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2.4.3 Total Load Current Accuracy

In the TS1105/06’s linear region where VOUT(min) < VOUT < VOUT(max), there are two specifications related to the circuit’s accuracy: a)

the TS1105/06 CSA’s input offset voltage (VOS(max) = 150 μV), b) the TS1105/06 CSA’s gain error (GE(max) = 1%). An expression for

the TS1105/06’s total error is given by:

VOUT = VBIAS + GAIN × (1 ± GE) × VSENSE ± (GAIN × VOS)

A large value for RSENSE permits the use of smaller load currents to be measured more accurately because the effects of offset voltag-

es are less significant when compared to larger VSENSE voltages. Due care though should be exercised as previously mentioned with

large values of RSENSE.

2.4.4 Circuit Efficiency and Power Dissipation

IR loses 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 TS1105/06 CSA’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.

2.4.5 RSENSE Kelvin Connections

For optimal VSENSE accuracy in the presence of large load currents, parasitic pcb track resistance should be minimized. Kelvin-sense

pcb connections between RSENSE and the TS1105/06’s RS+ and RS– terminals are strongly recommended. The drawing below illus-

trates the connections between the current-sense amplifier and the current-sense resistor. The pcb layout should be balanced and sym-

metrical to minimize wiring-induced errors. In addition, the pcb layout for RSENSE should include good thermal management techniques

for optimal RSENSE power dissipation.

Figure 2.4. Making PCB Connections to RSENSE

2.4.6 RSENSE Composition

Current-shunt resistors are 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 recommen-

ded.

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 injected into the circuit inductively or ca-

pacitively) 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 in-

jection 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. Against common-mode injection noise, the current-sense amplifier’s in-

ternal common-mode rejection ratio is 130 dB (typ).

To counter the effects of externally-injected noise, the TS1105-06 incorporates a 50 kHz (typ), 2nd-order differential low-pass filter as

shown in the TS1105-06’s block diagram, thereby eliminating the need for an external low-pass filter which can generate errors in the

offset voltage and the gain error.

TS1105/06 Data Sheet

System Overview

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