PROFET+ : Current Sense with Low Battery Voltage

Application note, Rev 0.0, June 2010
PROFET+ Current Sense with
Low Battery Voltage
PROFET +
Application note
By Stéphane Fraissé
Automotive Power
IIS at Low VBAT
Abstract
1
Abstract
Note: The following information is given as a hint for the implementation of the device only and shall not be
regarded as a description or warranty of a certain functionality, condition or quality of the device.
This application note provide information about the current sense SENSE behaviour during low battery voltage. It
shows possibilities and limitation of the PROFET + devices during phases where the battery voltage falls down
close to the undervoltage shutdown of the PROFET +.
2
Introduction
All terms and physical quantity discussed in the current application note are sumed up in Figure 1. The schematic
is simplified in regards to the real application to ease up the understanding. The real application schematic can be
found in the PROFET + data sheet in usage.
The schematic consists in a PROFET + device, supplied on its VS pin via the battery voltage called VBAT. GND pin
of the PROFET + is connected to the GND of the system via a GND network. The current consumption of the
PROFET + is called IGND. The voltage drop which is resulting of the GND network and IGND is called VGND_SHIFT.
The PROFET + switches a load L and a load current IL flows. L has a certain impedance resulting a voltage VOUT.
The PROFET + provides load diagnosis via the IS pin. The output current is called IIS. The IIS current is converted
to a voltage VIS via a RIS resistor. The voltage is read by a micro controller for system diagnosis with a A/D
converter. To protect the A/D stage of the micro controller, a resistor RA_D is used.
Two additional voltages related to the battery voltage VBAT are defined. VS, effective supply voltage of the PROFET
+ and VIS_DROP, representing the voltage between battery voltage and current sense pin IS.
The micro controller is supplied by a voltage VDD, referenced to GND.
VIS_DROP
VBAT
VS
IIS
IIS(FAULT)
VS
VDD
VDD
OUT
IA_D
RA_D
IS
A/D
VA_D
IL
GND
VIS
IIS
VOUT
IGND
GND
VGND_SHIFT
RIS
GND Network
L
SF_Circuit_equivalent.svg
Figure 1
Hardware simplified Schematic
Application note
PROFET +
2
Rev 0.0, 2010-06-30
IIS at Low VBAT
SENSE Behaviour in Nominal Operating Range
3
SENSE Behaviour in Nominal Operating Range
The SENSE provided at the IS pin is proportional to the load current IL. Figure 2 shows as an example the function
IIS = f (IL) of the BTS5020-2EKA. Because the current sense generator IIS is not perfect and is not matched to
IIS_FAULT, the maximum diagnosable load current is limited. This limitation is called SENSE dynamic. It is in the
range of 19A for the BTS5020-2EKA. At this load current, the SENSE is at least 6mA. With higher load current,
the BTS5020-2EKA cannot guarantee analog diagnosis and the information is mixed with failure (short circuit)
information. The voltage representation of the Figure 2 can be found in the Figure 3. The resistor RIS is
coresponding to the datasheet suggested value so 1.2kΩ. In such a representation, it is assumed the is not
connected to the micro controller. Therefore, no voltage limitation of the IS pin takes place. The second
assumption is the battery voltage is high enough to provide such an high voltage shown on the graphic, up to
16.8V. Figure 4 is the representation of the SENSE pin and its representation at the A/D converter, the VA_D
potential. Figure 5 becomes the software compatible representation of the load current assuming a 10bit AD.
16
I sense all max
I sense max I fault min
14
I sense typical
I sense min I fault max
Sense Current IIS (mA)
12
I sense all min
10
8
LINEAR
6 BEHAVIOR
SATURATION
BEHAVIOR OF
IIS
OF I IS
4
SWITCH TO
IIS_FAULT
2
0
0
20
#17A
Figure 2
40
60
Load Current IL (A)
80
100
SF_sense Normal .vsd
SENSE Function of the Load Current with BTS5020-2EKA
18
V sense all max
16
V sense max I fault min
SENSE Voltage VIS (V)
14
V sense typical
V sense min I fault max
12
V sense all min
10
8
6
4
2
0
0
Figure 3
20
40
60
Load Current IL (A)
80
100
SENSE Voltage Function of the Load Current with BTS5020-2EKA
Application note
PROFET +
3
Rev 0.0, 2010-06-30
IIS at Low VBAT
SENSE Behaviour in low Battery Operating Range
6
V sense all max
V sense max I fault min
5
V sense typical
V sense min I fault max
A/D Voltage VA_D (V)
V sense all min
4
3
2
1
0
0
Figure 4
5
#13A 15
10
Load Current IL (A)
20
25
SF_sense Voltage _ avec _Limitation .vsd
A/D Voltage Function of the Load Current with BTS5020-2EKA
SENSE Voltage (DIGITAL)
1024
960
V sense all max
896
V sense min I fault max
832
V sense typical
V sense min I fault max
768
V sense all min
704
640
576
512
448
384
320
256
192
128
64
0
0
5
10 #12A
15
Load Current IL (A)
20
25
SF_sense Voltage _ DIGITAL.vsd
Figure 5
A/D representation Function of the Load Current with BTS5020-2EKA
4
SENSE Behaviour in low Battery Operating Range
During low battery operation, the behaviour of the SENSE doesn’t differ so much from the Nominal Operating
Range. The limitation comes from the maximum voltage the IS pin can provide. The PROFET + datasheet defines
the parameter VIS_RANGE (parameter P_7.5.6). This voltage should be at least 3V. The voltage drop between VS
and IS pins (VIS_DROP) should be at least 3V to have a proper diagnosis. In a theoretical possible but application
useless case, with IS pin shorted to GND, the Figure 2 remains correct down to the under voltage shutdown of
the PROFET + so 3.5V typical VS.
Let’s consider for example, VBAT = 6V, resulting in a VS voltage of 5V (assuming the VGND_SHIFT = 1V). VIS cannot
the be higher than VBAT = 6V and can be down to VBAT - 3V = 3V min. With such assumption, Figure 3 becomes
much different and it shown on Figure 6. The SENSE dynamic is now 8.5A instead of the former 19A. Figure 4
becomes Figure 7 and Figure 8 show the SENSE dynamic translated to the micro controller port A/D.
Application note
PROFET +
4
Rev 0.0, 2010-06-30
IIS at Low VBAT
SENSE Behaviour in low Battery Operating Range
6
SENSE Voltage VIS (V)
5
4
3
V sense all max
V sense max I fault min
2
V sense typical
V sense min I fault max
1
V sense all min
0
0
Figure 6
#8.5A
20
40
60
Load Current IL (A)
80
100
SF _sense Voltage _ sans _Limitation_LOWVBAT .vsd
SENSE Voltage Function of the Load Current with BTS5020-2EKA
6
V sense all max
V sense max I fault min
5
V sense typical
A/D Voltage VA_D (V)
V sense min I fault max
V sense all min
4
3
2
1
0
0
5
10
15
Load Current IL (A)
20
25
SF_sense Voltage _ avec _Limitation_LOWVBAT . vsd
Figure 7
A/D Voltage Function of the Load Current with BTS5020-2EKA
1024
960
V sense all max
896
V sense typical
832
SENSEVoltage (DIGITAL)
768
V sense all min
704
640
576
512
448
384
320
256
192
128
64
0
0
Figure 8
5
# 8,5A
10
15
Load Current IL (A)
20
25
SF_sense Voltage _ DIGITAL_LOWVBAT . vsd
A/D Voltage Function of the Load Current with BTS5020-2EKA
Application note
PROFET +
5
Rev 0.0, 2010-06-30
IIS at Low VBAT
Conclusion
The SENSE dynamic is limited during low battery operation by the VIS voltage. Taking the BTS5020-2EKA as an
example, from a SENSE dynamic of 16.5A, the value can goes down to 8.5A so half the value. In other words, the
BTS5020-2EKA signals a load current higher than 8.5A the same way than a short circuit with a VBAT of 6V. The
micro controller should interpret a voltage of 3V at the A/D converter as a short circuit event if the battery voltage
is 3V.
At first sight, this value can appear low but it should be balanced to the effective load current at 6V VBAT. Table 1
sums up the SENSE dynamic of all PROFET + devices and Table 2 shows the maximum current the planned load
exhibits at 6V battery. There is no overlapping and even good safety margin.
Table 1
PROFET + SENSE Dynamic
mΩ
8
10
12
16
20
30
45
kILIS_TYP
IL_MAX SENSE dynamic
4500
4250
4000
3500
3000
2150
1500
27.4
25.8
24.3
21.3
16.6
11.9
8.3
9.8
9.2
8.7
7.6
6.9
4.9
3.4
90
120
180
1500
550
550
8.3
3.0
3.0
3.4
1.2
1.2
13.5V
IL_MAX SENSE dynamic
6V
Table 2
PROFET + Load Current
mΩ
8
10
12
16
20
30
45
90
120
180
LOAD (W)
65
55
3x27+
5
55
2x27+
5
2x21
27
21
10
5
IL_MAX in DC (no PWM)
5.3
4.5
7.3
4.5
5.0
3.9
2.3
2.0
0.8
0.4
3.5
3.0
4.9
3.0
3.3
2.6
1.5
1.3
0.5
0.3
13.5V supply
IL_MAXin DC (no PWM)
6V supply
5
Conclusion
Using PROFET + at low battery voltage is possible. The diagnosis works with limitation. The micro controller
should monitor the current battery voltage and interprets A/D information accordingly.
6
Revision History
PROFET+ Current Sense with Low Battery Voltage
Revision History: Rev 0.0, 2010-06-30
Previous Version(s):
Rev. 0.0, 2010-06-30
Page
Subjects (major changes since last revision)
Application note
PROFET +
6
Rev 0.0, 2010-06-30
Edition 2010-06-30
Published by
Infineon Technologies AG
81726 Munich, Germany
© 2010 Infineon Technologies AG
All Rights Reserved.
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