isl28006 Intersil Corporation, isl28006 Datasheet - Page 17

• Manufacturer Part Number: isl28006
• Description: Micropower, Rail To Rail Input Current Sense Amplifier With Voltage Output
• Manufacturer: Intersil Corporation

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 54). The
comparator monitors the voltage on R
the sense amplifier from the low-side gm amp to the
high-side gm amplifier whenever the input voltage at
R
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
ISL28006 the proprietary ability to sense current all the
way to 0V. Negative voltages on the R
beyond the sensing voltage range of this amplifier.
Typical Application Circuit
Figure 56 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
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
protection for a 2V transient with the maximum of 20mA
S
+ increases above the 1.35V threshold. Conversely, a
FIGURE 54. GAIN ACCURACY vs V
-0.1
-0.2
-0.3
-0.4
-0.5
0.5
0.4
0.3
0.2
0.1
0
0
0.2
P
0.4
resistor value of 100Ω will provide
CURRENT IN
0.6
Current In
Current In
17
0.8
P
FIGURE 55. PC BOARD CURRENT SENSE KELVIN CONNECTION
) in Figure 56 may be
V
RS+
1.0
NON-UNIFORM
CURRENT FLOW
(V)
Current Flow
Current Flow
Non-uniform
Non-uniform
1.2
RS+
S
S
1.4
+ or R
+ and switches
1.6
= 0V TO 2V
KELVIN V
Kelvin V
Kelvin V
S
1.8
- are
CURRENT SENSE RESISTOR
2.0
S
S
ISL28006
S
Current Sense Resistor
Current Sense Resistor
Contacts
Contacts
CONTACTS
1mΩ TO 10mΩ
1 to 10mO
1 to 10mO
flowing through the input while adding only an additional
13µV (worse case over-temperature) of V
following formula:
((R
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
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 55). 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
voltage errors. If the transient protection resistor is
added, an additional V
voltage due to input bias current. The limiting resistor
should only be added to the R
high-side gm amplifier (gm
amps of current through the RS+ pin.
Layout Guidelines
The Kelvin Connected Sense Resistor
The source of Kelvin voltage errors is illustrated in
Figure 55. The resistance of 1/2 Oz copper is
~1 milli-Ohm per square with a TC of ~3900ppm/°C
(0.39%/°C). When you compare this unwanted parasitic
resistance with the total of 1 to 10 milli-ohms 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
output voltage of 10V (G = 100). Two side contacts of
only 0.25 square per contact puts the V
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.
P
x I
RS-
PC BOARD
PC Board
PC Board
) = (100Ω x 130nA) = 13µV)
1/2 Oz COPPER TRACE
Copper Trace
Copper Trace
30mO/Sq.
30mO/Sq.
1mΩ /SQ
OS
OS
CURRENT OUT
of the amplifier and Kelvin
error can result from the IxR
Current Out
Current Out
M
HI
and C
SENSE
) sinking several micro
S
- input, due to the
D
= 0.1 and a full scale
filter the common
SENSE
OS
. Refer to the
February 4, 2010
input
FN6548.1