Data Sheet

DNS001, 2015-04-27, RKr Page 2 of 7
R E F E R E N C E D E S I G N N O T E
LED Driver with MagI
3
C Power Module
MagI
3
C
Power Modules
VDRM
- LED Driver Application
3. Description __________________________________________________
The power consumption of LEDs on the lighting market is constantly increasing. LEDs require currents of 700
mA, 1 A or higher. High output currents are supported commonly with voltage regulated modules. Constant
high current led drivers are uncommon on the market. This design note describes how to convert a voltage
regulated module to a current regulated module. Typical supplies for LEDs are modules operating as current
sources. They include dual protection against output short circuits. A current regulated circuit is - if designed
properly - short circuit protected by itself due to its nature of forcing a defined current into the load regardless
if the load is an LED or a low resistance like a short circuit. The second overcurrent protection circuit is
integrated in the module commonly through a cycle by cycle current limit.
A MagI
3
C VDRM module is used in this application. Natively it is voltage regulated. The current through a
specific number of LEDs will flow from LED+ pin through the LEDs to the LED- pin. The LED current flows
through a shunt resistor, R3, and the voltage across R3 is compared with a dimming voltage. A 5 V dimming
voltage at the DIM terminal results in a voltage of 75 mV at the regulation loop amplifier (IC3, pin4) with which
the shunt voltage (IC3, pin 3) is compared. IC3 is used for an outer and slower regulation circuit which drives
the feedback pin of the VDRM module. The compensation network consists of C8, C9 and R6, which may be
trimmed by the design engineer depending on the customer PCB layout if modified from the reference design
PCB layout.
An external dimming (adjusting) voltage is always required. This can be derived directly from V
IN
if stable or
from a resistor divider from the 3.3 V of the linear regulator (IC2).
The supply of the operational amplifier is derived from V
IN
with a linear regulator (IC2) with a capability of abs.
max. 40V or 60V input voltage. Its output voltage is set to 3.3V. The Input voltage of the design ranges from 7
V to 50 V (only with the 60 V version of the LM2936, which is not available in this small package) and the
output current ranges from 0 A to 2.5 A. Note that the output power shall not exceed 22.5 W. The compensation
may be adapted for various operating points.
This Reference Design is developed using the following specification:
V
in
= 7 V - 24 V V
out
= 0 V - 15 V I
out
= 10 mA - 1.5 A