Application Note, 1.0, Jul. 2008
BTS5590G, BTS5576G and
BTS5566G
SPI Power Controller
Application Note
Short Circuit to VBB and Open Load Detection
Automotive Power
BTS5590G, BTS5576G, BTS5566G
Application Note
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 is intended to provide detailed application hints regarding the short circuit to
V
BB
detection
of the BTS5590G, BTS5576G and BTS5566G SPI Power Controllers. Furthermore, an open load in off detection
can be implemented with the use of an external resistor. General information about the SPI Power Controller can
be found in the BTS5590G, BTS5576G and BTS5566G data sheets.
2
Introduction
The BTS5590G, BTS5576G and BTS5566G are five channel high-side power switches, which are especially
designed to control standard exterior front and rear lighting in automotive applications.
Configuration and status diagnosis is done via SPI. Additionally, there is a current sense signal available for each
channel that is routed via a multiplexer to one diagnostic pin.
This document will show how to use the internal function of the switch bypass monitor (SBM) to detect whether
the output is connected via a low resistance to
V
BB
or not. For this short circuit to
V
BB
detection no external
hardware is required. For the implementation of an open load in off detection an external resistor is required.
3
Functionality of Switch Bypass Monitor (SBM)
The switch bypass monitor (SBM) is a part of the device logic, which evaluates the
V
DS
-voltage of the DMOS.
There is only one SBM comparator on the device for all five channels. Thus, there is an additional multiplexer
implemented, which connects the outputs of the channels to the SBM comparator. This multiplexer is also
programmed by the current sense multiplexer bits of the diagnosis control register. For further details please refer
to the datasheet.
In case of a short circuit to
V
BB
and
V
DS
<
V
DS(SB)
the SBM-bit within the HWCR register indicates the short circuit
to
V
BB
. This can be seen in the following two tables for off- and on-state:
Table 1
SBM and Current Sense during off-state
Current sense
Z
Z
Z
Z
Z
SBM-bit
1
1
x
0
x
Operation mode
Normal operation
Short Circuit to GND
Over Temperature
Short Circuit to
V
BB
Open Load
Table 2
SBM and Current Sense during on-state
Current sense
SBM-bit
0
x
1
x
0
0
Operation mode
Normal operation
Current Limitation
Short Circuit to GND
Over Temperature
Short Circuit to
V
BB
Open Load
I
L
/
k
ILIS
Z
Z
Z
<
I
L
/
k
ILIS
Z
Z = high impedance, potential depends on leakage currents and external circuits, x = undefined
Application Note
2
1.0, 2008-07-14
BTS5590G, BTS5576G, BTS5566G
Application Note
Functionality of Switch Bypass Monitor (SBM)
Note: During off-state the SBM-bit depends on the potential at the output pin. The potential of the output pin
depends in open load case on leakage currents. As a result, the SBM bit can be set or reset during an open
load in off-state.
3.1
Use of SBM for Short Circuit to
V
BB
Detection
In case of a short circuit between the output-pin and the
V
BB
-pin a major part of the load current will flow through
the short circuit. As a result, a lower current compared with the normal operation will flow through the DMOS of
the BTS5590G, BTS5576G and BTS5566G, which can be recognized at the current sense signal.
In case of a current sense signal below the normal value, a software strategy is required to verify, whether an over
temperature failure or a short circuit to
V
BB
leads to the low current sense signal.
The following flow chart shows a possible way to implement a software strategy:
Legend:
Process
Switch MUX to
active channel x
Decision
Measure sense
voltage V
IS
Store V
IS
Display
Yes
Switch off channel
V
IS
< V
IS nom min
No
Store data
Read SBM
Store ERR-
Flag
Yes
V
IS
> V
IS nom max
No
Switch MUX to
next channel
Yes
SBM = 1
No
SC to V
BB
or
open load
Over load
Yes
ERR-Flag = 1
(over temp.)
No
Over
temperature
channel x
Yes
V
IS
< V
IS
worst case
No
Too low load
current
Open load
Figure 1
Flow Chart for the Verification, if a Short Circuit to
V
BB
or an Over Temperature Shutdown
occurred
With this software routine it is possible to distinguish between short circuit to
V
BB
and over temperature. A complete
differentiation between short circuit to
V
BB
and open load is not possible, because during open load the output
potential depends on leakage currents. This can also cause a
V
DS
voltage below
V
DS(SB)
. Therefore, the open load
case may behave like a short circuit to
V
BB
.
Application Note
3
1.0, 2008-07-14
BTS5590G, BTS5576G, BTS5566G
Application Note
Functionality of Switch Bypass Monitor (SBM)
For the determination of
V
IS nom min
and
V
IS nom max
please refer to the following two equations.
I
nom min
-
V
IS nom min
= ------------------
⋅
R
IS min
k
ILIS max
I
nom max
-
V
IS nom max
= ------------------
⋅
R
IS max
k
ILIS min
For the determination of
V
IS worst case
please refer to the following calculation.
For example: The nominal load of channel 0 is a 21 W bulb. But it is also possible to put a 10 W or 5 W bulb into
the socket of the 21 W load. Therefore, it might be interesting to recognize, if there is an open load or only a wrong
bulb connected. The implementation of a
V
IS
-threshold with a value of 0.5 V for a sense resistor
R
IS
= 4.7 kΩ is
recommended. So, if the
V
IS
voltage is below this threshold, then an open load failure is detected, otherwise a too
low load current caused the failure.
The following calculation shows an example with typical values in bulb-mode, how the minimum sense voltage can
be calculated. For devices with LED-mode higher accuracies can be achieved by switching into LED-mode.
Nominal current of a 10 W bulb:
I
nom 10 W
= 0.74 A
Typical
k
ILIS
ratio for channel 0 in bulb-mode:
k
ILIS
= 3100
Typical sense resistor:
R
IS
= 4.7 kΩ
I
nom 10 W
0.74 A
-
-
V
IS
= --------------------
⋅
R
IS
= ----------------
⋅
4.7 kΩ = 1.1 V
3100
k
ILIS
For the worst case calculation the following formula should be used:
I
nom 10 W min
-
V
IS worst case
= ----------------------------
⋅
R
IS min
k
ILIS max
The implemented threshold should be below this value.
(4)
(3)
(1)
(2)
3.2
Use of SBM for Differentiation between Short Circuit to
V
BB
and Open Load
If a short circuit to
V
BB
should be distinguished from an open load (e.g. broken bulb) an external output pull down
resistor is required. The following figure shows the circuit with the additional output pull down resistor
R
OUT pd
:
V
bb
OUT
R
OUT pd
Load
Figure 2
External Circuit for Open Load Detection in Off-state
Application Note
4
1.0, 2008-07-14
BTS5590G, BTS5576G, BTS5566G
Application Note
Functionality of Switch Bypass Monitor (SBM)
With the following formula the output pull down resistor
R
OUT pd
can be calculated to ensure during open load
condition that device and system leakage currents do not cause the SBM-bit to transition to a logic 0.
V
BB
–
V
DS(SB)max
R
OUT pd
<
--------------------------------------------------------------
I
L(OFF)max
+
I
Leakage system
Here an example:
9 V
–
2.5 V
-
R
OUT pd
<
----------------------------
=
162.5 kΩ
40 µA
(6)
(5)
So, a pull down resistor of
R
OUT pd
= 100 kΩ could be used, if the leakage currents of the system are neglectable.
The following figure shows the flow chart for the software strategy to distinguish between an open load or short
circuit to
V
BB
failure:
Legend:
Process
Switch MUX to
active channel x
Decision
Measure sense
voltage V
IS
Store V
IS
Display
Yes
Switch off channel
V
IS
< V
IS nom min
No
Store data
Read SBM
Store ERR-
Flag
Yes
V
IS
> V
IS nom max
No
Switch MUX to
next channel
Yes
SBM = 1
No
SC to V
BB
Over load
Yes
ERR-Flag = 1
(over temp.)
No
Over
temperature
channel x
Yes
V
IS
< V
IS
worst case
No
Too low load
current
Open load
Figure 3
Flow Chart for the Verification, if a Short Circuit to
V
BB
, an Over Temperature Shutdown or an
Open Load Failure occurred
In the figure above a the differentiation between open load and a too low load current can be done by the
implementation of a
V
IS
-threshold voltage as described in the
Chapter 3.1.
Application Note
5
1.0, 2008-07-14
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