AN11

Application Note 11
Issue 2 October 1995
Features and Applications of the FMMT618 and 619
High Current SOT23 replaces SOT89, SOT223 and D-PAK
David Bradbury
Switch 6A loads using a SOT23
transistor?
Zetex has developed this new range to
meet ever increasing demands for
higher performance in smaller
packages.
At first sight the performance of the
FMMT618 and FMMT619 transistors may
appear unbelievable for SOT23 packaged
devices, however the following note
describes how this has been achieved. The
n ote als o disc usses a number of
applications where they are ideally suited,
which until now were often forced to
utilise the larger SOT89 or SOT223
packaged devices. Figure1 provides a
graphic example of this performance.
VCE=2V
FMMT618
FE
400
FMMT619
h
200
BCP54 (SOT223)
100
0
0.001
0.01
0.1
Collector Current (A)
Figure 1
Comparison of hFE profiles.
1
* VCEO up to 50V
* ICM up to 6A
* IC CONT up to 2.5A
* Ptot of 625mW
* High hFE - e.g 300 min @ 200mA
* Very low VCE(sat) - 200mV max @ 2.5A
Comparison with SOT89 and
SOT223 Parts
Table 1 gives a quick comparison of the
performance of the Zetex FMMT618 &
FMMT619 with two industry standard
SOT89 and SOT223 parts.
ZETEX product
FMMT618 FMMT619
10
IndustryStandard
BCX54
BCP54
SOT23
SOT23
SOT89
SOT223
BVCEO
20V
50V
45V
45V
IC
2.5A
2A
1A
1A
1.5A
Package
500
300
Main Product Features
IC(max)
6A
6A
1.5A
hFE(min)
@ Ic
200
2A
200
1A
25
0.5A
25
0.5A
VCE(sat) max
@ IC
50mV
1A
200mV
1A
500mV
0.5A
500mV
0.5A
Ptot (mW)
625
625
1000
1500
Table 1
Parametric Comparison.
AN 11 - 1
Application Note 11
Issue 2 October 1995
Figure 2
Practical PD vs PCB area (FR4).
No other manufacturer in the world
offers similar performance parts to the
FMMT618 and FMMT619 in SOT23. Even
in the larger SOT89 and SOT223
packages, few devices are available that
can match the current handling and low
saturation voltage of the new Zetex
range. This level of performance has
been achieved by a unique combination
of package and chip development.
The Zetex SOT23 Package
The FMMT618 SOT23 series is capable of
dissipating 625mW measured on the
industry standard 15 x 15mm ceramic
substrate.
Most other manufacturers offer power
ratings of half this level. The key factor
in obtaining such a high power rating is
minimising the thermal resistance
between the back of the chip and the
solder points.
For a standard SOT23 package, this
thermal resistance is of the order of
280°C/W. Using a custom designed lead
frame and multi-metal plating system
Zetex have been able to reduce this to
only 100°C/W. On an infinite heatsink
(say if the test substrate is clamped to a
fan cooled heatsink) the package could
actually dissipate 1.25W safely but such
facilities are rarely available.
The graph shown in Figure 2 provides a
more practical demonstration, and
shows the power rating of the device
against PCB area.The Zetex SOT23
package requires far less PCB area than
standard SOT23 for a given power
dissipation. The practical advantages of
this are increased packing densities
and/or reduced chip temperatures, thus
giving
cost
and
reliability
improvements.
AN 11 - 2
Application Note 11
Issue 2 October 1995
Zetex Matrix Chip Technology
The saturation voltage of the FMMT618
is typically only 130mV at 2.5A and hFE
remains useful up to its peak current
limit of 6A.
Two major feature enhancements have
contributed to the very impressive
performance of the FMMT618 and
FMMT619 transistors.
Firstly a custom designed SOT23 frame
has allowed an extremely large chip to
be encapsulated ie. 34 x 36 thou.
Secondly and more importantly, the
Zetex pioneered ‘Matrix’ chip design has
been used.
In a matrix transistor it is possible to
utilise virtually all the die as active area
yet minimise the distributed base
resistance, ensuring all the area works
effectively.
The low saturation voltage obtained is
both useful to the circuit designer in
maximising circuit efficiency and power
Base
Contacts
Emitter
Region
Collect
Region
Base
Region
Figure 3
The Matrix Geometry
Distributed base resistance is minimised using a
large matrix of base contact holes. By keeping the
size of these holes small, little emitter area is lost
and so active chip area is maximised.
applied to the load, whilst also
extending current ratings. For most
SOT23 packaged bipolar devices, the
maximum current capability is defined
by power rating and saturation voltage.
By optimising both of these
characteristics Zetex has produced a
range of devices that outperforms many
SOT89 and SOT223 parts.
Replacing these larger package types with
the low saturation voltage FMMT618 and
FMMT619 transistors will reduce power
dissipation, improve circuit efficiency and
reliability, while saving on PCB area and
component costs.
FMMT618 Applications
Designed with heavy duty battery
powered applications in mind, the 20V
FMMT618 handles 2.5A continuously,
giving a typical saturation voltage of
only 130mV at this current. With a
minimum hFE at 2A of 200 and at 6A of
100, this transistor is an ideal driver of
heavy loads such as motors, lamps, IR
LEDs etc. It is also very useful in
saturation voltage critical applications
such as supply switching and low
voltage DC-DC converters.
In Figure 4 an FMMT618 is being used to
drive two ZME50 infra-red LEDs in a
domestic remote control link. When a
key is pressed on the 8 x 4 keypad, the
MV500 control IC generates a sequence
of pulses which includes synchronisation and key identity data. These
16ms wide pulses are amplified by the
FMMT618 and transmitted by the ZME50
diodes.To maximise range, each
photodiode must be pulsed at 1-2A so
the combined load current for the
FMMT618 is 2-4A.
AN 11 - 3
Application Note 11
Issue 2 October 1995
220uF
ZME50
ZME50
X1=500KHz
Ceramic Resonator
MV500
FMMT
618
8x4
Key
Pad
drivers must be capable of supplying
0.5A but as they are driven from low
power logic shift registers they must
have high gain. The FMMT618
transistors work well as column drivers,
needing only 5mA of base drive to
ensure a saturation voltage below 50mV.
47
100pF
X1
1
18
2
17
3
16
4
15
5
14
6
13
7
12
8
11
9
10
100pF
+3V
10K
0.1uF
Figure 4
IR Remote Control Transmitter.
Since the remote control unit must be
kept small, its power source is just two
1.5V cells giving a 3V nominal supply. To
achieve the required LED current, all of
this supply must be applied across them
so low saturation voltage is essential.
Replacing TO126 packaged devices
commonly used for this application the
FMMT618 provides lower saturation
voltages, lower component costs and
reduced size. It also allows more of the
PCB to be automatically assembled reducing assembly costs too.
Figure 5 shows the output stage of an
LED based ‘Moving Messages’ display.
These systems use thousands of LEDs so
it would be impractical to have
individual drivers for each LED. To avoid
this, the LEDs are wired in the form of a
matrix which ideally reduces the number
of drivers required to the square root of
the number of LEDs. The penalty for this
saving is that the drivers must pass
much higher currents. The circuit
shown is just a segment of a 32 x 32
matrix display. Each is given an average
current of 15mA by pulsing it at 0.5A
with a duty cycle of 3.125%. The column
For pagers and other 5V circuits that for
size reasons must be powered from a
1.5V cell, the FMMT618 can be used as
part of an efficient up-converter. The
standard boost converter shown in
Figure 6 exploits the fast switching and
low saturation voltage of the SOT23
transistor to give up to 300mA at
efficiencies over 70%.
Real applications are likely to require
lower current but significant board area
and efficiency gains are to be made by
replacing SOT89 and SOT223 transistors
in this application.
+5V
Columns
Row
Driver
Etc
Rows
Etc
3.9
3.9
3.9
Etc
Column
Driver
820
820
Figure 5
Lamp Matrix.
AN 11 - 4
FMMT
618
820
FMMT
618
FMMT
618
0V
Application Note 11
Issue 2 October 1995
220uH
Vcc
+5V
LL5818
Sense
+1.5V
PWM
Controller
FMMT
618
Drive
100uF
Gnd
0V
Figure 6
Step-Up converter.
FMMT619 Applications
Intended for higher voltage applications,
th e 5 0V F M M T 61 9 s t ill ha nd le s a
creditable 2A continuous and 6A peak.
Giving a typical saturation voltage of just
125mV at 1A for just 10mA of base drive,
this transistor can be used to replace
inefficient Darlingtons in stepper motor
drivers and print head drivers. Also,
hybrid DC-DC switching converters
where small size and low losses are
important can significantly gain by the
use of FMMT619 transistors.
Figure 7 shows all that is necessary to use
the FMMT619 in a printer’s stepper motor
driver. Running from a switched 24V-12V
Switched Supply
24V / 12V
The resulting power loss in the device is
so low that the four drivers required can
be packed into a PCB area limited by
component size rather than power
dissipation. Stepping motors can force
reverse collector currents through their
drivers but the high reverse beta of the
FMMT619 allows this to occur safely,
without risk to the transistor or it’s
driver.
Hybrid DC-DC converter manufacturers
are continually working to provide ever
higher output power from decreasing
package sizes. The push-pull converter
shown in Figure 8 is typical of
unregulated versions of this type of
product and one that can significantly
benefit from the use of FMMT619
transistors.
+5V
4x
LL5818
Transformer details:Core : FX3311
W1 : 23T
W2 : 23T
W3 : 5T
W4 : 11T
W5 : 11T
W4 W5
+12V
W1 W2
390
FMMT
619
390
390
Input
FMMT
619
390
FMMT
619
10uF
FMMT
619
2K2
W3
-5V
Outputs
2K2
FMMT
619
0V
0V
Figure 7
Stepper Motor Driver.
.047uF
T1
FMMT
619
Figure 8
Typical hybrid DC-DC converter.
AN 11 - 5
.047uF
0V
4-Phase
Stepper
Motor
+5V
Control
Logic
supply, the 4-phase motor takes a peak
current of around 0.8A. With just 10mA
base drive from a CMOS logic buffer the
FMMT619 gives a saturation voltage of
only 100mV.
Application Note 11
Issue 2 October 1995
T a k i n g 12 V in and pro duc ing ±5V
isolated supplies, this self oscillating
converter runs at a switching frequency
of around 40kHz. The transformer is
wound on a small toroidal core and the
complete circuit built on a small PCB or
ceramic substrate that can be
encapsulated in a DIL style package.
Conceivably by employing FMMT619
switching transistors this circuit could
give a power throughput of over 10W but
output rectifier losses and the low power
dissipation capability of the small
substrates used means a more
c o n s e r v ative throu ghp ut must be
specified. By providing lower saturation
voltages and lower thermal resistance
the FMMT619 transistors give this
standard circuit improved output, load
regulation and power.
Higher power converters no longer need
use SOT89 or SOT223 types as the
ZETEX SOT23 devices offer better
performance at lower cost.
One final application area to benefit from
the very low saturation voltage of the
FMMT619 is pin drivers for EPROM and
FPLA programmers.These drivers are
complex, since to provide the necessary
versatility such programmers must be
capable of switching logic signals,
supplies and programming pulses to
24-28-40 pin sockets. Despite a small
supply current requirement, low
saturation voltage is vital. Figure 9
shows the typical saturation voltage of
an FMMT619 against collector current
demonstrating how the transistor can be
used as a supply switch, introducing
only 10-20mV of offset.
Figure 9
FMMT619 VCE(sat) vs IC.
AN 11 - 6
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