Download AN-9758 - Open-Load Detection in Smart Switches

www.fairchildsemi.com
AN-9758
Open-Load Detection in Smart Switches
1. Introduction
The new family of smart high-side MOSFETs consists of
five-terminal and seven-terminal high-side devices.
Fairchild’s smart high-side switches are PowerTrench®
MOSFETs with integrated protection circuitry. This family
features efficient power MOSFETs with active clamp and
integrated protection functions, including: over-temperature,
current limitation, over-current and reverse battery.
The smart high-side switches include protection functions
such as under-voltage, over-voltage, short-circuit, and overtemperature shutdown, as shown in Figure 1. Some external
components are needed to operate a device as shown in
Figure 2.
This family features a control input and a diagnostic pin. An
internal charge pump circuit allows the MOSFET to be
driven in a high-side configuration without additional
external components.
This application note explains the features of the high-side
family, helps the designer select components, and provides
suggestions on how to detect open-load in both OFF and
ON state in automotive systems.
The open-load detection circuit can be implemented with
external resistors. The purpose of open-load detection is to
provide the vehicle driver an alert about the state of the
system. For example, if headlamps were to fail at night or
during a foggy day; it creates a dangerous situation. The
open-load detection functions during OFF state and issues a
warning when headlamps fail.
Figure 1. Block Diagram of Smart High-Side Switches
Figure 2. Typical Application Circuit for Open-Load Detection
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/19/12
www.fairchildsemi.com
AN-9758
APPLICATION NOTE
2. Possible Solution
Two types of open-load detection circuits are discussed:
during OFF state and during ON state.
2.1. Open-Load Detection during OFF State
The first method for open-load detection is to use resistors
to detect a voltage showing up at external resistor during
OFF state. There are cases where open-load detection is
requested also when the load is not activated. In this case,
the micro-controller is able to detect the open-load as soon
as it occurs during OFF state.
Figure 4. Open Load Detection During OFF State
Current flows into R1, R2, and R3.The output is connected
to R2 and R3 and a certain voltage is shown at output. The
voltage of Diag1 is almost zero.
Therefore, the open-load condition equation for applying
voltage to Diag2 pin can be approximated as:
Rtotal 1  R2  R3
Vload  (Bat  R1 ) /(R1  Rtotal 1 )
The smart high-side switches can detect normal condition
and open-load condition by the voltage of Diag1 and Diag2.
However, an external passive resistor; like R1, R2, and R3;
is needed to detect open-load state. To reduce leakage
current during OFF state, high-value resistors are used.
Assume the application circuit is applied to the conditions:



In normal condition, the load is connected to GND and
extremely low current (beside the output leakage) flows into
the load. The voltage of Diag1 and Diag2 is almost zero.
The normal condition equation for applying voltage to
Diag2 pin can be approximated as:
Vload  (Bat  Rtotal 2 ) /(R1  Rtotal 2 )
FDDS10H04A_F085A
Rload is OPEN
VBAT is 12 V.
In this case, voltage shown at R3 can be expressed as:
Rtotal1  30k  10k  40k
Vload  (12V  40k) /(100k  40k)  3.43V
(4)
VR 3  (Vload  10k) /(10k  30k)  0.86V
Rtotal 1  R2  R3
Rtotal 2  (Rtotal 1  Rload ) / Rtotal 1  Rload )
(3)
VR 3  (Vload  R3 ) / Rtotal 1
Figure 3. Normal Operating during OFF State
The result of open-load condition during OFF state is that
VDIAG1 is almost zero and VDIAG2 is 0.86 VBAT 12 VBAT.
(1)
VR 3  (Vload  R3 ) / Rtotal 1
Assume the application circuit is applied to the conditions:



FDDS10H04A_F085A
Rload is 6 Ω
VBAT is 12 V.
In this case, voltage at R3 can be expressed as:
Rtotal 1  30k  10k  40k
Rtotal 2  ( 40k  6 ) / 40k  6 )  5.98
Vload  (12V  5.98 ) /(100k  6 )  3mV
(2)
VR 3  (Vload  10k ) /(10k  30k )  1mV
Figure 5. Normal and Open-Load during OFF State
As the result; in normal condition, VDIAG1 and VDIAG2 are
almost zero.
If the load is disconnected during OFF state, an external
resistor pulls up the output so that the open-load condition is
detected via VDAIG2 and VDIAG1.
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/19/12
www.fairchildsemi.com
2
AN-9758
APPLICATION NOTE
2.2. Open-Load Detection during ON State
The second method for open-load detection is to use the
resistor network and check the voltage levels at Diag1 and
Diag2 during ON state.
Figure 8. Normal Operating during ON State
The smart high-side switches can detect normal condition
and open-load condition with voltage of Diag1 and Diag2.
However, an external passive resistor; like R1, R2, and R3;
is needed to detect open-load state.
Figure 6. Timing Diagram During OFF State
Even if the MOSFET is turned off, a certain voltage appears
at Diag2 in the case of an open-load, as described timing
diagram in Figure 6.
In normal condition, VOUTPUT is almost the same as VBAT.
The voltage of Diag1 and Diag2 is not zero.
The equation for applying voltage to the Diag2 pin can be
approximated as:
Figure 7 provides a flow chart for a possible method to
detect with a software strategy. With this software routine, it
is possible to distinguish between normal condition and
open-load condition during OFF state. If VBAT is taken into
consideration, a VDIAG2 range from 0.4 V to 1.9 V detects
open-load. VDIAG2 is 0.64 V at 9 VBAT and 1.14 V at 16 VBAT
in case of open-load state.
Rtotal 1  R2  R3
Vload  (VBAT  VD S )
(5)
VR 3  (Vload  R3 ) / Rtotal 1
where VDS is the voltage drop between drain and source.
Assume the application circuit is applied to the conditions:
 FDDS10H04A_F085A
 Rload is 6 Ω
 VBAT is 12 V.
In this case, the voltage at R3 can be expressed as:
R total 1  30k  10k  40k
Vload  (12V  Max. 0.065V )  11.9V
VR 3  (Vload  10k ) /(10k  30k )  2.98V
(6)
VIS  (12V / 6 ) / 10000  1k  0.2V
VR 3  (Vload  10k )(10k  30k )  2.98V
As the result, VDIAG1 is 0.2 V and VDIAG2 is 2.98 V under
normal condition.
If the load is disconnected during ON state, an external
resistor pulls up the output so the open-load condition is
detected by VDIAG2 and VDIAG1.
Figure 7. Open Load Detection During Off State
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/19/12
www.fairchildsemi.com
3
AN-9758
APPLICATION NOTE
Figure 9. Open-Load Detection during ON State
Output current flows into R2 and R3. The output is
connected to R2 and R3 and battery voltage is shown at
output. The voltage of Diag1 is almost zero.
The open-load condition equation for applying voltage to
the Diag2 pin can be approximated as:
Rtotal 1  R2  R3
Vload  (VBAT  VD S )
(7)
Figure 11. Timing Diagram during ON State
VR 3  (Vload  R3 ) / Rtotal 1
Even if the MOSFET is turned on, a certain voltage is not
reached at Diag1 in open-load condition, as shown in
Figure 11.
where VDS is the voltage drop between drain and source.
Assume the application circuit is applied to the conditions:



Figure 12 provides a flow chart for a possible method to
detect with a software strategy.
FDDS10H04A_F085A
Rload is 6 Ω
VBAT is 12 V.
In this case, voltage at R3 can be expressed as:
Rtotal 1  30k  10k  40k
Vload  (12B  Max. 0.065V ) ~ 11.9V )
VR 3  (Vload  10k ) /(10k  30k )  2.98V )
(8)
VIS ~ 0V
VR 3  (Vload  10k ) /(10k  30k )  2.98V )
As the result of those equations, VDIAG1 is 0 V and VDIAG2 is
2.98 V in open-load condition.
Figure 12. Open-Load Detection during ON State
With this software routine, it is possible to distinguish
between normal condition and open-load condition during
ON state. If VBAT is taken into consideration, the VDIAG2
range is from 2.2 V to 5V to detect open-load. VDIAG2 is
2.25 V at 9 VBAT and 4 V at 16 VBAT in open-load condition.
Figure 10. Normal and Open-Load during ON State
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/19/12
www.fairchildsemi.com
4
AN-9758
APPLICATION NOTE
3. Conclusion
In this application note, a method for the detection of openload conditions for smart high-side switches during ON and
OFF state was explained and a software strategy proposed.
The measured leakage currents during OFF state are shown
in Table 1.
Table 1. Total Leakage Current of MOSFET and
External Component (12 V Condition)
Total
External
R1
R2 R3
MOSFET
Leakage
Load
Components (kΩ) (kΩ) (kΩ)
Leakage
Current
Normal in
OFF State
100
30
10
6Ω
0.8 µA
120 µA
Open-Load in
100
OFF State
30
10
Open
0.8 µA
86 µA
Figure 13. Logic Diagram of Open Load State
A software strategy is required to distinguish between ON
state of open-load and OFF state of open-load. The best
solution for open-load detection is determined by the
requirements of the application.
For better understanding of the open-load detection process,
refer to logic diagram in Figure 13.
References
[1] AN-8039 — Using the FDDS100H06_F085 in Automotive Systems
[2] FDDS10H04A_F085A and FDBS09H04A_F085A Datasheets
Related Resources
FDDS10H04A — Smart High-Side Switch
FDBS09H04A — Smart, Protected, High-Side Switch
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS
HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE
APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS
PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION.
As used herein:
1.
Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, or (c) whose failure to perform
when properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to
result in significant injury to the user.
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/19/12
2.
A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
www.fairchildsemi.com
5