A Primer on Essentials of Output Voltage Limitations

Product Information
A Primer on Essentials of Output Voltage Limitations
Typically, overcoming the foremost roadblocks to successfully
and reliably applying power and “smart-power” ICs involves an
understanding of two critical specifications.
IC
RE
COLLECTOR CURRENT
SI
ST
V
CE(sus)
IV
E
LO
AD
V CE
LIN
OFF
E
"CUSP"
ON
V
V
COLLECTOR VOLTAGE
(BR)CBO
CC
V
CE(sat)
TIME
Dwg. GP-047
Figure 1 — Load Line with Resistive Load
V
CE(sus)
POTENTIALLY
DESTRUCTIVE
ENERGY
COLLECTOR CURRENT
The second consideration relates to the specified minimum
collector-emitter (or output) sustaining voltage for inductive loads.
Many inductive load applications involve substantial stored energy.
The output load line during turn-off resembles that of figure 2 and
this stored energy can cause instantaneous, catastrophic failure of
the device.
For safe, reliable operation with inductive loads, it is important
(even imperative) that the circuit load line not intersect the sustaining voltage curve. Without proper output voltage limiting or clamping, the turn-off voltage across the output can (will usually) far
exceed the device capability! Protection is easily accomplished by
providing:
INDUCTIVE LOAD LINE
IC
The first is the device’s specified maximum allowable output
voltage. Whether it is the maximum load supply voltage (VBB),
collector supply voltage (VCC), or output voltage (VOUT) the value
approximates V(BR)CBO. Typically, this absolute maximum rating
is guaranteed by a leakage current measurement at the specified
maximum voltage. This limit is satisfactory for resistive loads and
some capacitive loads (see figure 1). Should the output load line
cross the sustaining voltage curve, no “stored” energy is involved.
Thus, device damage is unlikely although output voltage waveshape aberrations (a “cusp”) during turn-off often occur.
(a) a diode clamp (usually called a “flyback” or “recirculating diode”) across the inductive load, or
(b) a Zener-diode clamp to supply or ground, or
(BR)CBO
V
CC
COLLECTOR VOLTAGE
V
V
CE(sat)
(c) an RC snubber network.
Dwg. GP-047-1
Figure 2 — Load Line with Inductive Load
29501-1-AN, Rev. C
For rapid current decay (fast turn-off speeds), using Zener diodes
will raise the flyback voltage during turn-off and improve performance. However, the transient/momentary/peak/spike voltage
must not exceed the specified minimum sustaining voltage.
V
CC
VZ
INDUCTIVE
LOAD
Further, for reliable operation, the peak current ratings of both
the flyback and Zener diodes must be at least as high as the load
current. Note – surge power ratings of Zener diodes are typically
greater than 10 times the continuous rating in repetitive, low-duty
cycle (5% to 10%) applications.
VF
IC
Dwg. EP-058
OUTPUT
VOLTAGE
VCC + VZ + VF
V CC + VF
V CC
Power supply and circuit values are chosen such that VCC
(supply) + VF (diode forward voltage) + VZ (Zener voltage, if used)
b VCE(sus).
The output voltage ratings of popular Allegro power drivers
follows. Some of these sustaining voltage ratings (bold faced) do
not presently appear in Allegro data sheets. These specifications
will be added at the next revisions. Additional assistance pertaining to sustaining voltage issues can be provided by Allegro Applications Engineering.
V CE(SAT)
OUTPUT
CURRENT
I OUT
ZENER CLAMP
DIODE CLAMP
I CEX
Dwg. WP-001
Figure 3 — "Protected" Inductive Load Driver
Part Number(s)
Max. VBB, VCC,
VOUT, or
VCEX
Min. VOUT(SUS),
VCE(SUS), or
VCE(sus)
Internal
clamp
diodes
2003 and 2004
2023 and 2024
2065
2068
2069
50
95
80
50
80
35
50
50
35
50
yes
yes
yes
yes
yes
2540
2543
2547
2549 and 2559
2557
50
25*
25*
25*
32
50
35
40
40
40
yes
yes
no
yes
yes
— cont'd —
29501-1-AN, Rev. C
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
Part Number
Max. VBB, VCC,
VOUT, or
VCEX
Min. VOUT(SUS),
VCE(SUS), or
VCE(sus)
Internal
clamp
diodes
2580
2585
2588
2596 and 2597
2803 and 2804
50
25
80
50
50
35
15
50
35
35
yes
yes
yes
yes
yes
2823 and 2824
2878
2879 and 2879-2
2916, 2917, 2918, and 2919
2936-120
95
50
80
45
45
50
35
50
45
45
yes
yes
yes
yes
yes
2944
2961
2962
2981 and 2982
2983 and 2984
60
45
45
50
80
35
45
45
35
45
yes
1 of 2 req’d
yes
yes
yes
2985
2987
2998
3951, 3952, and 3953
3955 and 3957
30
35
50
50
50
15
35
50
50
50
yes
yes
yes
yes
yes
33
30
25*
50
50
33
30
25
~35
35
yes
yes
yes
yes
yes
3964
3966 and 3968
5140 Hall
5800 and 5801
5804
5821
5822
5841
5842
5890
50
80
50
80
80
~35 (use 5841 for inductive loads)
~50 (use 5842 for inductive loads)
35
50
50
no
no
yes
yes
yes
5891
5895
6219
8902
50
50
45
14
35
35
45
14
yes
yes
yes
yes
* Maximum allowable supply voltage is specified higher. Listed here is maximum useful
operating voltage defined by an internal overvoltage shutdown or current limiting.
29501-1-AN, Rev. C
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
3
Copyright ©1994-2013, Allegro MicroSystems, LLC
The information contained in this document does not constitute any representation, warranty, assurance, guaranty, or inducement by Allegro to the
customer with respect to the subject matter of this document. The information being provided does not guarantee that a process based on this information will be reliable, or that Allegro has explored all of the possible failure modes. It is the customer’s responsibility to do sufficient qualification
testing of the final product to insure that it is reliable and meets all design requirements.
For the latest version of this document, visit our website:
www.allegromicro.com
29501-1-AN, Rev. C
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
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