TSM9938
or
I
S
I
LOA
x R
SENSE
R1
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)
25
50
100
200
R1 (Ω)
400
200
100
100
ROUT (Ω)
10k
10k
10k
20k
Part Number
TSM9938T
TSM9938F
TSM9938H
TSM9938W
Since the FET’s drain terminal is connected to
ROUT, the output voltage of the TSM9938 at the
OUT terminal is, therefore;
V
OUT
I
LOA
x R
SENSE
x
R
OUT
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
APPLICATIONS INFORMATION
and
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) V
OUT
Swing vs. Applied Input Voltage at V
RS+
and Desired V
SENSE
3) Total I
LOAD
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) V
OUT
Swing vs. Applied Input Voltage at V
RS+
and Desired V
SENSE
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,
V
OUT(max)
= V
RS+(min)
- V
SENSE(max)
– V
OH(max)
R
SENSE
V
OUT
max
GAIN I
LOA
max
where the full-scale V
SENSE
should be less than
V
OUT(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 V
SENSE
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 I
LOAD(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 V
SENSE
s above can be
increased.
3) Total I
LOAD
Accuracy
In
the
TSM9938’s
linear
region
where
V
OUT
< V
OUT(max)
, there are two specifications related
to the circuit’s accuracy: a) the TSM9938’s input
offset voltage (V
OS(max)
= 500μV) and b) its gain error
(GE(max) = 0.5%). An expression for the
TSM9938’s total output voltage (+ error) is given by:
V
OUT
= [GAIN x (1 ± GE) x V
SENSE
] ± (GAIN x V
OS
)
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
Page 8
TSM9938DS r1p0
RTFDS
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
