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TSM9938 Datasheet(PDF) 8 Page - Touchstone Semiconductor Inc

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

TSM9938 Datasheet(HTML) 8 Page - Touchstone Semiconductor Inc

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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 current-limiting 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 full-scale 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 full-scale ILOAD(MAX)
current in each application. For the case where the
minimum power supply voltage is higher than 3.6V,
each of the four full-scale 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


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