om5448 Hendon Semiconductors, om5448 Datasheet - Page 3

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

6
6.1
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 V
V
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 V
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.
2007 May 28, Revision 1.0
EE
. When the output transistor is
FUNCTIONAL DESCRIPTION
OM5448 function
CC
. For example
CC
and
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.
6.2
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 V
volts below V
the enable comparator.
If the battery voltage V
1.2 V it the reference follows V
When V
is clamped to 1.2 V.
6.3
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
Voltage reference
Enable input (EN pin)
CC
is greater than 1.2 volts it
BE
CC
voltage of about 0.6
to set the threshold of
3
CC
is less than
CC
.
Low Voltage LED Driver
the OM5448 is active. EN can also be
connected to V
If it is pulled low, for example by
connecting it to V
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.
6.4
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.
6.5
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
V
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
V
and Boost Converter
CE(sat)
BE
also demands an adequate
Set output current
threshold (SETI pin)
Saturation voltage
threshold detector
voltage of 450 mV, the
Product Specification
CC
OM5448
EE
.
, the OM5448 is