52 January & February 2021 http://www.elektormagazine.com
And then there are half H-bridges that require both a high-side
and a low-side switch... and PWM... Therefore, as always, before
settling on any technique, make sure that it is a valid choice for
your application.
190369-F-01
TWO METHODS FOR HIGH-SIDE CURRENT SENSING
Basically, the current sense amplifier (left) divides the current
through Rsense by a factor of R1/Rsense (= 1,000) and forces it
through R3. The voltage over R3 therefore is proportional to the
current through Rsense. A current of 1 A through Rsense produces
a current of 1 mA through R3 (1 kΩ), resulting in a voltage of 1 V
over R3.The difference amplifier (right) multiplies the
voltage over Rsense by R4/R2 (= 10). A current
of 1 A through Rsense produces a voltage of 0.1 V
over Rsense, which, amplified by 10, results in 1 V
at the output of the op-amp.Both circuits produce the same output, yet they
are not equivalent.Bandwidth
First, the bandwidth of the two circuits is not
the same. The difference amplifier usually has a
(much) lower bandwidth than the current sense
amplifier and is therefore more suitable for
measuring the average current flowing through
a load. The current sense amplifier, on the other
hand, is fast and can measure instantaneous load
current at high frequencies.CMR
Another important factor to consider is common-mode
rejection, or CMR. As the common mode voltage at the inputs is
high, even a small asymmetry between the two inputs will result
in an error at the amplifier’s output. The difference amplifier
therefore needs high-precision resistors to keep the CMR error
as low as possible. In the current sense amplifier, it is mainly
the opamp that determines CMR, a parameter controlled by its
manufacturer.Robustness
On the other hand, thanks to its external resistors, a difference
amplifier can more easily be adapted to very high common
mode voltages than the current sense amplifier. This brings
us to the subject of robustness of the current sensing circuit.If transients producing out-of-bounds voltages over the sense
resistor can be present, the difference amplifier is protected by
its high-value resistors. In such cases the current sense amplifier
must mainly rely on any input overvoltage protection circuitry
built into its opamp. Similarly, a difference amplifier is usually
also more robust in the case of an inverted supply.Power Consumption
Finally, there is power consumption, especially important in
(ultra) low-power applications. If connected directly to the
supply, the difference amplifier with its resistors will always
consume some current even when the opamp is unpowered.
The current sense amplifier features a much higher input
impedance, which helps making batteries last longer.Special ICs
Because of all these subtleties, semiconductor manufacturers
sell special current sensing ICs based on different topologies,
making it much easier for designers to add current sensing to
their applications.Several methods exist for doing high-side current sensing. The current sense amplifier
and the current shunt monitor or difference amplifier shown here are two of them.Questions or Comments?
Do you have questions or comments about his article? Email
the author at [email protected] or contact Elektor
at [email protected].