Application Note 15
Issue 1 November 1995
Features and Applications of the FMMT617 and
FMMT717 “SuperSOT” SOT23 Transistors
3A NPN and 2.5A PNP SOT23 Bipolar Devices
David Bradbury
Neil Chadderton
Introduction
The following note describes some of
the features, benefits and applications of
the FMMT617 NPN and FMMT717 PNP
SOT23 transistors developed by Zetex.
These devices extend the FMMT620 and
FMMT720 high performance surface
mount bipolar ranges. Specially
optimised for the stringent requirements
of battery powered systems, these tiny
SOT23 packaged devices will replace
muc h lar ger bipolar and MOS FET
trans is tors, leadin g to s i gnificant
savings in component costs and PCB
sizes. Indeed the FMMT617 and
Parameter
Polarity
BV
CEO
I
C continuous
I
C maximum
Mid-band h
FE
Typical h
FE
@I
C
Typical V
ce(sat)
@I
C
Ptot
FMMT617
NPN
15
3
12
450
320
3
150
3
625
FMMT717 transistors outperform many
SOT223 and SOT89 types plus all SOT23
trans istors presently available
world-wide in terms of current handling
and low losses.
Features
As can be seen in Table 1, the FMMT617
is a 15V NPN transistor capable of
switching loads of up to 3A continuous,
12A peak. Designed to give a high
mid-band gain of 450, the matrix chip
geometry ensures this level of
FMMT717
PNP
12
2.5
10
450
275
2.5
160
2.5
625
A
mV
A
mW
V
A
A
Units
Table 1
Parametric Overview.
AN 15 - 1
Application Note 15
Issue 1 November 1995
The FMMT717, though not quite as good
as it’s NPN counterpart, still gives
excellent performance. It is a 12V PNP
device rated at a collector current of 2.5A
continuous, 10A peak. Also designed to
give a mid-band gain of 450, the h
FE
is still
around 275 at 2.5A I
c
. The saturation
voltage of this part is a low 160mV at 2.5A.
The key features vital in battery powered
equipment are low V
CE(sat)
and high h
FE
.
Both these parameters are important
determinants of losses and hence
battery life. Consider the simple motor
driver circuit using the FMMT617 shown
in Figure 1.
Conduction losses and the much lower
base current losses occurring in the
FMMT617 are charted in Figure 2.
Conduction losses are given by V
CE(sat)
x
I
c
and base current losses by I
b
x V
be.
Load
Battery
Controller
Rb
FMMT
617
Figure 1
Generic Load Driver For Text Discussion.
performance is maintained well above
the 3A rating. For example, it’s typical
h
FE
at 5A is still around 240 and even at
12A the h
FE
is a creditable 80, thus
allowing the devices to handle very high
current pulses, and tolerate switching
transients. Giving a saturation voltage of
only 150mV at 3A for a forced gain of 60,
the FMMT617 is a highly efficient switch.
600
Mosfet L
osses
Power Losses (mW)
500
400
300
200
100
0
0
BC
81
8)
Lo
ss
es
(
Typical S
OT23
To
tal
5t
h
Ge
n
SO
T2
3
)
17
T6
M
(FM
s
es
sse
ss
Lo
o
lL
tion
ta
duc
To
Con
e Losses
Total Driv
0.5
1
MO
SF
ET
Base Losses
lo
ss
es
1.5
2
Load Current (A)
Figure 2
Graph of Power Losses (mW) vs Load Current (A).
AN 15 - 2
Application Note 15
Issue 1 November 1995
However, the necessary base current
must be taken from the 4.8V supply,
incurring additional base drive losses in
the driver circuit, raising the driver
losses to a level comparable with
conduction losses. To even approach
such low conduction losses, competitive
bipolar types would require far more
base drive than the FMMT617 and hence
cause base drive losses many times
higher. To demonstrate this, the total
losses of a FMMT617 and a competitive
dri ver transistor (BC 818 ) a re also
charted in Figure 2.
Using MOSFETs would eliminate the
drive power losses, but as can be seen
by reviewing the performance of the
best MOSFET presently available in
SOT23 (see Figure 2), the on-resistance
makes conduction losses very much
higher than the total losses obtained
using the FMMT617. Furthermore,
battery supplies rarely provide sufficient
gate-drive voltage for the MOSFETs to
obtain their minimum on-resistance
values, a factor that rapidly gets worse
as the batteries reach their end of life
voltage.
The low losses obtained using the
FMMT617 in this motor driver circuit can
only be matched by competitive bipolar
or MOSFET devices in much larger
packages, e.g. TO220, D-PAK, the best
performance examples of SOT223 or
SO8 etc.
These alternatives are considerably
more expensive, eat up far more PCB
area, are more difficult to mount and
often unable to match the reliability of
SOT23 packaged devices. The motor
d r i v e r c i r c u i t d e m o n st r a t e d i s a n
example of the kind of high current,
efficiency sensitive applications for
which the FMMT617 and FMMT717 are
particularly advantageous. Following
are further application examples where
these devices are ideally suited.
Motor Drivers
Providing bi-directional motor drivers
for battery powered equipment requires
either half-bridge controllers with
centre-tapped battery packs or a full
‘H ’-br idge c ir c uits w i th untapped
batteries. Half-bridge controllers are
simple and can be very cost-effective, but
for a given motor power their drivers must
pass twice the current of equivalent
‘H’-bridge circuits. Also, as the operating
voltage of the motor is only half of the total
supply, the saturation voltage of the driver
transistors must be kept to a minimum to
maintain efficiency and battery life. The
high current capability and low saturation
voltage of the FMMT617 and FMMT717
ma k e t h e s e t r a n si s to rs i d e al for
half-bridge controllers.
The circuit shown in Figure 3 is suitable
for motors with peak currents up to 2.5A,
giving saturation losses of only 90mV at
1A and 250mV at 2.5A. Using only 25mA
base drive for the PNP and 15mA for the
NPN, (current levels easily supplied by
servo controllers and many logic ICs
without the need for buffer stages), base
drive losses are even smaller than
conduction losses. Total conduction and
driver losses are only a fraction of those
ob tained us ing indus try s tandar d
transistors such as the BC818 and
BC808, ensuring most of the energy
taken from the battery is supplied to the
load. High reverse gain eliminates the
need for catch diodes in this circuit to
protect the drivers from their inductive
load, thus further reducing component
counts. This half-bridge motor driver
AN 15 - 3
Application Note 15
Issue 1 November 1995
utilising the FMMT617 and FMMT717
gives useful savings in component count
a n d c o s t s w i t h o u t c o m p r o m i s i ng
eff i c i e nc y o r b a t te r y l i fe . T y pic a l
applications are in positional systems,
linear motor drives, servo and actuator
drivers and toys.
In applications where the motor is driven
predominately in one direction, it is
possible for half-bridge circuits to
discharge it’s two battery sections
unequally, wasting capacity and money
when only partly discharged batteries are
discarded. For these situations, a full
bridge circuit is preferable. Although full
bridge circuits halve the motor current
required for a given power, two driver
transistors are in the motor current path so
low saturation voltage is still very
important. Consequently, the FMMT617
and FMMT717 are equally applicable to
this circuit topology too. Figure 4 shows a
full bridge circuit using the FMMT617 and
FMMT717 that will drive a 2.5A peak motor
bi-directionally with exceptionally low
losses. Note again that the buffers and
catch diodes required by some competitive
solutions are not required with this driver
circuit, enhancing it’s cost-effectiveness.
+4.8V
150
150
Controller
DC MOTOR
FMMT
FMMT
617
617
0V
FMMT
FMMT
717
2.5A
717
270
270
Figure 4
’H’ bridge Motor Driver.
Active Power Switches
(Including Mobile
Communications and PCMCIA)
To maximise the endurance of complex
battery powered products such as
Mobile telecoms, Pagers, Laptop and
Notebook computers etc., it is frequently
necessary to switch the power supply of
intermittently used circuit sections. To
achieve low losses and ensure
compliance with standard IC power
supply specifications it is vital that these
switches give very low voltage drops.
For instance, 5% tolerance on a 3.3V
supply equates to only 165mV. However,
load currents can be high, for example
the supply current demands of mobile
phone RF output stages can peak at 2A,
PCMCIA memory cards and hard disk
drive modules can demand 300mA to
1A. To make the designer’s life even
harder, circuit constraints usually dictate
th a t t h e + v e s u p pl y r a i l m u s t b e
switched, making it difficult to use NPN
bipolar transistors or N-channel
MOSFETs which perform better than
their P type counterparts. Meeting these
stringent requirements usually
nec essitate s the use of la rge and
ex pens ive P-channe l MOSF ETs or
switch ICs.
150
Controller
FMMT
717
2.5A
DC MOTOR
270
FMMT
617
Figure 3
Half-bridge Motor Driver.
AN 15 - 4
Application Note 15
Issue 1 November 1995
The PCMCIA interface standard in
particular adds to the cost and
complexity as not only are it’s supplies
switched, they can also be set to several
voltage levels , caus ing isolation
problems that double the number of
active switches needed.
C o n si d e r ab le s a v in gs in cost and
component count can be made by
s u b s ti t u t i n g
FMMT717 bipolar
transistors for the alternative solutions
currently used as supply switches. The
low saturation voltage of the FMMT717
makes it an excellent low loss switch.
Figures 5 and 6 show two PCMCIA
interface supply switches. The PCMCIA
i n t e r f a c e s t a n d a r d p r o v i d e s tw o
switched outputs, V
cc
a n d V
dd
. Th e
supply V
cc
must be switchable between
3.3V and 5V, and V
dd
between 0V, 5V
and 12V. The V
cc
switch in Figure 5 is
capable of sourcing 1A continuous and
will supply peaks of over 3A. Controlled
simply via logic drive signals, the typical
voltage drop of either the 3.3V or 5V
switch at 1A is around only 80mV. To
achieve comparable performance using
P-cha nnel MOSFETs sw itch would
require chips so large they would only fit
in SO8, D-PAK, TO220 etc. and cost many
times more than the SOT23 FMMT717
bipolar transistors. Also, when
MOSFETs are used as the active switch
element, two devices, one reversed and
wired in series, must be used for the 3.3V
switch to isolate the 3.3V input supply
from the circuit’s output when 5V out is
selected. This is because the MOSFET’s
body diode conducts if the device
becomes reverse biased, so a second
device, (reverse wired) is required to
block this effect. Not only does the use
of MOSFETs for the 3.3V switch double
its cost, it also doubles on-resistance
and hence on-voltage drop. The
FMMT717 can safely block a reverse
voltage of 1.7V (5V-3.3V) hence no
special measures are necessary.
The V
dd
switch has been designed to
+5V In
FMMT
717
FMMT
717
1M
100
1M
+5V/
+3.3V Out
220
+3.3V In
5V Sel.
3.3V Sel.
0V
15K
8K2
BC
846B
BC
846B
0V Out
Figure 5
Typical Positive Line Switch - 1.
FMMT
717
FMMT
717
1M
82
12V Sel.
5V Sel.
0V
0V Out
39K
15K
1M
+12V/
+5V Out
+12V In
+5V In
1K
BC
846B
BC
846B
Figure 6
Typical positive line switch - 2.
AN 15 - 5
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