1
Application Note 1480
ISL28133ISENSEV1Z Evaluation Board User’s Guide
The ISL28133ISENSEV1Z evaluation board contains a
complete precision current sense amplifier using the
ISL28133 chopper amplifier in a high side, low side of floating
supply current sense application.
The advantages of the ISL28133 in this application include very
low offset voltage (2µV typical) and offset drift (0.07µV/°C),
rail-to-rail input and output and low power consumption (18µA
typical). These features provide a very high degree of precision
for use in 5V, low power applications.
Reference Documents
• ISL28133 Data Sheet (
FN6560
)
Evaluation Board Key Features
The ISL28133ISENSEV1Z operates from a single 1.65VDC to
+5.5VDC supply. The current sense function is configured as a
single stage, balanced input, trans-impedance amplifier. A
0.1
Ω
0.1% current sense resistor is used convert the incoming
current to a voltage which is applied to a precision differential
amplifier with a gain of 100. The overall current to voltage
transfer ratio is 10V/A, and a bidirectional current range of
±0.25A is achieved when operating from a +5V supply, using a
2.5V reference voltage (V
REF
). At +25°C, the ISL28133
achieves a total full scale offset error of 0.038% (when a
precision external reference is used) and approximately 0.15%
using the internal 0.1% resistors.
The total current draw from the amplifier power is less than
35µA and the total leakage current from the current sense
input is less than 3µA.
Power Supply and Protection Features
External power connections are made through the +V, and
ground jacks. The single supply input is overvoltage protected
using a series 100
Ω
resistor and a 5.6V zener diode (D1).
Reverse polarity protection uses the 100
Ω
resistor and two
protection diode pairs (D3, D4). These also provide input
common mode voltage protection to the op amp.
Input Protection Clamp Options
Included on the evaluation board are optional input protection
circuits that illustrate the best methods to limit input common
mode and differential transient voltage spikes in exposed or
electrically hazardous applications. The ISL28133 can handle
input common mode and differential transients to a diode drop
beyond the rails, or to a range of -0.5V to +5.5V when operating
from a single 5V supply.
Clamp diodes D3A, B, D4A, B and input resistors R6, R7, R8,
R9 form a current limiting, 6V common mode and differential
voltage clamp. This clamp provides sufficient protection for the
ISL28133 for common mode fault voltages far beyond the
power supply rails. These diodes also provide the same 6V
protection against large differential transients.
High speed current sensing may involve filtering repetitive
current transients with high peak to average values. In these
cases, limiting these transient even further will improve
amplifier response and overload settling time. The optional
Schottky diode transient clamp D2 reduces transients let
through by the common mode clamp from 6V down to
~ ±0.3V, which improves overload recovery time. Capacitors
C7 and C9 provide common mode noise filtering and capacitor
C8 can be used to filter the current sense signal.
Current Sense Gain Equations
The current sense amplifier in Figure 1 forms a
trans-impedance amplifier whose gain is in units of V/A and is
determined by the ratios of resistor pairs. R11:R8 and R14:
R9. Resistor R11 is set equal to R14 and R8 is set equal to R9.
This matching cancels the input offset voltage errors caused
by the op amp input bias currents, leaving behind only the
offset voltage errors caused by the ISL28133 input offset
current (I
OS
).
The I
SENSE
to V
OUT
DC transfer function is given by Equation 1:
where:
R11 = R14 and R6 + R8 = R7 + R9
On the evaluation board, the R12, R9 resistor ratio is 100:1
(499k
Ω
: 4.9k
Ω
), and R
SENSE
is 0.1
Ω
for a trans-impedance
gain given by Equation 2:
Input Range and Offset Error Analysis
The ISL28133 output swings from rail-to-rail, and the evaluation
board has a bi-directional output range of V
REF
±2.5V when
operated from a +5V supply and V
REF
is set to V
S
/2. The
evaluation board contains a jumper-selectable, internal V
S
/2
reference or an external reference (V
REF
pin). With V
REF
set to
+2.5, the full scale output range is 0 ±0.25A.
The very low offset voltage (±8µV +25°C max) and offset
current (300pA +25°C max) of the ISL28133 enable the use
of very high values resistors for low current consumption while
maintaining excellent precision in battery operated circuits.
The total offset voltage contribution of the ISL28133 is the
sum of the input offset voltage (V
OS
) and the offset voltage
produced by the input offset current (IOS) through the gain
resistors and using the evaluation board resistor values and
the data sheet m25°C V
OS
gives the total input
offset voltage as shown in Equations 3, 4 and 5:
V
OUT
I
SENSE
R
SENSE
×
R14
R9
-----------
×
V
REF
+
=
(EQ. 1)
(EQ. 2)
V
SENSE
I
SENSE
--------------------
10
V
A
---
=
V
OS Total
(
)
IOS
R14 R9
×
R14 R9
+
-------------------------
×
⎝
⎠
⎛
⎞
VOS
+
=
(EQ. 3)
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Copyright Intersil Americas Inc. 2011. All Rights Reserved
Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries.
All other trademarks mentioned are the property of their respective owners.
July 18, 2011
AN1480.2
