High Accuracy Current Sensing with TI High Side Switches

High Accuracy Current Sensing with TI High Side Switches 2020 Asia FAE Summit Timothy Logan TI Information Selective Disclosure TI Information Selective Disclosure 1

Agenda Introduction to High Side Switches Current Sense Design Considerations Example Application LED Headlight Live Demo TI Information Selective Disclosure 2

TI High Side Switch Features Higher reliability protect power supply during short circuit or clamp inrush current charging a capacitive load as in an off-board ECU Lower system cost reduce cost/weight in the wiring harness and electrical connectors Adjustable Current Limiting Mode High Accuracy Current Sense Accurate, real-time load diagnostics even in light loads (such as LEDs) Reduce system cost - eliminates the need for current sense calibration AEC Q100-012 Grade A, 1M times short to GND test (TPS1H100, TPS2H160) Designed for automotive environments - EMC, ISO7637-2 & ISO16750-2 IEC-61004-2 ESD protection to 8 kV/ 15 kV Full Diagnostics TI Information Selective Disclosure

Current Sense Accuracy High Accuracy Current Sense Accurate, real-time load diagnostics even in light loads (such as LEDs) Enhanced fault detection through monitoring changes in load current Reduce system cost - eliminates the need for current sense calibration Innovative design with accurate sense FET ratio and low offset amplifier. Current Sense Ratio is calibrated in device production test at 25 C by trimming gain at fixed current. In addition to load current, FET Temperature can also be sensed by configuring SELx pin (TPSxHBxx ONLY) TI Information Selective Disclosure 4

Fault Reporting on SNS Pin Fault conditions such as current limit are reported via SNS pin TPSxHxx devices output a voltage (VCS,H) TPSxHBxx/TPSxHAxx devices output a current (ISNSFH) Zener diode recommended to ensure voltage on SNS pin within specification of ADC analog input TPSxHxx TPSxHAxx / TPSxHBxx VCS,H ISNSFH TPS1H100-Q1: 4.3V to 4.9V TPS1HB08-Q1: 4mA to 5.3mA TI Information Selective Disclosure 5

Example TPS1HB08-Q1 and MSP430F5529 TPS1HB08-Q1 MSP430F5529 12-bit ADC 1k 5.3mA Absolute maximum voltage for MSP430 device is rated at AVCC (typically 3.3V) Fault condition on TPS1HB08 would likely result in damage to ADC input ????= 0.0053 1000 ????= 5.3? ??????= 3.3? Measureable Voltage 0.0v to 3.3V TI Information Selective Disclosure 6

Determining KSNS on TI High Side Switches For TPSxHxx devices the current sense accuracy is specified by percentages in datasheet For TPSxHAxx/TPSxHBxx devices ISNSI values are specified at varying load currents KSNS value can bet determined by linear extrapolation TPS1HB08-Q1 TPS1H100-Q1 IOUT 10A Typical ISNS 2.0mA Effective KSNS 5000 3A 0.6mA 5000 1A 0.2mA 5000 300mA 0.06043mA 4964 100mA 0.0206mA 4854 50mA 0.0106mA 4717 20mA 0.0046mA 4348 TI Information Selective Disclosure 7

ADC Best Practices Decouple analog and digital supply voltage sources and eliminate any potential ground loops Add capacitor to analog input pin to mitigate inrushes associated with start of initial sampling Sample multiple times (usually enabled by a sequence of conversion) and take median value of collected samples Make sure to factor in any calibrated trim values (typically stored in INFO flash) for ADC software calculations TI Information Selective Disclosure 8

Current Sense and PWM When using a PWM to toggle the high side switch the time it takes for the current sense to settle needs to be taken into account: Toggling the EN line via PWM faster than the settling time of the current sense can lead to unreliable data reported from SNS pin TI Information Selective Disclosure 9

Considerations to Maximize Accuracy Make sure that the device performance is correlated to the current sense If poor quality layout or incorrect configuration of load exists in design current sense will only report the current flowing through the device When measuring SNS pin discretely with scope minimize the ground path GND CS Having a long path between positive and negative probes of the oscilloscope will lead to potentially noisy reading If measuring with an oscilloscope choose a ground path close to probe point for CS GND TI Information Selective Disclosure 10

Picking the Right Device TPS1H100 Higher load current in the device s supported current range will lead to better current sense accuracy If current sense accuracy at a specific load profile is important make sure to check device accuracy specification TI Information Selective Disclosure 11

Picking the Right Device TPS1HB08-Q1 (8m ) TPS1H100-Q1 (100m ) +80% +57.3% +5% +6.2% +10% +30.3% +3% +5.3% +7% +15.1% -7% -13.6% -3% -5% -10% -28.3% -5% -4.6% -80% -56% 5mA 25mA 50mA 100mA 1A 20mA 50mA 100mA 300mA 1A TI Information Selective Disclosure 12

Picking the Right Device TPS4H000-Q1 (1 ) TPS2HB35-Q1 (35m ) +70% +13% +5% +45% +7% +3% +5% +15% +6% -15% -6% -3% -5% -45% -7% -5% -70% -13% 1mA 2mA 5mA 25mA 100mA 1A 50mA 100mA 300mA TI Information Selective Disclosure 13

Example Application LED Headlight Description Purely resistive load of LED string used in automotive applications for headlight MSP430F5529 w/ 12-bit ADC as microcontroller Requirements Nominal load current (total): 250mA Number of LEDs on load: 5 (50mA per LED) Diagnostics Required: Short-to-Ground, Open Load, Short to Battery, High Accuracy Current Sense Advantages of using TI High Side Switch for Infotainment High accuracy current sense can be used to detect individual LEDs burning out Current limit can be set to programmable lower limit and reduce cost of downstream power components TI Information Selective Disclosure 14

Part Selection What about the TPS1H100 (100m RON)? Calculate how hot the high side switch s junction will get ??= ??+ (???? ????) or ??= ??+ ((????? ? ???) ????) Knowns TA = 85 ILOAD = 250mA RON = 166m (max from datasheet) R? ?JA = 41 / W (from datasheet) MaximumTJ = 150 Calculation ??= ?? + ((?.??? .???) ??) ??= ??.?? 85.43 < 150 It works but for the nominal load current the TPS1H100 is not optimal. TI Information Selective Disclosure 15

Part Selection What about the TPS4H000 (1 RON)? Calculate how hot the high side switch s junction will get ??= ??+ (???? ????) or ??= ??+ ((????? ? ???) ????) Knowns TA = 85 ILOAD = 250mA RON = 2 (max from datasheet) R? ?JA = 38.5 / W (from datasheet) MaximumTJ = 150 For this application the TPS4H000 is a better value and fit. Calculation ??= ?? + ((?.??? ?) ??.?) ??= 89.81 89.81 < 150 TI Information Selective Disclosure 16

Choosing the Right SNS Resistor Without Accuracy Variations Calculations ????= 250?? Determine max current you want to sense ???????=0.250 = 3.13?? 80 ????=??????? 3.0? 3.13??= 958 953 ???????=??? ??????? = ????= 3.3? ???,?= 4.3? 4.9? ??????? ???? ????= 80 Min Max Fault Output Current 0A 250mA SNS Current 3.13mA 0A 3.0V SNS Voltage 0V 3.3V TI Information Selective Disclosure ADC Reading 0x0 0xE8B 0xFFF 17

Accounting for Tolerances Each LED has roughly 25mA running through it with a total system load current of 250mA At over 100mA load currents the TPS4H000 has a sense accuracy of +/- 3% TI Information Selective Disclosure 18

Accounting for Tolerances/Accuracies ? ?? ?? ?? ? Accuracy ( 3%) Ideal Parameters Resistor Tolerances (1%) ADC Parameters ??????,???= 3.5LSBs ???????= 50?? ???????????= 3% ??????= 12???? ??????= 3.3? ????= 953 ???= 80 ???????= 242.5?? ????= 250?? ???????= 257.5?? ???????= 943.47 ????= 953 ???????= 967.53 Normalized Calculation ??????? ??? ?????= ??????? ??????? ??????? = 2.86? .97 ? = 2.86? ?????? ??????????? 2?????? 1 3.3 0.97 212 1 = 0.78mV/step ???????= = LEDs Output Current Measured Voltage Voltage Delta LSB Drop 1 50mA 0.56V to 0.63V 0.56V to 0.63 72 LSBs 2 100mA 1.12V to 1.25V 3 150mA 1.70V to 1.87V 0.58V to 0.62V 74 LSBs 4 200mA 2.28V to 2.49V 5 250mA 2.86V to 3.11V TI Information Selective Disclosure FAULT 19 N/A 3.3V (Zener) N/A N/A

Accuracy Comparisons Without Accuracy Adjustment Min Max 250mA Fault Output Current 0A SNS Current 3.13mA 0A 3.0V 3.3V SNS Voltage 0V ADC Reading 0x0 0xE8B 0xFFF With Accuracy Adjustment Min Max 257.5mA Fault Output Current 0A SNS Current 3.22mA 0A 3.11V 3.3V SNS Voltage 0V ADC Reading 0x0 0xF13 0xFFF TI Information Selective Disclosure

Resources High Accuracy Current Sense of Smart High Side Switches How To Drive Resistive, Inductive, Capacitive, and Lighting Loads With Smart High Side Switches 11 Ways to Protect Your Power Path Adjustable Current Limit of Smart Power Switches E2E Power Management Forum https://e2e.ti.com/support/power-management/f/196 TI Information Selective Disclosure 21