www.nutsvolts.com/index.php?/magazine/article/november2011_Day
■ FIGURE 2. Laptop adaptor's DC
voltage wiring, stripped and connected to
the TPS54160 PWB.
integrated DC/DC power supply. A
DELL Latitude ™ AC adaptor
generates 18V at 3.5A. You can
access the DC voltage by cutting off
the connector that plugs into the
laptop. Peel back the outer insulation
to reveal the power and return wires
as shown in Figure 2.
The AC adaptor directly powers
a standard DC/DC power supply that
has been converted to provide a
constant current output. Almost any
standard DC/DC converter can be
used. The converter’s input voltage
rating must be higher than the laptop
adaptor output voltage. It also must
have an appropriate output current
rating. A power supply controller
typically is a cheaper solution, while
a fully integrated converter simplifies the design process.
The chosen power supply must provide the user with
access to the feedback resistors to allow for modification
from constant output voltage to constant output current.
■ FIGURE 3. The power supply's top feedback resistor
replaced with the LED load.
Power Supply
Vout
Vout
SHEDDING LIGHT
ON THE BUILD
Figure 3 shows how to convert a standard power
supply into a constant output current supply. By replacing
the top feedback resistor with the LED load and then
changing the bottom feedback resistor to set the LED
current, the power supply now regulates the voltage
across R2 to its internal reference voltage. Because
relatively no current flows into the feedback (FB) pin, the
fixed voltage across R2 sets the LED current. The output
capacitor, Cout, return connection is moved from ground
to the FB pin to minimize the ripple current in the LEDs
while still stabilizing the power supply’s feedback loop.
Equation 1 calculates the correct value of R2. Vref
can be found in the power supply’s datasheet. This project
■ EQUATION 1.
Vref R2 = ILED
Vfb
Vfb
PARTS LIST
ITEM SOURCE
LEDs CREE XRE
Lenses L2 Optics
Heatsinks Wakefield Solutions
TPS54160EVM Texas Instruments
PWBs Misc.
Laptop adaptor Junk drawer
takes advantage of the TPS54160 LED driver EVM. This is
a standard DC/DC power supply that already has been
converted into a constant current source.
The circuit in Figure 3 automatically adjusts its output
voltage to keep a constant current flowing through the
LEDs. It compensates for changes in the input power
supply voltage and for changes in the LED forward voltage
drop. You can add additional LEDs in series, as long as
the sum of the LED forward voltage drops, plus, Vref is
less than the voltage supplied by the AC adaptor. Select
an LED color
temperature that
compliments
the area being
illuminated.
This design uses
a cool white Cree®
1-A XLamp® LED to
bring out the blue
November 2011 31
Download a datasheet or
other technical documents:
www.ti.com/product/
tps54160.
For more information
about LEDs, visit:
www.ti.com/led-ca.
See videos, ask questions,
and share knowledge
on the TI
E2E™ Community:
www.ti.com/e2e-ca.
REFERENCES
