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TSM9938 Datasheet(PDF) 8 Page  Touchstone Semiconductor Inc 

TSM9938 Datasheet(HTML) 8 Page  Touchstone Semiconductor Inc 
8 / 11 page TSM9938 Page 8 TSM9938DS r1p0 RTFDS or I S ILOA x RSENSE R1 Since the FET’s drain terminal is connected to ROUT, the output voltage of the TSM9938 at the OUT terminal is, therefore; VOUT ILOA x RSENSE x ROUT R1 The current sense amplifier’s gain accuracy is therefore the ratio match of ROUT to R1. For each of the four gain options available, Table 1 lists the values for ROUT and R1. The TSM9938’s output stage is protected against input overdrive by use of an output currentlimiting circuit of 3mA (typical) and a 7V internal clamp protection circuit. Table 1: Internal Gain Setting Resistors (Typical Values) GAIN (V/V) R1 ( Ω) ROUT ( Ω) Part Number 25 400 10k TSM9938T 50 200 10k TSM9938F 100 100 10k TSM9938H 200 100 20k TSM9938W APPLICATIONS INFORMATION 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 1) RSENSE Voltage Loss For lowest IR voltage loss in RSENSE, the smallest usable value for RSENSE should be selected. 2) VOUT Swing vs. Applied Input Voltage at VRS+ and Desired VSENSE As there is no separate power supply pin for the TSM9938, 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 TSM9938. Therefore, VOUT(max) = VRS+(min)  VSENSE(max) – VOH(max) and RSENSE VOUT max GAIN ILOA max where the fullscale VSENSE should be less than VOUT(MAX)/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: a) 120mV (for the 25V/V GAIN option), b) 60mV (for the 50V/V GAIN option), c) 30mV (for the 100V/V GAIN option), or d) 15mV (for the 200V/V GAIN option) at the fullscale ILOAD(MAX) current in each application. For the case where the minimum power supply voltage is higher than 3.6V, each of the four fullscale VSENSEs above can be increased. 3) Total ILOAD Accuracy In the TSM9938’s linear region where VOUT < VOUT(max), there are two specifications related to the circuit’s accuracy: a) the TSM9938’s input offset voltage (VOS(max) = 500μV) and b) its gain error (GE(max) = 0.5%). An expression for the TSM9938’s total output voltage (+ error) is given by: 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 
