AN2259 Datasheet, PDF (1/38 Pages) STMicroelectronics – High intensity LED driver using

AN2259
APPLICATION NOTE
High intensity LED driver using the L5970D/L5973D
Introduction
High brightness LEDs are becoming a prominent source of light and often have better efficiency
and reliability than conventional light sources. While LEDs can operate from an energy source
as simple as a battery and resistor, most applications require an efficient energy source not
only for the reduction of losses, but also to maintain the brightness of the LED itself. For
applications that are powered from low voltage AC sources typically used in landscape lighting
or low voltage DC sources that may be used in automotive applications or to meet safety
requirements, high efficiency DC-DC converters configured for constant output current provide
a high efficiency driver that can operate over a relatively wide range of input voltages to drive
series strings of one to several LEDs
This application note describes a DC-DC converter circuit that can easily be configured to drive
LEDs at several different output currents and can be configured for either AC or DC input. The
circuit uses the L5973D monolithic step down converter configured to drive a series string of
LEDs in a constant current mode.
L5970D is a step down monolithic power switching regulator capable of delivering 1A while the
L5973D is able to deliver 2A at output voltages from 1.25V to 35V. Both devices use internal P-
Channel D-MOS transistors (with typical R_DS(on) of 250mΩ) as the switching element to
minimize the size of external components. An internal oscillator fixes the switching frequency
at 250kHz.
The brightness of the LED (Light Emitting Diode), or light intensity as measured in Lumens, is
proportional to the forward current flowing through the LED. Since the forward voltage drop of
the LED can vary from device to device it is important to drive the LEDs with a constant current
driver to be able to get good matching of the light output, especially when they are located side
by side where variations in light intensity are quickly noticed. A typical way to drive LEDs in the
constant current mode is to use a DC-DC converter configured to give a constant current
output. The circuit shown in Figure 7. uses the L5973D in a constant current configuration to
drive LEDs
AN2259/1105
Rev 1.0
1/38
www.st.com
38