TS1100 Datasheet, PDF (7/20 Pages) Touchstone Semiconductor Inc – A 1uA, +2V to +25V SOT23 Precision Current-Sense Amplifier

TS1100/01/02/03 Data Sheet
System Overview
2.4.5 R_SENSE 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 R_SENSE and the current-sense amplifier’s RS+ and RS– terminals are strongly recommended. The drawing
below illustrates the connections between the current-sense amplifier and the current-sense resistor. The PCB layout should be bal-
anced and symmetrical to minimize wiring-induced errors. In addition, the pcb layout for R_SENSE should include good thermal manage-
ment techniques for optimal R_SENSE power dissipation.
Figure 2.5. Making PCB Connections to R_SENSE
2.4.6 R_SENSE Composition
Current-shunt resistors are available in metal film, metal strip, and wire-wound constructions. Wire-wound current-shunt resistors con-
sist of a wire spirally wound onto a core. As a result, these types of current shunt resistors exhibit the largest self inductance. In applica-
tions where the load current contains high-frequency transients, metal film or metal strip current-sense resistors are recommended.
2.4.7 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 introduced into the circuit inductively or
capacitively) can produce a differential-mode voltage drop across the external current-shunt resistor (R_SENSE). An example of external-
ly-generated, common-mode noise is the high-frequency output ripple of a switching regulator that can result in the injection of com-
mon-mode noise 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 P–N junction diodes were
used as RF envelope detectors in early radio designs. The amplifier’s internal common-mode rejection is usually sufficient to defeat
injected common-mode noise.
To counter the effects of externally-injected noise, it has always been good engineering practice to add external low-pass filters in ser-
ies 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 to compensate for any input-bias-current-generated offset voltage and
gain errors.
With the advent of monolithic current-sense amplifiers, the addition of external low-pass filters in series with the current-sense amplifi-
er’s inputs only introduces additional offset voltage and gain errors. To minimize or altogether eliminate the need for external low-pass
filters and to maintain low input offset voltage and gain errors, the current-sense amplifiers incorporate a 50 kHz (typ) 2nd-order differ-
ential low-pass filter as shown in the Block Diagrams.
2.4.8 Output Filter Capacitor
If the current-sense amplifiers are a part of a signal acquisition system in which their OUT terminal is connected to the input of an ADC
with an internal, switched-capacitor track-and-hold circuit, the internal track-and-hold’s sampling capacitor can cause voltage droop at
V_OUT. A good-quality 22 to 100 nF ceramic capacitor from the OUT terminal to GND forms a low-pass filter with the current-sense am-
plifier’s R_OUT and should be used to minimize voltage droop (holding VOUT constant during the sample interval. Using a capacitor on
the OUT terminal will also reduce the small-signal bandwidth as well as band-limiting amplifier noise.
2.4.9 PC Board Layout and Power Supply Bypassing
For optimal circuit performance, the current-sense amplifiers should be in very close proximity to the external current-sense resistor,
and the PCB tracks from R_SENSE to the RS+ and the RS– input terminals should be short and symmetric. Also recommended are a
ground plane and surface mount resistors and capacitors.
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