TSM9938F A 1µA, SOT23 Precision Current-Sense Amplifier FEATURES DESCRIPTION ♦ Second-source for MAX9938F ♦ Ultra-Low Supply Current: 1μA ♦ Wide Input Common Mode Range: +1.6V to +28V ♦ Low Input Offset Voltage: 500μV (max) ♦ Low Gain Error: <0.5% (max) ♦ Voltage Output ♦ Gain Option Available: TSM9938F: Gain = 50V/V ♦ 5-Pin SOT23 Packaging The voltage-output TSM9938F current-sense amplifier is electrically and form-factor identical to the MAX9938F current-sense amplifier. Consuming a very low 1μA supply current, the TSM9938F high-side current-sense amplifier exhibits a 500-µV (max) VOS and a 0.5% (max) gain error, both specifications optimized for any precision current measurement. For all high-side current-sensing applications, the TSM9938F features a wide input common-mode voltage range from 1.6V to 28V. APPLICATIONS Notebook Computers Power Management Systems Portable/Battery-Powered Systems PDAs Smart Phones The SOT23 package makes the TSM9938F an ideal choice for pcb-area-critical, low-current, highaccuracy current-sense applications in all batterypowered portable instruments. The TSM9938F is specified for operation over the -40°C to +85°C extended temperature range. TYPICAL APPLICATION CIRCUIT Input Offset Voltage Histogram 35 PERCENT OF UNITS - % 30 25 15 15 10 5 0 0 10 20 30 40 50 INPUT OFFSET VOLTAGE - µV Page 1 © 2014 Silicon Laboratories, Inc. All rights reserved. TSM9938F ABSOLUTE MAXIMUM RATINGS RS+, RS- to GND- .............................................-0.3V to +30V OUT to GND- ......................................................-0.3V to +6V RS+ to RS- ..................................................................... ±30V Short-Circuit Duration: OUT to GND .................... Continuous Continuous Input Current (Any Pin) ............................ ±20mA Continuous Power Dissipation (TA = +70°C) 5-Pin SOT23 (Derate at 3.9mW/°C above +70°C).. 312mW Operating Temperature Range ...................... -40°C to +85°C Junction Temperature ................................................ +150°C Storage Temperature Range ....................... -65°C to +150°C Lead Temperature (Soldering, 10s) ........................... +300°C Soldering Temperature (Reflow) ............................ +260°C Electrical and thermal stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only and functional operation of the device at these or any other condition beyond those indicated in the operational sections of the specifications is not implied. Exposure to any absolute maximum rating conditions for extended periods may affect device reliability and lifetime. PACKAGE/ORDERING INFORMATION ORDER NUMBER PART MARKINGCARRIERQUANTITY TSM9938FEUK+T TADA Tape & Reel 3000 Lead-free Program: Silicon Labs supplies only lead-free packaging. Consult Silicon Labs for products specified with wider operating temperature ranges. Page 2 TSM9938F Rev. 1.0 TSM9938F ELECTRICAL CHARACTERISTICS VRS+ = VRS- = 3.6V; VSENSE = (VRS+ - VRS-) = 0V; TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C. See Note 1 PARAMETER SYMBOL Supply Current (Note 2) ICC Common-Mode Input Range Common-Mode Rejection Ratio VCM Input Offset Voltage (Note 3) Gain CMRR VOS CONDITIONS VRS+ = 5V, TA = +25°C VRS+ = 5V, -40°C < TA < +85°C VRS+ = 28V, TA = +25°C VRS+ = 28V, -40°C < TA < +85°C Guaranteed by CMRR , -40°C < TA < +85°C 1.6V < VRS+ < 28V, -40°C < TA < +85°C MIN 1.1 1.6 94 TA = +25°C -40°C < TA < +85°C GE Output Resistance OUT Low Voltage OUT High Voltage ROUT VOL VOH 50 ±0.1 TA = +25°C -40°C < TA < +85°C (Note 5) Gain = 50 VOH = VRS- - VOUT (Note 6) 7.0 MAX 0.85 1.1 1.8 2.5 28 130 ±100 G Gain Error (Note 4) TYP 0.5 10 3 0.1 UNITS μA V dB ±500 ±600 μV V/V ±0.5 ±0.6 13.2 30 0.2 % kΩ mV V Note 1: All devices are 100% production tested at TA = +25°C. All temperature limits are guaranteed by product characterization. Note 2: Extrapolated to VOUT = 0. ICC is the total current into the RS+ and the RS- pins. Note 3: Input offset voltage VOS is extrapolated from VOUT with VSENSE set to 1mV. Note 4: Gain error is calculated by applying two values for VSENSE and then calculating the error of the actual slope vs. the ideal transfer characteristic: For GAIN = 50, the applied VSENSE is 10mV and 60mV. Note 5: The device is stable for any capacitive load at VOUT. Note 6: VOH is the voltage from VRS- to VOUT with VSENSE = 3.6V/GAIN. TSM9938F Rev. 1.0 Page 3 TSM9938F TYPICAL PERFORMANCE CHARACTERISTICS VRS+ = VRS- = 3.6V; TA = +25°C, unless otherwise noted. Gain Error Histogram Input Offset Voltage Histogram 35 30 PERCENT OF UNITS - % PERCENT OF UNITS - % 30 25 15 15 10 5 0 10 20 30 40 15 10 5 -0.4 50 0 0.2 0.4 GAIN ERROR - % Supply Current vs Temperature Input Offset Voltage vs Common-Mode Voltage 40 INPUT OFFSET VOLTAGE - µV 28V 0.8 1.8V 0.6 3.6V 0.4 0.2 35 30 25 20 0 -40 -15 10 35 60 0 85 5 10 15 20 25 30 TEMPERATURE - °C SUPPLY VOLTAGE - Volt Input Offset Voltage vs Temperature Supply Current vs Common-Mode Voltage 1 60 0.8 SUPPLY CURRENT - µA 80 40 20 0 -20 0.6 0.4 0.2 0 -40 -15 10 35 60 TEMPERATURE - °C Page 4 -0.2 INPUT OFFSET VOLTAGE - µV 1 SUPPLY CURENT - µA 20 0 0 INPUT OFFSET VOLTAGE - µV 25 85 0 5 10 15 20 25 30 SUPPLY VOLTAGE - Volt TSM9938F Rev. 1.0 TSM9938F TYPICAL PERFORMANCE CHARACTERISTICS VRS+ = VRS- = 3.6V; TA = +25°C, unless otherwise noted. Gain Error vs. Temperature Gain Error vs Common-Mode Voltage 0.3 0.5 GAIN ERROR - % GAIN ERROR - % 0.4 0.2 0.1 0.3 0.2 0.1 0 -0.1 -40 0 5 10 20 15 25 30 35 60 TEMPERATURE - °C VOUT vs VSENSE @ Supply = 3.6V VOUT vs VSENSE @ Supply = 1.6V 4 1.6 3.5 1.4 3 1.2 2.5 1.0 2 85 0.8 1.5 0.6 1 0.4 0.5 0.2 0 0 50 0 100 0 150 20 40 60 80 100 VSENSE- mV VSENSE- mV Small-Signal Gain vs Frequency Common-Mode Rejection vs Frequency 0 COMMON-MODE REJECTION - dB 5 0 SMALL-SIGNAL GAIN -dB 10 -15 SUPPLY VOLTAGE - Volt VOUT - V VOUT - V 0 -5 -10 -15 -20 -25 -30 -35 0.001 0.01 0.1 1 10 FREQUENCY - kHz TSM9938F Rev. 1.0 100 1000 -20 -40 -60 -80 -100 -120 -140 0.001 0.01 0.1 1 10 100 1000 FREQUENCY - kHz Page 5 TSM9938F TYPICAL PERFORMANCE CHARACTERISTICS VRS+ = VRS- = 3.6V; TA = +25°C, unless otherwise noted. Large-Signal Pulse Response, Gain = 50 VOUT VOUT VSENSE VSENSE Small-Signal Pulse Response, Gain = 50 200µs/DIV Page 6 200µs/DIV TSM9938F Rev. 1.0 TSM9938F PIN FUNCTIONS PIN SOT23 5 4 1, 2 3 LABEL FUNCTION RS+ RSGND OUT External Sense Resistor Power-Side Connection External Sense Resistor Load-Side Connection Ground. Connect this pin to analog ground. Output Voltage. VOUT is proportional to VSENSE = VRS+ - VRS- BLOCK DIAGRAMS DESCRIPTION OF OPERATION The internal configuration of the TSM9938F – a unidirectional high-side, current-sense amplifier - is based on a commonly-used operational amplifier (op amp) circuit for measuring load currents (in one direction) in the presence of high-common-mode voltages. In the general case, a current-sense amplifier monitors the voltage caused by a load current through an external sense resistor and generates an output voltage as a function of that load current. Referring to the typical application circuit on Page 1, the inputs of the op-amp-based circuit are connected across an external RSENSE resistor that is used to measure load current. At the non-inverting input of the TSM9938F (the RS- terminal), the applied voltage is ILOAD X RSENSE. Since the RS- terminal is the non-inverting input of the internal op amp, op-amp feedback action forces the inverting input of the internal op amp to the same potential (ILOAD x RSENSE). Therefore, the voltage drop across TSM9938F Rev. 1.0 RSENSE (VSENSE) and the voltage drop across R1 (at the RS+ terminal) are equal. To minimize any additional error because of op-amp input bias current mismatch, both R1s are the same value. Since the internal p-channel FET’s source is connected to the inverting input of the internal op amp and since the voltage drop across R1 is the same as the external VSENSE, op amp feedback action drives the gate of the FET such that the FET’s drain current is equal to: IDS = VSENSE R1 Page 7 TSM9938F or ILOAD x RSENSE IDS = R1 Since the FET’s drain terminal is connected to ROUT, the output voltage of the TSM9938F at the OUT terminal is, therefore; VOUT =ILOAD x RSENSE x lists the values for ROUT and R1. The TSM9938F’s output stage is protected against input overdrive by use of an output current-limiting circuit of 3mA (typical) and a 7V internal clamp protection circuit. Table 1: Internal Gain Setting Resistors (Typical Values) ROUT R1 GAIN (V/V) 50 R1 (Ω) 200 ROUT (Ω) 10k Part Number TSM9938F The current-sense amplifier’s gain accuracy is therefore the ratio match of ROUT to R1. Table 1 APPLICATIONS INFORMATION and Choosing the Sense Resistor Selecting the optimal value for the external RSENSE is based on the following criteria and for each commentary follows: 1) RSENSE Voltage Loss 2) VOUT Swing vs. Applied Input Voltage at VRS+ and Desired VSENSE 3) Total ILOAD Accuracy 4) Circuit Efficiency and Power Dissipation 5) RSENSE Kelvin Connections RSENSE = VOUT max GAIN × ILOAD max where the full-scale VSENSE should be less than VOUT/GAIN at the application’s minimum RS+ terminal voltage. For best performance with a 3.6V power supply, RSENSE should be chosen to generate a VSENSE of 60mV at the full-scale ILOAD current in each application. For the case where the minimum power supply voltage is higher than 3.6V, the full-scale VSENSE above can be increased. 1) RSENSE Voltage Loss 3) Total Load Current Accuracy For lowest IR voltage loss in RSENSE, the smallest usable value for RSENSE should be selected. In the TSM9938F’s linear region where VOUT < VOUT(max), there are two specifications related to the circuit’s accuracy: a) the TSM9938F’s input offset voltage (VOS = 500μV, max) and b) its gain error (GE(max) = 0.5%). An expression for the TSM9938F’s total error is given by: 2) VOUT Swing vs. Applied Input Voltage at VRS+ and Desired VSENSE As there is no separate power supply pin for the TSM9938F, the circuit draws its power from the applied voltage at both its RS+ and RS- terminals. Therefore, the signal voltage at the OUT terminal is bounded by the minimum supply voltage applied to the TSM9938F. Therefore, VOUT = [GAIN x (1 ± GE) x VSENSE] ± (GAIN x VOS) A large value for RSENSE permits the use of smaller load currents to be measured more accurately because the effects of offset voltages are less significant when compared to larger VSENSE voltages. Due care though should be exercised as VOUT(max) = VRS+(min) - VSENSE(max) – VOH(max) Page 8 TSM9938F Rev. 1.0 TSM9938F previously mentioned with large values of RSENSE. 4) Circuit Efficiency and Power Dissipation IR losses in RSENSE can be large especially at high load currents. It is important to select the smallest, usable RSENSE value to minimize power dissipation and to keep the physical size of RSENSE small. If the external RSENSE is allowed to dissipate significant power, then its inherent temperature coefficient may alter its design center value, thereby reducing load current measurement accuracy. Precisely because the TSM9938F’s input stage was designed to exhibit a very low input offset voltage, small RSENSE values can be used to reduce power dissipation and minimize local hot spots on the pcb. 5) RSENSE Kelvin Connections For optimal VSENSE accuracy in the presence of large load currents, parasitic pcb track resistance should be minimized. Kelvin-sense pcb connections between RSENSE and the TSM9938F’s RS+ and RS- terminals are strongly recommended. The drawing in Figure 1 illustrates the connections between the current-sense amplifier and the currentsense resistor. The pcb layout should be balanced and symmetrical to minimize wiring-induced errors. In addition, the pcb layout for RSENSE should include good thermal management techniques for optimal RSENSE power dissipation. Optional Output Filter Capacitor If the TSM9938F is part of a signal acquisition system where its OUT terminal is connected to the input of an ADC with an internal, switched-capacitor track-and-hold circuit, the internal track-and-hold’s sampling capacitor can cause voltage droop at VOUT. A 22nF to 100nF, good-quality ceramic capacitor from the OUT terminal to GND should be used to minimize voltage droop (holding VOUT constant during the sample interval). Using a capacitor on the OUT terminal will also reduce the TSM9938F’s small-signal bandwidth as well as band-limiting amplifier noise. Using the TSM9938F in Bidirectional Load Current Applications Figure 1: Making PCB Connections to the Sense Resistor (drawing is not to scale). In many battery-powered systems, it is oftentimes necessary to monitor a battery’s discharge and charge currents. To perform this function, a bidirectional current-sense amplifier is required. The circuit illustrated in Figure 2 shows how two TSM9938Fs can be configured as a bidirectional current-sense amplifier. As shown in the figure, the Figure 2: Using Two TSM9938Fs for Bidirectional Load Current Detection TSM9938F Rev. 1.0 Page 9 TSM9938F RS+/RS- input pair of TSM9938F #2 is wired opposite in polarity with respect to the RS+/RSconnections of TSM9938F #1. Current-sense amplifier #1 therefore measures the discharge current and current-sense amplifier #2 measures the charge current. Note that both output voltages are measured with respect to GND. When the discharge current is being measured, VOUT1 is active and VOUT2 is zero; for the case where charge current is being measured, VOUT1 is zero, and VOUT2 is active. Page 10 PC Board Layout and Power-Supply Bypassing For optimal circuit performance, the TSM9938F should be in very close proximity to the external current-sense resistor and the pcb tracks from RSENSE to the RS+ and the RS- input terminals of the TSM9938F should be short and symmetric. Also recommended are a ground plane and surface mount resistors and capacitors. TSM9938F Rev. 1.0 TSM9938F PACKAGE OUTLINE DRAWING 5-Pin SOT23 Package Outline Drawing (N.B., Drawings are not to scale) Patent Notice Silicon Labs invests in research and development to help our customers differentiate in the market with innovative low-power, small size, analog-intensive mixed-signal solutions. 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Other products or brandnames mentioned herein are trademarks or registered trademarks of their respective holders. Silicon Laboratories, Inc. 400 West Cesar Chavez, Austin, TX 78701 +1 (512) 416-8500 ▪ www.silabs.com Page 11 TSM9938F Rev. 1.0 Smart. Connected. Energy-Friendly Products Quality Support and Community www.silabs.com/products www.silabs.com/quality community.silabs.com Disclaimer Silicon Laboratories intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the Silicon Laboratories products. Characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "Typical" parameters provided can and do vary in different applications. Application examples described herein are for illustrative purposes only. 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