om5448 Hendon Semiconductors, om5448 Datasheet - Page 6

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

9
9.1
A series of circuit examples are
suggested. These offer a number of
application ideas for use of the
OM5448. They have been chosen to
illustrate the different ways in which
the OM5448 can be used, and show
how different circuit options can be
chosen to emphasise the useful
possibilities of this low voltage circuit.
The first circuit is the simplest, using a
single LED load driven by a pulsed
forward current drive (without filtering)
during the flyback OFF time of the
OM5448. Care must be taken to
ensure that the peak pulse current
remains within the pulse current
rating of the LED data sheet.
9.2
This application circuit in figure 3 has
the lowest component count. In this
circuit it is shown driving a single LED
with pulsed current drive. When the
output is ON, and pin OP is held low,
the current increases through the
choke L1 until the internally set
current threshold is reached and the
2007 May 28, Revision 1.0
APPLICATION INFORMATION
Circuit design
considerations
Simple pulse driven LED
circuit
Fig.3 Simple pulse driven LED circuit
Different circuit features discussed
include the following:
• Pulsed LED drive
• DC LED drive
• Multiple LEDs, in series or parallel
• Active control of the SETI input
• Battery voltage compensation for
• Simple boost power supply circuits
• Enable input control
• Regulated power supply
• Voltage multiplier circuit
• Regulated voltage multiplier
The OM5448 operation sees
increasing current in the inductor
while it is being driven during the ON
period of the output drive. During the
OFF time this same current flows into
the load voltage with the inductive
OP transistor is turned OFF. The
inductive action seeking to maintain
the current flow in L1 raises the
voltage on the junction of OP and the
LED above the supply rail VCC until
there is sufficient voltage for the LED
to conduct. Thus the current in the
inductance continues to flow in the
LED for a fixed period while OP is
OFF. During this time the current falls
LED current drive
6
Low Voltage LED Driver
effect generating the necessary
voltage above the V
drive the load. Thus a circuit used to
drive a 2.4 volt load from a 1.2 volt
supply will at a first approximation,
have a duty cycle of 50% with equal
times for the output ON and OFF.
If the load is increased to 4.8 volts,
then the ON time becomes 75% of the
cycle, and the OFF time 25%. Thus
the proportion of the cycle time spent
driving the output decreases as it is
required to drive a higher output
voltage.
In addition, this decreasing duty cycle
implies that the output current
available must decrease as the
voltage is increased.
at a rate determined by the LED
forward voltage plus the effective
voltage drop in the inductor’s
resistance less the supply voltage.
As the LED current is pulsed, the
peak current must remain below the
maximum peak current rating of the
LED. If ratings are exceeded the
operational life of the LED may be
threatened.
and Boost Converter
Product Specification
OM5448
CC
supply rail to