isl28005 Intersil Corporation, isl28005 Datasheet - Page 11

• Manufacturer Part Number: isl28005
• 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 18). 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
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
beyond the sensing voltage range of this amplifier.
Typical Application Circuit
Figure 20 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 18. 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
0.4
P
resistor value of 100Ω will provide
0.6
CURRENT IN
Current In
Current In
0.8
11
P
FIGURE 19. PC BOARD CURRENT SENSE KELVIN CONNECTION
) in Figure 20 may be
V
RS+
1.0
NON-UNIFORM
CURRENT FLOW
(V)
Current Flow
Current Flow
Non-uniform
Non-uniform
1.2
RS+
S
+ or R
1.4
= 0V TO 2V
KELVIN V
1.6
Kelvin V
Kelvin V
S
- are
1.8
CURRENT SENSE RESISTOR
S
S
ISL28005
S
Current Sense Resistor
Current Sense Resistor
Contacts
Contacts
2.0
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
recovery 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 19). 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
Kelvin Connected Sense Resistor
The source of Kelvin voltage errors is illustrated in
Figure 19. 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
(G = 100). Two side contacts of only 0.25 square per
contact puts the V
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.
SENSE
P
x I
RS-
= 0.1 and a full scale output voltage of 10V
PC BOARD
PC Board
PC Board
) = (100Ω x 130nA) = 13µV)
1/2 Oz COPPER TRACE
Copper Trace
Copper Trace
30mO/Sq.
30mO/Sq.
SENSE
1mΩ /SQ
OS
OS
CURRENT OUT
of the amplifier and Kelvin
input about 0.5 x 1mΩ away
error can result from the IxR
Current Out
Current Out
HI
M
) sinking several micro
and C
S
- input, due to the
D
filter the common
OS
. Refer to the
February 3, 2010
FN6973.1