TS1101
The other attribute of the SIGN comparator’s
behavior is its propagation delay as a function of
applied V
SENSE
[(V
RS+
- V
RS-
) or (V
RS-
- V
RS+
)]. As
shown in Figure
, the SIGN comparator’s
propagation delay behavior is symmetric regardless
of current-flow direction and is inversely proportional
to V
SENSE
.
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) 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 power dissipation in RSENSE, the
smallest usable resistor 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
TS1101, the circuit draws its power from the voltage
at its RS+ and RS- terminals. Therefore, the signal
voltage at the OUT terminal is bounded by the
minimum voltage applied at the RS+ terminal.
Therefore,
V
OUT(max)
= V
RS+(min)
- V
SENSE(max)
– V
OH(max)
and
R
S
NS
minimum power supply voltage is higher than 3.6V,
each of the four full-scale V
SENSE
s above can be
increased.
3) Total Load Current Accuracy
In
the
TS1101’s
linear
region
where
V
OUT
< V
OUT(max)
, there are two specifications related
to the circuit’s accuracy: a the TS1101’s input offset
voltage (V
OS(max)
= 100μV and b) its gain error
(GE(max) = 0.6%). An expression for the TS1101’s
total 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
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 TS1101’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 V
SENSE
accuracy in the presence of large
load currents, parasitic pcb track resistance should
be minimized. Kelvin-sense pcb connections
between RSENSE and the TS1101’s RS+ and RS-
terminals are strongly recommended. The drawing in
Figure 3 illustrates the connections between
V
O T
max
GAIN I
LOAD
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
current in each application. For the case where the
TS1101DS r1p0
Page 9
RTFDS
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