TSM9938 TOUCHSTONE [Touchstone Semiconductor Inc], TSM9938 Datasheet - Page 8

• Manufacturer Part Number: TSM9938
• Description: A 1uA, SOT23 Precision Current-Sense Amplifier
• Manufacturer: TOUCHSTONE [Touchstone Semiconductor Inc]

TSM9938
or
Since the FET’s drain terminal is connected to
ROUT, the output voltage of the TSM9938 at the
OUT terminal is, therefore;
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
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
For lowest IR voltage loss in RSENSE, the smallest
usable value for RSENSE should be selected.
2) V
and Desired V
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,
Page 8
1) RSENSE Voltage Loss
2) V
3) Total I
4) Circuit Efficiency and Power Dissipation
5) RSENSE Kelvin Connections
OUT
V
OUT(max)
Swing vs. Applied Input Voltage at V
and Desired V
OUT
V
OUT
Swing vs. Applied Input Voltage at V
LOAD
= V
SENSE
I
RS+(min)
S
I
Accuracy
LOA
I
LOA
SENSE
x R
- V
x R
R1
SENSE
SENSE(max)
SENSE
x
R
R1
– V
OUT
OH(max)
RS+
RS+
stage is protected against input overdrive by use of
an output current-limiting circuit of
a
Table 1: Internal Gain Setting Resistors (Typical
and
where the full-scale V
V
terminal voltage. For best performance with a 3.6V
power supply, RSENSE should be chosen to
generate a V
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 I
current in each application. For the case where the
minimum power supply voltage is higher than 3.6V,
each of the four full-scale V
increased.
3) Total I
In
V
to the circuit’s accuracy: a) the TSM9938’s input
offset voltage (V
(GE(max)
TSM9938’s total output voltage (+ error) is given by:
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
OUT(MAX)
OUT
7V
V
OUT
GAIN (V/V)
internal clamp protection
< V
the
= [GAIN x (1 ± GE) x V
100
200
25
50
/GAIN at the application’s minimum RS+
OUT(max)
Values)
LOAD
R
TSM9938’s
=
SENSE
SENSE
Accuracy
, there are two specifications related
0.5%).
OS(max)
R1 (Ω)
400
200
100
100
of: a) 120mV (for the 25V/V GAIN
SENSE
GAIN
= 500μV) and b) its gain error
ROUT (Ω)
An
V
OUT
should be less than
SENSE
linear
10k
10k
10k
20k
I
SENSE
circuit.
LOA
expression
max
s above can be
TSM9938DS r1p0
] ± (GAIN x V
3mA
max
Part Number
region
TSM9938W
TSM9938H
TSM9938T
TSM9938F
(typical) and
LOAD(MAX)
for
where
RTFDS
OS
the
)