Bridging Theory in Practice

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Bridging Theory in Practice

• Transferring Technical Knowledge
• to Practical Applications

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Protected Low Side Drivers
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Protected Low Side Drivers
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Protected 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

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Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

6
Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

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MOSFET 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
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MOSFET 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
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Low 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
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HITFET High Integration Temperature protected
FET
Short Circuit Protection
HITFET
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High Integration Temperature protected FET
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Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

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Rugged 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

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HITFET Protection Features

• Electrostatic Discharge (ESD) Protection
• Load Dump Tolerant
• Inductive and Over voltage Output Clamp
  Protection
• Current Limit Protection
• Thermal Shutdown Protection

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Block Diagram Including Protection Features
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ESD Protection

• Maximum Ratings at Tj 25ºC, Unless Otherwise
  Specified

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Load Dump Protection
Vld Voltage load dump
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Inductive And Over Voltage Clamp
Inductive and Over Voltage Output Clamp
Over voltage condition usually occurs in the
presence of an inductive switching action
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Thermal Shutdown Protection
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Current Limit Protection
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Thermal Shutdown Latch Behavior
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Thermal 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
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Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

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HITFET 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

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HITFET 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

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HITFET Diagnostic Feedback Iin current monitor
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Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

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PWM 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?

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HITFET Block Diagram
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HITFET Turn-On/Turn-Off Slew Rate Controlled

• Turn on / Turn off rate control.
• Slew rate is controlled

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HITFET Turn-on/turn-off Slew Rate Controlled
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Typical HITFET Radiated Emissions Evaluation
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Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

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Reverse 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

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Reverse 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

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Reverse 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!!

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High 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
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Source Follower

• Advantage
  
• Custom edge control (EMC)
• Disadvantage
• -- Complexity
• -- Profet may be better

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HITFET 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)

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HITFET 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

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HITFET 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
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HITFET Edge Shaping Simple Low Pass Filter
Input Voltage
Drain Voltage
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HITFET 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

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HITFET Edge Shaping Miller Capacitor
HITFET
Drain feedback (Miller) capacitor method
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HITFET Edge Shaping Miller Capacitor
Input Voltage
Drain Voltage
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HITFET 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

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HITFET 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

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HITFET 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.

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HITFET 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).

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HITFET 5V vs. 10V operation
Additional Vin
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Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

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Frequently 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?

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Frequently 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.)?

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What 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

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Is 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

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Is 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
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What 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

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What 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

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If 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

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Protected Low Side Drivers

• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions

61
Protected Low Side Drivers
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Thank You!

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