are the electrical input requirements and output capabilities.

VIN: 11V to 28V
PWMIN: 10% to 100%
VLED configuration: 7.6VDC (min) to 10VDC (max), 350mA


Figure 1. Schematic of the driver design.

Table 1. Bill of Materials

Designator	Component	Description	Footprint	Quantity
C1, C2, C3, C4, C5, C6	Nonpolarized capacitor	0.1μF, 25V	0603	6
C10, C11, C12, C13, C14, C15	Nonpolarized capacitor	0.1μF, 50V	0603	6
Q1, Q2, Q3, Q4, Q5, Q6	Dual PNP transistor	DMMT5401	6-SOT23	6
R1, R5, R9, R13, R17, R21	Resistor	0.5Ω	0603	6
R2, R6, R10, R14, R18, R22	Resistor	866Ω	0603	6
R3, R7, R11, R15, R19, R23	Resistor	1.00kΩ	0603	6
R4, R8, R12, R16, R20, R24	Resistor	40.2Ω	0603	6
U1, U2, U3, U4, U5, U6	MAX16836ATE	High-brightness LED driver	5mm x 5mm, 16-TQFN with exposed pad	6