Title: Bridging Theory in Practice
1Bridging Theory in Practice
- Transferring Technical Knowledge
- to Practical Applications
2Protected Low Side Drivers
3Protected Low Side Drivers
4Protected Low Side Drivers
- Intended Audience
- Electrical engineers with a knowledge of simple
electrical circuits - An understanding of MOSFETs and low side drivers
is assumed - Topics Covered
- What is a Protected Low Side Driver?
- What type of protection does a HITFT have?
- What type of diagnostics does a HITFET have?
- How does a HITFET impact system EMI?
- How is a HITFET circuit implemented?
- HITFET Selection Questions
- Expected Time
- Approximately 90 Minutes
5Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
6Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
7MOSFET Review
MOSFET ? Metal Oxide Semiconductor Field Effect
Transistor
D
G
VGS
S
VGS
S
S
G
G
P-Channel MOSFET (Enhancement)
(
Enhancement)
MOSFET
D
D
N-Channel
8MOSFET Regions of Operation
- A positive (for N-Channel) or negative (for
P-Channel) VGS produces a conducting channel
between the Drain and Source - The MOSFET is then able to operate in two
regions - 1) Linear region The MOSFET behaves like a
resistance. - 2) Saturation region The MOSFET behaves like a
current source.
VDS VGS-VT
VGS gt 0V N-Channel MOSFET (NMOS)
IDS
VDS
9Low Side Drive (LSD)Configuration
The switch is on the low side of the load
14V
Load
MOSFET Switch
Drain
Vgate _at_ 5V to 10V Vgs Vg - Vs Vgs 5V to 10V
Drain voltage is small ex. 0.1volt
Gate
Source Vs 0V
10HITFET High Integration Temperature protected
FET
Short Circuit Protection
HITFET
11High Integration Temperature protected FET
12Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
13Rugged vs. Protected
- Rugged
- MOSFETs
- Achieved through process manufacturing
technology - Protection Not Built in
- Protected
- HITFETs
- Achieved through design and utilization of more
advanced integrated circuit technologies - Available CMOS, DMOS and Bipolar devices allow
for the integration of ESD protection, active
clamping, current limit, temperature sensing,
etc. - Protection Built in
14HITFET Protection Features
- Electrostatic Discharge (ESD) Protection
- Load Dump Tolerant
- Inductive and Over voltage Output Clamp
Protection - Current Limit Protection
- Thermal Shutdown Protection
15Block Diagram Including Protection Features
16ESD Protection
- Maximum Ratings at Tj 25ºC, Unless Otherwise
Specified
Electrostatic discharge voltage (Human Body Model) VESD 2 KV
17Load Dump Protection
Drive (Vin) Param Voltage Vp Vbatt Vs Pulse param Pulse Type
Load dump protection Vin low or high (8V) Vld 80 47 13.5 Vload dump Vp Vs Exponential DC offset
Vld Voltage load dump
18Inductive And Over Voltage Clamp
Inductive and Over Voltage Output Clamp
Over voltage condition usually occurs in the
presence of an inductive switching action
19Thermal Shutdown Protection
20Current Limit Protection
Paramater and Conditions at Tj25, Vbb12V unless otherwise specified Symbol Values Values Values Unit
Paramater and Conditions at Tj25, Vbb12V unless otherwise specified Symbol min typ max
Current Limit Vin 10V, Vds 12V IDlim 1 1.5 1.9 A
21Thermal Shutdown Latch Behavior
22Thermal Shutdown Latch Behavior
Input Vin is driven high
Input current reflects latch current consumption
Current is switched off when latch engages
Silicon temperature drops when current is
switched off
23Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
24HITFET Diagnostic Feedback
-
- HITFET does not contain internal structures that
Are specifically intended for diagnostic feedback - Diagnostic feedback can be obtained by using
external Components in conjunction with - Drain high current output pin true conductive
state of device can be estimated by monitoring
the drain voltage - Iin input drive bias can be detected and
evaluated against over current or thermal
shutdown bias current this method requires that
input current be evaluated. - NOTE Care must be taken so as not to
significantly reduce - the available Vin voltage
-
- Drain high current output pin true conductive
state of device can be - estimated by monitoring the drain voltage
- Vin input drive bias can be detected and
evaluated against over current ,or thermal
shutdown bias current this method requires that
input current be evaluated. - NOTE Care must be taken so as not to
significantly reduce
25HITFET Diagnostic Feedback Drain Sense
- Diagnostic feedback is sensed at the drain and
applied to a micro processor Analog to Digital
Input - Advantage
- Low cost diagnostic and fault detection
- Disadvantage
- Slower than integrated solution
- Uses one A/D channel per sensed
- drain
26HITFET Diagnostic Feedback Iin current monitor
27Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
28PWM Definitions
- Frequency (frequency domain) What is the rate
of repetition of a wave form? - Duty cycle (Time domain) What amount time is
spent on with respect to what amount of time is
spent off?
29HITFET Block Diagram
30HITFET Turn-On/Turn-Off Slew Rate Controlled
- Turn on / Turn off rate control.
- Slew rate is controlled
Dynamic characteristics Param min nom max units
Turn on time Vin to 90ID Rl 22 O, Vin 0 to 10V, Vbb12V Ton -- 10 10 µs
Turn-off time Vin to 10 ID Rl 22 O, Vin 0 to 10V, Vbb12V Toff -- 10 20 µs
Slew rate on 70 to 50 Vbb Rl 22 O, Vin 0 to 10V, Vbb12V -dvds/dton -- 4 10 µs
Slew rate off 70 to 50 Vbb Rl 22 O, Vin 0 to 10V, Vbb12V dvds/dtoff -- 4 10 µs
31HITFET Turn-on/turn-off Slew Rate Controlled
32Typical HITFET Radiated Emissions Evaluation
33Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
34Reverse Battery Tolerance
- Reverse load current through the intrinsic drain
diode in series with the - load.
- Power dissipation is Higher compared to normal
operating - conditions due to the voltage drop across the
drain to source diode - Source diode current is limited by the load
35Reverse Battery ToleranceNormal Operation
- Recall
- TJ TAmbient PD RTHJA V IR
- PD ILOAD2RDS(on)
- Therefore
- ILOAD ((TJ TAmbient) / (RDS(on) RTHJA )1/2
- Given
- TJMAX 150 C
- TAmbient 95 C
- RDS(on) 0.068 O
- Rthja 55 C/W
- VBattery 14 V
- Results
- ILOAD_MAX 3.8A and RLOAD_MIN 3.6 O
36Reverse Battery ToleranceBody Diode
- Recall
- PD VDiode ILOAD
- Therefore
- ILOAD ((TJ TAmbient) / (RTHJA VDiode)
- Given
- TJMAX 150 C
- TAmbient 95 C
- VDiode 0.7 V
- Rthja 55 C/W
- VBattery 14 V
- Results
- ILOAD_MAX 1.4 A and RLOAD_MIN 9.5 O!!
- Compared to 3.8A and 3.6 O when used in normal
operation!!
37High Side Drive (HSD) Configuration
The switch is on the HIGH side of the load
14V
MOSFET Switch
If the MOSFET gate is pulled to a higher voltage
Load
38Source Follower
- Advantage
- Custom edge control (EMC)
- Disadvantage
- -- Complexity
- -- Profet may be better
39HITFET Edge Shaping
- Edge rise and fall time can only be increased by
the addition of external components - Slew rate can not be made faster by the addition
of external components - Potentially can modify EMC characteristics
(Electro Magnetic emissions) - Allows for symmetrical or asymmetrical adjustment
to rise and fall times as well as slew rate
modification - Additional power is consumed by changing
transition times (operation in linear region)
40HITFET Edge ShapingSimple low pass filter
- Advantages
- Simple in terms of calculating RC values
- Is effective at controlling rise and fall
time of the device - Disadvantage
- Adding a low pass filter to the input
- Will insert a turn on delay and a turn off
delay (dead time) which may modify the intent of
PWM applications
41HITFET Edge Shaping Simple RC
- External components can be added to a HITFET to
modify rise and fall time and slew rate.
HITFET
Simple low pass filter
42HITFET Edge Shaping Simple Low Pass Filter
Input Voltage
Drain Voltage
43HITFET Edge Shaping Miller Capacitor
- Advantages
- Is effective at controlling rise and fall time
of the device - Does not insert a significant turn on or turn off
delay - Disadvantages
- Calculation or the RC components is more
complicated - Must now consider the resistance of the load
44HITFET Edge Shaping Miller Capacitor
HITFET
Drain feedback (Miller) capacitor method
45HITFET Edge Shaping Miller Capacitor
Input Voltage
Drain Voltage
46HITFET Edge Shaping
- Edge shaping can only be used to increase the
rise and fall time of the respective edge - Slowing the edge rates will result in additional
heat being dissipated in the part - In both simple and miller edge shaping
approaches, the series limiting resistor must be
sized to allow proper bias of the over current
protection functions
47HITFET Input protection
- Input protection usually not needed if driven
directly from a micro processor - Input protection is needed in cases were drive is
sourced from a non regulated or out boarded
signal source
Test condition Parameter Limit Unit
Continuous input current -0.2V ? VIN ? 10V VIN lt-0.2V or VIN gt10V IIN self limited IIN ? 2 ma
48HITFET Input protection
- The HITFET input drive circuitry must provide
adequate voltage to the gate (4.5V or more) and
must not exceed the maximum allowable input
voltage (typically 10V). - The maximum specified current allowed to sink or
source from the HITFET in pin is 2.0 mA. Current
up to 2mA may be required to operate internal
HITFET input protection circuitry.
49HITFET 5 volt versus 10 volt operation
- HITFETs comply to a specification which uses a
nominal 5 Vin drive voltage as a specified
operating point. - Further device enhancement lower Rdson and
higher output current may be achieved by
operating the device at a higher Vin voltage
(10V).
50HITFET 5V vs. 10V operation
Additional Vin
51Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
52Frequently Asked Questions
- What is the load current?
- Is the load capacitive and what is the inrush
current? - Is the load inductive and the inductance and/or
energy during turn-off? - Will load be on/off or PWM? What is PWM
frequency (load states) ? - What is ambient temperature?
- Can a HITFET be operated as a high side switch?
53Frequently Asked Questions
- What happens if ground (drain leg) opens?
- What type of package - surface mount or
through-hole? - If surface mount, how much copper area for Vbb /
tab connection? - How is inductive energy evaluated and controlled
by the HITFET? - If through-hole, what type of heat sink will be
provided for package? - What diagnostics are needed?
- What application extremes will the device /
system be subjected to (reverse battery, load
dump, over voltage etc.)?
54What Is the Load Current?
- What is the maximum load current?
- When does the maximum occur?
- What is the typical load current?
- Alternative Question What is the load
resistance? - Alternative Question If the load is a lamp,
what is its wattage? - Recall, the load current is fundamental in
determining the Rdson value
55Is the Load Capacitive?What Is the In-rush
Current?
- Recall, the in rush current for lamps and RC
networks may be an order of magnitude higher than
the steady state current
56Is the Load Inductive? Inductance and/or Energy
During Turn-Off?
- MOSFETs are rated for the max absorbable energy
when turning off inductive loads
The equations relate the energy absorption
Capability regarding a Single pulse
Where L load inductance Ipk short
circuit load current Vcl over voltage clip
voltage of HITFET Vbat supply voltage
57What Is the Ambient Temperature?
- Minimum automotive ambient temperatures is
usually -40C - Maximum ambient temperature ranges from 85C to
125C for most applications - 85C for most non - power train applications
- 105C for some in - dashboard applications
- 125C for most power train applications
58What Type of Package?Surface Mount or
Through-hole?
- Many applications require all surface mount
components - Surface mount components typically only have
excess copper board space heat sinks - Through-hole components can have large heat sinks
for improved power dissipation
59If Surface Mount - How Much Board Area Is
Available for Heat sinks?
- Engineers must trade-off the cost and size of the
heatsink vs. the Rdson (and hence, the cost) of
the HITFET
60Protected Low Side Drivers
- Introduction to Protected Low Side Drivers
- HITFET Protection Features
- HITFET Diagnostic Features
- EMI/EMC Considerations
- System Implementation
- Frequently Asked Questions
61Protected Low Side Drivers
62Thank You!