om5448 Hendon Semiconductors, om5448 Datasheet - Page 7

• Manufacturer Part Number: om5448
• Description: Low Voltage Led Driver And Boost Converter
• Manufacturer: Hendon Semiconductors

The value of the current setting
resistor R1 must be chosen to give
the required average LED current and
hence brightness.
The required value of R1, and the
operating duty cycle will change
depending on the forward voltage
drop of the LED. For example, 2.1 volt
9.3
The circuit shown in figure 4 adds a
Schottky diode and filter capacitor to
the previous circuit. The Schottky
diode carries current when the
voltage on the junction of pin OP, the
inductor, and the anode of the
Schottky diode is positive with respect
to V
current flows through D1 to provide
stored charge in filter capacitor C1.
Therefore the LED in this circuit is
driven with a near DC current, and
there are no problems with respect to
its pulse rating being exceeded.
A normal small signal silicon diode
can be used, but its increased forward
voltage drop will result in lower circuit
efficiency. As has been mentioned
2007 May 28, Revision 1.0
CC
while OP is OFF. The inductor
DC LED drive
amber LED will run with a longer
conduction time each cycle when
compared with a white LED with a
typical forward voltage drop of 3.2
volts. For a similar average current
the peak current in the white LED will
be higher due to its lower duty cycle
(shorter conduction time).
elsewhere, the effective series
resistance of capacitor C1 is also a
factor in how efficiently the circuit will
perform.
The value for R1 is chosen to give the
target LED current at nominal
operating voltage.
For example the component values
shown in this circuit (figure 4) are for
an amber LED carrying an average
current of 20 mA.
To drive a white LED at 20 mA the
value of R1 becomes 5.6 k.
If the battery voltage is increased to
2.4 volts, then the values of R1
needed for a 20 mA white LED drive
becomes 82 kilohms. As 2.4 volts
exceeds the sum of the forward
Fig.4 DC driven LED circuit
7
Low Voltage LED Driver
This circuit will also drive a white LED
from a 2.4 volt supply.
Note that 2.4 volts is greater than the
forward voltage of an amber LED, so
the LED will already be conducting at
the nominal battery voltage, and the
circuit will not work.
voltage of an amber LED and the
forward voltage of the Schottky diode
D1, at 2.4 volts this circuit is
unsuitable for driving an amber LED.
9.3.1
In the following graphs, shown in
figures 5, 6 and 7, efficiencies are
compared for a number of variants of
the circuit in figure 3 and figure 4.
Efficiency is plotted for the average
LED current x LED voltage at an
equivalent DC current as a proportion
of the input power (average input
current x DC battery voltage).
The first two are for white LED drive,
first (fig 5) in pulse mode, and the
second (fig 6) in DC mode.
and Boost Converter
P
DC
ERFORMANCE GRAPHS FOR
LED DRIVE
Product Specification
OM5448
,
EFFICIENCY