DN340 - DC/DC Converter Drives Lumileds White LEDs from a Variety of Power Sources

DC/DC Converter Drives White LEDs from a
Variety of Power Sources – Design Note 340
Keith Szolusha
Introduction
LEDs are usually driven with a constant DC current
source in order to maintain constant luminosity. However, most DC/DC converters are designed to deliver
a constant voltage by comparing a feedback voltage
to an internal reference via an internal error amplifier.
The easiest way to turn a simple DC/DC converter into
a constant current source is to use a sense resistor to
turn the output current into a voltage and use it as the
feedback. The problem with this method is its reduced
efficiency—700mA across a 1.2V (typical reference voltage) drop produces an 840mW power loss. One solution
is to use an external op amp to amplify the voltage of a
low value resistor to the given reference voltage. This
saves converter efficiency, but significantly increases
the cost and complexity of a simple converter with the
additional components and board space.
A superior solution is to use the LT®1618 constant
current, constant voltage converter, which combines
a traditional voltage feedback loop and a unique current feedback loop to operate as a constant-voltage,
constant-current DC/DC converter. No external op amps
are required for this extremely compact solution. The
IADJ (current adjustment) pin provides the capability
to dim the LED during normal operation by varying
the resistor setting or injecting a PWM signal. Access
to both the positive and negative inputs of the internal
constant-current amplifier allows the sense resistor to be
placed anywhere in the converter’s output or input path
and provides constant output or input current. Without
access to both inputs, either a ground referenced sense
resistor or some additional level-shifting transistors or
operational amplifier would be required.
Lithium-Ion Source (3.3V to 4.2V)
The LT1618EDD is a 1.4MHz constant-current, constantvoltage boost converter in a tiny 10-pin thermally
enhanced DFN package. The monolithic (onboard) low
side switch has a maximum peak current limit of 1.5A.
This enables extremely compact high-current solutions
06/04/340_conv
RSENSE
0.15Ω
1%
320mA
LXHL-BW02
VIN
3.3V TO 4.2V
L1
15μH
LED
ON
OFF
SHDN
D1
VIN
SW
SHDN
ISN
4.7μF
10V
ISP
1μF
6.3V
LT1618
IADJ
GND
866k
FB
VC
124k
0.1μF
D1: PHILIPS PMEG2010EA
L1: COOPER SD25
DN340 F01
Figure 1. The LT1618 Powers the LXHL-BW02 White LED
from a Single Lithium-Ion Battery with 70% Efficiency
for portable and battery-powered applications. The
high switching frequency allows the input and output
capacitors and the inductor to be extremely small.
Although the LT1618 is conventionally used as a high
frequency boost converter with the load being driven
between VOUT and ground, the unique method, shown
in Figure 1, of tying the load from VOUT back to VIN
allows it to be used to drive the LXHL-BW02 1W white
LED from a lithium-ion battery input. Tying the load
back to VIN allows the forward voltage of the LED (the
load voltage) to be either above or below the input
voltage as the battery voltage changes. This topology
avoids the need for an additional inductor as would be
required in other buck-and-boost topologies such as
SEPIC or flyback.
The single inductor used here is extremely small and
low cost, matching the tiny all-ceramic capacitors and
low-profile IC. Tying the load back to VIN increases the
inductor current by summing both the input and output
currents. Due to increased switch losses, the overall
efficiency of the solution is approximately 70% over the
input voltage range. Nevertheless, at this efficiency, it
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is difficult to match the compactness and low cost of
this solution. The LED is turned off by grounding the
SHDN pin and input or output disconnect is not required.
The constant-current sensing resistor, RSENSE, is only
150mΩ. At 320mA, it dissipates about 15mW. RSENSE
is tied directly to the positive and negative input pins of
the LT1618 and is placed in the path of the LED, returning to VIN. The LT1618’s access to both the positive and
negative pins keeps this circuit simple and compact.
2-Alkaline Cell Source (1.8V to 3.0V)
Powered from two alkaline cells, the circuit in Figure 2
can illuminate a 1W LXHL-BW02 Lumileds white LED
with a constant 320mA current at 85% efficiency. The
forward voltage of the 1W LED, 3.6V typical, requires
a boosted voltage from the 2-cell input. In order to turn
off the LED, an input disconnect switch is required. Typically in handheld lighting applications, this is a simple
pushbutton or switch. Without input disconnect, the
shutdown pin would only prevent the IC from switching,
still providing a direct path of current from the input
to the output through the inductor and catch diode.
LED
ON/OFF
L1
10μH
VIN
1.8V TO 3V
VIN
D1
RSENSE
0.15Ω
1%
320mA
LXHL-BW02
SW
SHDN
ISP
Automotive Power Source (9V to 16V)
With floating sense resistor inputs, the constant-current
LED can be placed in DC/DC converter circuits where one
would not normally think to place a load. For example,
Figure 3 shows the LXHL-PW09 white LED placed in
the input path of what appears to be a boost converter
with VOUT connected to VIN. In fact, the forward voltage drop across the LED allows the boost topology to
provide what would be an output of VIN while it appears
to see an input of VIN – VF (3.6V typical forward voltage). However, the load that is driven is the white LED
with a constant 700mA current at approximately 70%
efficiency. This circuit is therefore a buck that uses a
boost converter. One advantage of this topology is that
only a single input/output capacitor is required, but
an increased inductor size is needed to limit the ripple
seen by both the LED and the input/output capacitor.
Once again, the SHDN pin can be used to turn the LED
on and off without the need for an input or output
disconnect switch.
Conclusion
The LT1618 has many features that make it an ideal IC
for providing constant current to Lumileds white LEDs
from a variety of power sources.
VIN
9V TO 16V
RSENSE
0.07Ω
1%
ISN
1μF
6.3V
LT1618
IADJ
GND
332k
D2
700mA
L1
47μH
D1
FB
VC
0.1μF
D1: PHILIPS PMEG2010EA
L1: COOPER SD25
1μF
6.3V
124k
DN340 F02
Figure 2. The LT1618 Powers the LXHL-BWO2 White LED
from Two Alkaline Cells with 85% Efficiency
ISN
4.7μF
25V
LED
ON
OFF
ISP
SW
LT1618
VIN
SHDN
SHDN
IADJ GND
FB
VC
10k
2.2nF
220pF
D1: PHILIPS PMEG2010EA
D2: LUMILEDS LXHL-PW09
L1: TOKO D104C
DN340 F03
Figure 3. The LT1618 Powers the LXHL-PW09 White LED
from an Automotive Battery with 70% Efficiency
Data Sheet Download
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