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TSM9938 Datasheet(PDF) 7 Page - Touchstone Semiconductor Inc

Part No. TSM9938
Description  A 1uA, SOT23 Precision Current-Sense Amplifier
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Manufacturer  TOUCHSTONE [Touchstone Semiconductor Inc]
Direct Link  http://www.touchstonesemi.com
Logo TOUCHSTONE - Touchstone Semiconductor Inc

TSM9938 Datasheet(HTML) 7 Page - Touchstone Semiconductor Inc

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TSM9938
TSM9938DS r1p0
Page 7
RTFDS
PIN FUNCTIONS
PIN
LABEL
FUNCTION
SOT23
5
RS+
External Sense Resistor Power-Side Connection
4
RS-
External Sense Resistor Load-Side Connection
1, 2
GND
Ground. Connect these pins to analog ground.
3
OUT
Output Voltage. VOUT is proportional to VSENSE = VRS+ - VRS-
BLOCK DIAGRAMS
DESCRIPTION OF OPERATION
The internal configuration of the TSM9938
– 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 TSM9938 (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
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-
source current is equal to:
I S
VSENSE
R1


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