TS1102
An expression for the TS1102’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
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 TS1102’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
6) RSENSE Composition
Current-shunt resistors are made available in metal
film, metal strip, and wire-wound constructions.
Wire-wound current-shunt resistors are constructed
with wire spirally wound onto a core. As a result,
these types of current shunt resistors exhibit the
largest self inductance. In applications where the
load current contains high-frequency transients,
metal film or metal strip current sense resistors are
recommended.
Internal Noise Filter
In power management and motor control
applications, current-sense amplifiers are required to
measure load currents accurately in the presence of
both externally-generated differential and common-
mode noise. An example of differential-mode noise
that can appear at the inputs of a current-sense
amplifier is high-frequency ripple. High-frequency
ripple – whether injected into the circuit inductively
or capacitively - can produce a differential-mode
voltage drop across the external current-shunt
resistor (RSENSE). An example of externally-
generated, common-mode noise is the high-
frequency output ripple of a switching regulator that
can result in common-mode noise injection into both
inputs of a current-sense amplifier.
Even though the load current signal bandwidth is
DC, the input stage of any current-sense amplifier
can rectify unwanted, out-of-band noise that can
result in an apparent error voltage at its output. This
rectification of noise signals occurs because all
amplifier input stages are constructed with
transistors that can behave as high-frequency signal
detectors in the same way pn-junction diodes were
used as RF envelope detectors in early radio
designs. Against common-mode injected noise, the
amplifier’s internal common-mode rejection is
usually sufficient.
To counter the effects of externally-injected noise, it
has always been good engineering practice to add
external low-pass filters in series with the inputs of a
current-sense amplifier. In the design of discrete
current-sense amplifiers, resistors used in the
external low-pass filters were incorporated into the
circuit’s overall design so errors because of any
input-bias current-generated offset voltage errors
and gain errors were compensated.
With the advent of monolithic current-sense
amplifiers, like the TS1102, the addition of external
Page 9
RTFDS
Figure 1:
Making PCB Connections to the Sense
Resistor.
between RSENSE and the TS1102’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.
TS1102DS r1p0
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
