User Guide
12 of 46
Rev. 1.0
2022-03-09
48V Battery Switch Reference Design
R 48V BATT Switch10
System and functional description
In general, all the extensive diagnosis as well as configuration and control of the gate driver is handled via the
SPI interface.
Figure 9
Dual channel MOSFET gate driver 2ED4820-EM
2.2.5
Shunt based current sensing
With continuous currents up to 300 A power dissipation in the shunt resistor can be a concern. Even with a
shunt as low as 100 µ
W
this amounts to of 9 W power loss in the resistor. Since we wanted to use SMD mounted
shunts we decided to use two of them in parallel. Of course, this comes to the cost of halving the sensitivity, but
thanks to the programmable gain amplifier implemented in the
2ED4820-EM
this can easily be compensated.
Another concern is the parasitic inductance of the shunt. In the event of a short circuit the current is limited
only by the inductance of the cables. Assuming 1 m cable with an inductance of 1 µH, the slewrate dI/dt will be
48 A/µs for a supply voltage of 48V (dI/dT = V/L). The high current slew rate will lead to a voltage drop in the
shunt inductance. This is added to the voltage drop at the resistive part and introduces a measurement error.
According to the datasheet the parasitic inductance of the selected shunt resistor ranges between 0.5 nH and 5
nH. Calculated based on the physical dimensions of the section between the sense terminals it should be about
1.8 nH [1]. For two shunts in parallel the inductance is halved, which amounts to 0.9 nH.
With the given current slew rate this inductance induces an error voltage of
V
L
=L
dI
dt = 0.9 nH 48
A
µs = 43.2 mV
which would translate to a measurement error of
𝐼
𝑒𝑟𝑟
=
𝑉
𝐿
𝑅
𝑠ℎ𝑢𝑛𝑡
=
43.2
𝑚𝑉
50 µΩ
= 864
𝐴
!
In order to compensate that error, a RC low pass filter has to be added between the shunts and the gain
amplifier of the 2ED4820-EM. The time constant of that RC filter should be equal to the time constant formed by
the shunt resistor and the parasitic inductance:
