ISL28006_10 Datasheet, PDF (20/25 Pages) Intersil Corporation – Micropower, Rail to Rail Input Current Sense Amplifier with Voltage Output

ISL28006
Hysteretic Comparator
The input trans-conductance amps are under control of a
hysteretic comparator operating from the incoming
source voltage on the R_S+ pin (Figure 66). The
comparator monitors the voltage on R_S+ 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 R_S+ 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
ISL28006 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 68. GAIN ACCURACY vs V_RS+ = 0V TO 2V
Typical Application Circuit
Figure 70 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 70 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
Equation 3:
((R_P × I_RS-) = (100Ω × 130nA) = 13μV)
(EQ. 3)
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
recover 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 69). 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 R_S+ pin.
Layout Guidelines
The Kelvin Connected Sense Resistor
The source of Kelvin voltage errors is illustrated in
Figure 69. 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 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 69. PC BOARD CURRENT SENSE KELVIN CONNECTION
20
FN6548.4
September 16, 2010