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ISL28005 Datasheet(PDF) 11 Page - Intersil Corporation

Part No. ISL28005
Description  Micropower, Rail-to-Rail Input Current Sense Amplifier
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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ISL28005 Datasheet(HTML) 11 Page - Intersil Corporation

 
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ISL28005
11
FN6973.4
July 1, 2011
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 20). 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 RS+ 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 RS+ or RS- are beyond the sensing voltage range
of this amplifier.
Typical Application Circuit
Figure 22 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 (RP) in Figure 22 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 RP 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 VOS. Refer
to the following formula:
((RP x IRS-) = (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 CM and CD 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 21). 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 VOS of the amplifier and
Kelvin voltage errors. If the transient protection resistor is added,
an additional VOS error can result from the IxR voltage due to
input bias current. The limiting resistor should only be added to
the RS- input, due to the high-side gm amplifier (gmHI) 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 21. The
resistance of 1/2 oz. copper is ~1m
Ω per square with a TC of
~3900ppm/°C (0.39%/°C). When you compare this unwanted
parasitic resistance with the total of 1m
Ω 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 0.005
Ω sense resistor,
generating a VSENSE = 0.1 and a full scale output voltage of 10V
(G = 100). Two side contacts of only 0.25 square per contact puts
the VSENSE 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 0.001
Ω/sq. = 10mV) added to the 100mV sense
voltage for a sense voltage error of 10% (0.110V - 0.1)/0.1V) x
100.
FIGURE 20. GAIN ACCURACY vs VRS+ = 0V TO 2V
VRS+ (V)
-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
FIGURE 21. PC BOARD CURRENT SENSE KELVIN CONNECTION
PC Board
Non-uniform
Current Flow
Current Sense Resistor
1 to 10mO
Current In
Current Out
Kelvin V
S Contacts
Copper Trace
30mO/Sq.
PC Board
Non-uniform
Current Flow
Current Sense Resistor
1 to 10mO
Current In
Current Out
Kelvin V
S Contacts
Copper Trace
30mO/Sq.
CURRENT SENSE RESISTOR
1mΩ TO 10mΩ
1mΩ /SQ
CURRENT OUT
CURRENT IN
NON-UNIFORM
CURRENT FLOW
PC BOARD
KELVIN VS CONTACTS
1/2 Oz COPPER TRACE


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