om5448 Hendon Semiconductors, om5448 Datasheet - Page 9

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

The third graph in figure 7 shows the
efficiency against LED current for an
amber LED. Both the pulsed circuit
(figure 3) and the DC drive circuit
(figure 4) are shown on this graph.
9.3.2
The graphs in figures 8 and 9 show
the value of the resistor R1 connected
to the SETI pin in the DC LED drive
circuit of figure 4.
As can be seen for the graphs, the
choice of LED current and colour set
the output power, and therefore all
factors which contribute to losses
(and thus efficiency) will need to be
compensated for in the choice of R1
(R
2007 May 28, Revision 1.0
SETI
E ffi c i e n c y (% )
) for that given output
1 00
90
80
70
60
50
40
30
20
Fig.7 Comparison of efficiency of 1.2 volt amber LED circuit, pulsed drive and DC drive (figures 3 & 4).
SETI
CURRENT
5
GRAPHS AGAINST
10
220uH (No Cap, Diode)
LED
LE D Curre nt vs E ffic ie nc y for OM54 4 8 with Amber LED, 1.2 V supply
15
It should be noted that in the pulsed
circuit the efficiency falls more quickly
at high currents because of the
relatively high resistance of the
220 µH inductor.
requirement. Thus each possible
inductor value (and series resistance)
and power supply voltage will result in
a different R
curve.
9.4
The circuit in figure 10 is able to be
switched between two levels of LED
brightness. By using switch SW1 on
the SETI pin with a second current
setting resistor R2, actuation of the
1 00uH (No Cap, Diode)
Switched LED brightness
20
SETI
L E D C u r r e n t (mA )
vs LED current
9
25
220uH (10uCap, Diode)
Low Voltage LED Driver
As the typical forward voltage drop of
an amber LED is 2.1 volts at 20 mA,
and significant forward current flows
at about 1.6 volts, the amber LED
cannot be driven from a 2.4 volt
supply.
switch changes the set current and
hence the brightness.
Other ideas might be suggested for
setting brightness: for example use of
a potentiometer in series with a fixed
resistor will offer continuously
variable light levels between a
minimum and maximum figure.
30
and Boost Converter
100uH (10uCap, Diode)
Product Specification
35
OM5448
40