om5448 Hendon Semiconductors, om5448 Datasheet - Page 4

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

voltage margin to ensure proper
operation at low temperatures.
6.6
When the V
output transistor is reached, and
ON/OFF latch is switch to its OFF
state. This initiates a time delay by
applying the turn-OFF switching
edge to a series string of injection
logic gates (integrated injection
logic, I
reaches the end of this series string
of gates (the sum of the gate delays)
it resets the ON/OFF latch, and turns
ON the drive to the output transistor
again.
6.7
The output drive transistor has been
designed to have a low saturation
voltage and high current gain. For
most of the ON time it is well in
saturation with only a low voltage
drop lost across the transistor.
Towards the end of the conduction
7
In accordance with the Absolute Maximum Rating System (IEC 134).
All voltages are specified with respect to V
Note
1. Mounted on an FR4 printed circuit board 8mm x 10mm x 0.7 mm.
2007 May 28, Revision 1.0
V
I
V
I
V
I
V
I
P
T
T
T
R
supply
OP
SETI
EN
SYMBOL
stg
amb
jmax
CC
OP
SETI
EN
tot
thj-a
LIMITING VALUES
2
Monostable time delay
Output drive transistor
L). When the OFF transition
CE(SAT)
supply voltage
maximum current, V
output voltage, OP pin
maximum current, OP pin
input voltage, SETI pin
maximum current, SETI pin
input voltage, EN pin
maximum current, EN pin
total power dissipation
storage temperature
operating ambient temperature
maximum junction temperature
thermal resistance from junction to ambient
trip level of the
PARAMETER
CC
pin
part of the cycle when it pulls out of
saturation, the voltage rises quickly
with the increasing current
minimising this inefficient part of its
operating cycle. It is possible to
obtain efficient performance from the
OM5448 output circuit and its
chosen switching threshold.
6.8
The optimum choke inductance for
the OM5448 has been found to be
220 µH. This offers the best balance
between cost and performance. In
addition to the optimum value of the
inductance, the core must not
saturate at the peak current. Also the
winding resistance can add
significantly to the energy losses,
and therefore for the best efficiency
of the circuit the choke resistance
needs to be minimised.
Simple calculations indicate that
some of the source of losses in the
OM5448 circuit can be identified. For
EE
.
Choke selection:
Efficiency
4
current in
current in
current out
note 1
CONDITIONS
Low Voltage LED Driver
−0.5
−1
−0.5
−1
V
−1
V
−1
−
−40
−40
−
−
EE
EE
MIN.
example, operating from a 1.2 volt
battery an average conduction time
saturation voltage of 60 mV across
the output transistor will lose 5% of
the battery voltage (and hence
energy) in the transistor.
In addition a peak current of 200 mA
and a coil resistance of 1.6 Ohms
means that a further 320 mV of
battery voltage is effectively lost.
If an LED is driven with an average
current of 20 mA, with a peak current
of 50 mA, the peak voltage loss
across the inductor’s effective
resistance is 80 mV, and of a
comparable order of magnitude to
the transistor losses.
A further source of losses arises
from any internal series resistance
provided by the power supply and
the load. For example, the effective
series resistance of a power supply
filter capacitor can offer a significant
loss of efficiency: a low effective
series resistance capacitor is
needed.
and Boost Converter
− 0.5
− 0.5
3.5
500
14
500
V
50
V
1
200
+150
+85
+150
+500
CC
CC
Product Specification
MAX.
OM5448
+ 0.5
+ 0.5
V
mA
V
mA
V
mA
V
mA
mW
°C
°C
°C
deg K/W
UNIT