|
|
PDF OM5448 Data sheet ( Hoja de datos )
| Número de pieza | OM5448 | |
| Descripción | Low Voltage LED Driver and Boost Converter | |
| Fabricantes | IES | |
| Logotipo |  |
|
Hay una vista previa y un enlace de descarga de OM5448 (archivo pdf) en la parte inferior de esta página. Total 28 Páginas | ||
|
No Preview Available !
Data Sheet
INTEGRATED CIRCUIT
2004 Oct 15
OM5448
LowwwVowlta.DgeaLtaESDhDerievet4r UaCn.odcnoBvemorotesrt
Integrated Electronic Solutions
1 Butler Drive
Hendon SA 5014
Australia
www.DataSheet4U.com
1 page  
Integrated Electronic Solutions, Hendon, South Australia
Low Voltage LED Driver and Boost
Converter
Data Sheet
OM5448
7 FUNCTIONAL DESCRIPTION
7.1 OM5448 function
The OM5448 is an inductive boost
converter IC designed to operate to a
very low voltage. Using a bipolar
process, it will operate down to a
supply voltage of 1 volt, making it
suitable for use from a single 1.2 volt
rechargable battery.
It operates by switching ON an
NPN output drive transistor which is in
series with a choke between VCC and
VEE. When the output transistor is
switched ON, the current flowing in
the choke increases at a rate
determined by the voltage across the
choke and its inductance.
Incorporated in the OM5448 is a
threshold detector monitoring the
collector voltage of the output
transistor. The base drive current of
the output transistor is limited and as
the transistor becomes base current
starved and it pulls out of saturation
the collector voltage starts to rise
towards the internally set switching
threshold.
When the collector voltage reaches
this threshold, the base drive is
switched OFF, open circuiting the OP
transistor. The current flowing in the
inductance of the choke acts to
ensure that the saturation current
flowing at that time continues to flow,
and raises the collector voltage on the
transistor.
The collector voltage must not be
allowed to increase to the maximum
rated output voltage, and is usually
caught by the load circuit at a voltage
more positive than VCC. For example
the load in the most simple circuit can
be LED diodes having a forward
conduction voltage greater than the
available battery voltage but less than
the maximum voltage permitted on
OP.
In the OM5448 the OFF time is fixed
internally in the IC chip, and after this
time has elapsed the output drive is
turned ON again, allowing the
inductance current to increase again
towards the threshold.
Depending on the external
components used (current setting
resistor, choke inductance, and choke
resistance) the OM5448 inverter may
operate either with the choke current
falling to zero before the internally
generated OFF period has elapsed,
or in the mode where the current does
not fall to zero before OP is again
switched ON and is pulled low. In this
second mode the choke current is
modulated between the peak
switching threshold set by the SETI
resistor, and the current level to which
it has fallen at the end of the internally
set OFF period.
7.2 Voltage reference
A voltage reference circuit in the
OM5448 offers a voltage clamped to
1.2 volts to provide a reference for
two purposes: first is to give a stable
reference for the output collector
threshold detector. The 1.2 V
reference is divided in a resistive
voltage divider to give a reference of
150 mV for the collector saturation
voltage threshold comparator.
The second use of the reference
provides a VBE voltage of about 0.6
volts below VCC to set the threshold of
the enable comparator.
If the battery voltage VCC is less than
1.2 V it the reference follows VCC.
When VCC is greater than 1.2 volts it
is clamped to 1.2 V.
7.3 Enable input (EN pin)
The enable control comparator has a
small constant current pull-up source
driving the EN pin; so that if this pin is
not connected it is pulled high, and
the OM5448 is active. EN can also be
connected to VCC.
If it is pulled low, for example by
connecting it to VEE, the OM5448 is
inactive, and the output is held
switched off.
The EN pin can be also used in an
active control circuit to close a
regulating loop, offering a controlled
output voltage or current.
7.4 Set output current
threshold (SETI pin)
The base drive to the output transistor
is set by a resistor connected from the
SETI pin to VEE. This resistor can be
varied over a wide range to allow the
output power to be adjusted to suit the
intended load.
7.5 Saturation voltage
threshold detector
A long tailed pair comparator
monitors the collector voltage of the
output transistor. The saturation
voltage of the transistor is divided by
a resistor network so that when the
collector voltage reaches the desired
VCE(sat) voltage of 450 mV, the
voltage at the comparator input is
reduced to typically 150 mV.
This overcomes the problem that
without this divider, the sum obtained
by adding the saturation voltage of
the output transistor voltage to the
voltage of the long-tail comparator’s
emitter current source (100 mV) plus
its VBE for the comparator input
transistor (600 mV) giving a total of
1.15 volts; well above the required
1 volt target minimum operating
voltage for single cell rechargeable
operation. By dividing the saturation
voltage to 150 mV the total is reduced
to 850 mV, and 1 volt performance is
assured.
The temperature coefficient of the
VBE also demands an adequate
voltage margin to ensure proper
operation at low temperatures.
7.6 Monostable time delay
When the VCE(SAT) trip level of the
output transistor is reached, and
2004 Oct 15
5
5 Page 
Integrated Electronic Solutions, Hendon, South Australia
Low Voltage LED Driver and Boost
Converter
Data Sheet
OM5448
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.
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.
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.
E ffic ie n c y (% )
100
90
80
70
60
50
40
30
20
5
LE D Curre nt vs E ffic ie nc y for O M5 4 4 8 wit h Ambe r LE D, 1.2 V supply
10 15
220uH (No Cap, Diode)
20 25
L E D C u r r e n t (mA )
30
100uH (No Cap, Diode)
220uH (10uCap, Diode)
35
100uH (10uCap, Diode)
40
Fig.7 Comparison of efficiency of 1.2 volt amber LED circuit, pulsed drive and DC drive (figures 3 & 4).
10.3.2 SETI GRAPHS AGAINST LED
CURRENT
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
(RSETI) for that given output
requirement. Thus each possible
inductor value (and series resistance)
and power supply voltage will result in
a different RSETI vs LED current
curve.
10.4 Switched LED brightness
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
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.
2004 Oct 15
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet OM5448.PDF ] | |
Hoja de datos destacado
| Número de pieza | Descripción | Fabricantes |
| OM5448 | Low Voltage LED Driver and Boost Converter | IES |
| Número de pieza | Descripción | Fabricantes |
| SLA6805M | High Voltage 3 phase Motor Driver IC. |
 Sanken |
| SDC1742 | 12- and 14-Bit Hybrid Synchro / Resolver-to-Digital Converters. |
 Analog Devices |
|
DataSheet.es es una pagina web que funciona como un repositorio de manuales o hoja de datos de muchos de los productos más populares, |
| DataSheet.es | 2020 | Privacy Policy | Contacto | Buscar |