Bridging Theory in Practice

Bridging Theory in Practice Transferring Technical Knowledge to Practical Applications

Protected Low Side Drivers

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

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

Gate Source Source n+ n+ p+ p+ n- n+ Drain MOSFET Review MOSFET MetalOxideSemiconductorFieldEffectTransistor D G VGS S VGS S S G G P-Channel MOSFET (Enhancement) ( Enhancement) MOSFET D D N-Channel

MOSFET Regions of Operation VGS increases • 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 > 0V N-Channel MOSFET (NMOS) IDS VDS

To turn on, the MOSFET gate is pulled high Low Side Drive (LSD)Configuration The switch is on the “low” side of the load 14V Load MOSFET Switch Drain Vgate @ 5V to 10V Vgs = Vg - Vs Vgs = 5V to 10V Drain voltage is small – ex. 0.1volt Gate Source Vs = 0V

HITFET = High Integration Temperature protected FET Diagnostics Requires external components Current Limit Over Voltage Protection MOSFET Over Temperature Protection Short Circuit Protection HITFET

High Integration Temperature protected FET

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

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

Block Diagram Including Protection Features

ESD Protection Maximum Ratings at Tj = 25ºC, Unless Otherwise Specified

Load Dump Protection * Vld = Voltage load dump

Inductive And Over Voltage Clamp Inductive and Over Voltage Output Clamp Over voltage condition usually occurs in the presence of an inductive switching action

Thermal Shutdown Protection

Current Limit Protection

Thermal Shutdown Latch Behavior

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

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 the available Vin voltage 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 • 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 the available Vin voltage

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

HITFET Diagnostic Feedback Iin current monitor Vcc Vs Id_out R1 Id_in D DO Iin/Diag AI S + Micro processor HITFET Vd1 - Gnd

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?

HITFET Block Diagram

HITFET Turn-On/Turn-Off Slew Rate Controlled Turn on / Turn off rate control. Slew rate is controlled

HITFET Turn-on/turn-off Slew Rate Controlled

Typical HITFET Radiated Emissions Evaluation

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

Reverse Battery Tolerance:Normal Operation Recall: TJ = TAmbient + PD* RTHJA V = I*R PD = ILOAD2RDS(on) Therefore: ILOAD = ((TJ –TAmbient) / (RDS(on)* RTHJA )1/2 Given: TJMAX = 150 C ° TAmbient = 95 C ° RDS(on) = 0.068 Ω Rthja = 55 C/W VBattery = 14 V Results: ILOAD_MAX = 3.8A and RLOAD_MIN = 3.6 Ω

Reverse Battery Tolerance:Body 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 Ω!! Compared to 3.8A and 3.6 Ω when used in normal operation!!

To turn on the HSD, the MOSFET gate is pulled high 28V VS ~ 14V VGS ~ 14V The source voltage is now approximately Vsupply ILOAD The high value of VGS translates into a large value of ILOAD (linear region) 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

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

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)

HITFET Edge Shaping:Simple 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

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

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

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

HITFET Edge Shaping: Miller Capacitor HITFET Drain feedback (Miller) capacitor method

HITFET Edge Shaping: Miller Capacitor Input Voltage Drain Voltage

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

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

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.

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).

HITFET 5V vs. 10V operation Additional Vin

Bridging Theory in Practice