ISL28005_14 Datasheet, PDF (12/15 Pages) Intersil Corporation – Micropower, Rail-to-Rail Input Current Sense Amplifier with Voltage Output

ISL28005
Hysteretic Comparator
The input trans-conductance amps are under control of a
hysteretic comparator operating from the incoming source
voltage on the RS+ pin (see Figure 28). The comparator monitors
the voltage on RS+ and switches the sense amplifier from the
low-side gm amp to the high-side gm amplifier whenever the
input voltage at R_S+ increases above the 1.35V threshold.
Conversely, a decreasing voltage on the RS+ pin, causes the
hysteric comparator to switch from the high-side gm amp to the
low-side gm amp as the voltage decreases below 1.35V. It is that
low-side sense gm amplifier that gives the ISL28005 the
proprietary ability to sense current all the way to 0V. Negative
voltages on the R_S+ or R_S- are beyond the sensing voltage range
of this amplifier.
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
V_RS+ (V)
FIGURE 28. GAIN ACCURACY vs V_RS+ = 0V TO 2V
Typical Application Circuit
Figure 30 shows the basic application circuit and optional
protection components for switched-load applications. For
applications where the load and the power source is permanently
connected, only an external sense resistor is needed. For
applications where fast transients are caused by hot plugging the
source or load, external protection components may be needed.
The external current limiting resistor (R_P) in Figure 30 may be
required to limit the peak current through the internal ESD
diodes to < 20mA. This condition can occur in applications that
experience high levels of in-rush current causing high peak
voltages that can damage the internal ESD diodes. An R_P resistor
value of 100Ω will provide protection for a 2V transient with the
maximum of 20mA flowing through the input while adding only
an additional 13µV (worse case over-temperature) of V_OS. Refer
to the following formula:
((R_P x I_RS-) = (100Ω x 130nA) = 13µV)
Switching applications can generate voltage spikes that can
overdrive the amplifier input and drive the output of the amplifier
into the rails, resulting in a long overload recovery time.
Capacitors C_M and C_D filter the common mode and differential
voltage spikes.
Error Sources
There are 3 dominant error sources: gain error, input offset
voltage error and Kelvin voltage error (see Figure 29). The gain
error is dominated by the internal resistance matching
tolerances. The remaining errors appear as sense voltage errors
at the input to the amplifier. They are V_OS of the amplifier and
Kelvin voltage errors. If the transient protection resistor is added,
an additional V_OS error can result from the IxR voltage due to
input bias current. The limiting resistor should only be added to
the R_S- input, due to the high-side gm amplifier (gm_HI) sinking
several micro amps of current through the RS+ pin.
Layout Guidelines
Kelvin Connected Sense Resistor
The source of Kelvin voltage errors is illustrated in Figure 29. The
resistance of 1/2 oz. copper is ~1mΩ per square with a TC of
~3900ppm/°C (0.39%/°C). When you compare this unwanted
parasitic resistance with the total of 1mΩ to 10mΩ resistance of the
sense resistor, it is easy to see why the sense connection must be
chosen very carefully. For example, consider a maximum current of
20A through a 0.005Ω sense resistor, generating a V_SENSE = 0.1
and a full scale output voltage of 10V (G = 100). Two side contacts
of only 0.25 square per contact puts the V_SENSE input about 0.5 x
1mΩ away from the resistor end capacitor. If only 10A the 20A total
current flows through the kelvin path to the resistor, you get an error
voltage of 10mV (10A x 0.5sq x 0.001Ω/sq. = 10mV) added to the
100mV sense voltage for a sense voltage error of 10%
(0.110V - 0.1)/0.1V) x 100.
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FIGURE 29. PC BOARD CURRENT SENSE KELVIN CONNECTION
12
FN6973.5
October 24, 2013