om5448 Hendon Semiconductors, om5448 Datasheet - Page 13

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

9.6
As the base drive to the output
transistor in the OM5448 is set by the
input current to the SETI pin, the
resulting LED current is therefore
dependent on the voltage across the
SETI resistor R1.
The graph in figure 14 shows this
dependence for a 1.2 volt circuit
9.7
It is possible to use a simple active
circuit external to the OM5448
connected to the SETI pin to
compensate for the variation in supply
voltage.
The compensating circuit is shown in
figure 15, giving the resulting graph in
figure 16 in which the compensated
and uncompensated LED current
against supply voltage is compared
2007 May 28, Revision 1.0
Supply voltage sensitivity
Compensation of output
drive current against supply
voltage variation
LED C u r r e n t ( m A )
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
0.6
0.7
Fig.14 Variation of output current against supply voltage.
0.8
Whit e LED curr ent , 1.2V Supply, 5k4 SETI resist or
driving a white and an amber LED
using the DC drive circuit of figure 4.
Each circuit was set to 20 mA LED
current at 1.2 V, and then the
variation of output current was plotted
as the supply voltage was changed.
Of course a real battery supply will not
normally offer this wide variation in
operating voltage.
for a white LED set up for 20mA LED
current from a 2.4 volt supply.
In this circuit a 1.2 volt reference is
generated by the two series forward
biased diode in D1a and D1b. This
then applied to the base of TR1. The
emitter of TR1 is connected to the
junction of R1 and R2 which form a
voltage divider from the supply
voltage. This is equivalent to the
divided voltage in series with a
resistor to the emitter equal to the two
resistors in parallel.
S u p p l y V o l t a g e vs A ve r a g e O u t p u t C u r r e n t
0.9
S up p l y Vo l t a g e ( v o l t s)
1.0
13
1.1
Amber LED cur r ent , 1.2V Supply, 9k38 SETI r esist or
1.2
Low Voltage LED Driver
A similar graph for a white LED
operating at 2.4 volts is shown in the
following graph, figure 16. This graph
also includes a curve for a circuit in
which the voltage sensitivity is
compensated with a simple external
circuit.
As the supply voltage varies, the input
current to SETI is almost equal to the
current in this equivalent emitter
resistor. This has been designed to
track the variation in supply voltage
and compensate so that the load
current in the LED remains constant
over the battery operating range.
As can be seen from figure 16, from
2.2 to 3 volts the LED current is nearly
constant.
and Boost Converter
1.3
Product Specification
1.4
OM5448
1.5