Specifications

Driving a power LED from a battery
Unlike a bulb, you cannot connect an LED directly to a voltage
source, as this would result either in the LED not lighting up or,
potentially, blowing up. You need to control the current flowing
through the LED and the simplest way to do this is with a
current-limiting resistor.
Generally high-power LED flash lights or torches use a
3-cell alkaline battery system. If you use a resistor drive, your
battery system has to provide a
voltage high enough to overcome the
LED’s maximum forward voltage (V
F
).
LEDs have a wide variation in forward
voltages: the V
F
of a LUXEON
®
high-
power, white LED is typically 2.50 to
3.99V at 350mA. This value increases
with the drive current and is also
dependent on temperature.
In order to get a consistent level of
light across your project, you have to
match the limiting resistor value with
the V
F
of the LED, since the light
output will reduce as the battery
discharges and its voltage drops.
Furthermore, the lamp itself will turn off when the battery
voltage falls below the LED’s V
F
, regardless of the amount of
energy available in
the battery.
A switching
converter,
however, will
adjust its output
voltage to the LED
V
F
regardless of
the battery
voltage. This gives
you the option to
keep the light
output fairly
constant across
the battery discharge cycle or to mimic resistor-drive behaviour.
The effects of variations in the LED V
F
become less of an issue,
as you can drain the battery completely flat before the system
cuts off, or even make use of a 2-cell or 1-cell system.
Switching DC-DC converter vs voltage-resistor drive
In a resistor drive for a LXK2-PW12 LED, with a typical forward
voltage of 3.4V at 350mA, operating from a 4.5V supply (3
AAA alkaline batteries), a 3Ω current-setting resistor is
needed.
However, to illustrate the significance of forward-voltage
variance, Figure 2 shows the LED current for this circuit driving
a LXK2-PW12 LED with a forward voltage of 3.65V instead of
3.4V. The current obtained at a 4.5V supply drops from
350mA to 290mA, which represents a greater than 10% light-
flux reduction as shown in Figure 1. This illustrates that resistor
tuning is required for LEDs with varying forward voltages.
Low-cost, constant-current DC-DC converters help to solve
issues related to LED drive, battery life and overall size and
weight. Typical ZXSC400 and ZXSC310 boost converter
circuits for a 2-cell battery system are available via email from
[e-mail]. The ZXSC400
operates from voltages down to 1.8V while the ZXSC310
operates down to 0.8V as shown in Figure 3.
When analysing battery voltage against discharge time for
a 2-cell alkaline circuit using a
ZXSC400 to drive a LED at a
nominal 350mA, the time for the
battery voltage to drop to 2V is
approximately 22 minutes. This is
the point where forward current
drops to 300mA, giving a 10%
reduction in luminous flux. In the
same test, using a ZXSC310, the
time for the battery voltage to
drop to 2V is approximately 56
minutes. This gives a forward
current of 150mA, resulting in a
50% reduction in luminous flux.
Conclusion
A 2-cell AAA battery with a ZXSC400 constant-current boost
converter gives longer operational times at higher flux output
as well as more constant luminous flux than a 3-cell AAA
battery using a resistor drive. For a 50% luminous flux cut-off
point, a 2-cell AAA battery ZXSC310 system gives a similar
run time to a 3-cell AAA battery with a resistor drive.
Therefore, the use of a switch-mode boost converter
improves energy utilisation, battery life and is generally a
‘greener’ solution than that offered by resistor drives.
With the switching
regulator, the variation
in forward voltage of
the LED is no longer
an issue and use of a
2-cell instead of a 3-
cell system offers
significant reduction
in overall size and
weight.
Switching regulators
reduce battery count in portable lamps
Fig. 1: Normalised luminous flux vs LED current
Fig. 3: Battery voltage vs discharge time for 2-cell
AAA alkaline battery
Fig. 2: LED current vs battery discharge time for a 3-cell
voltage-resistor drive using a 3Ω resistor
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