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TSM9938 Datasheet(PDF) 9 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) 9 Page - Touchstone Semiconductor Inc

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TSM9938
TSM9938DS r1p0
Page 9
RTFDS
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 TSM9938’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 TSM9938’s RS+ and RS-
terminals are strongly recommended. The drawing in
Figure 1 illustrates the connections between the
current-sense amplifier and the current-sense
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 TSM9938 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 TSM9938’s small-
signal bandwidth as well as band-limiting amplifier
noise.
Using the TSM9938 in Bidirectional Load Current
Applications
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
TSM9938s can be configured as a bidirectional
current-sense amplifier. As shown in the figure, the
Figure 1: Making PCB Connections to the Sense
Resistor (drawing is not to scale).
Figure 2: Using Two TSM9938s for Bidirectional Load Current Detection


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