AD8214 Data Sheet
Rev. A | Page 12 of 16
APPLICATIONS
TYPICAL SETUP AND CALCULATIONS
The key feature of the AD8214 is its ability to detect an overcurrent
condition on the high side of the rail and provide a signal in less
than 100 ns. This performance protects expensive loads, FETs, and
shunt resistors in a variety of systems and applications. This section
details a typical application in which the normal current in the
system is less ≤10 A and an overcurrent detection is necessary
when 15 A is detected in the path.
If we assume a shunt resistance (R
SHUNT
) of 0.005 Ω and a
common-mode voltage range of 5 V to 65 V, the typical voltage
across the shunt resistor is
10 A × 0.005 Ω = 50 mV
The voltage drop across the shunt resistor, in the case of an
overcurrent condition is
15 A × 0.005 Ω = 75 mV
The threshold voltage, must therefore be set at 75 mV,
corresponding to the overcurrent condition. R1 and R2 can be
selected based on this 75 mV threshold at the positive input of
the comparator.
A low load current across the regulator corresponds to optimal
regulator performance; therefore, the series resistance of R1 and
R2 must be relatively large. For this case, the total resistance can
be set as
R1 + R2 = 200 kΩ
To have a 75 mV drop across R1, the following calculations apply:
R1 k 6.25
µA 12
=Ω=
R2 = (200 kΩ – R1) = 193.75 kΩ
The values for R1 and R2 are set; correspondingly, the threshold
voltage at +IN is set at 75 m V.
Under normal operating conditions, the current is 10 A or less,
corresponding to a maximum voltage drop across the shunt of
50 mV. This means that the negative input of the comparator is
50 mV below the battery voltage. Since the positive input is
75 mV below the battery voltage, the negative input is at a
higher potential than the positive; therefore, the output of the
AD8214 is low.
If the current increases to 15 A, the drop across the shunt is
75 mV. As the current continues to increase, the positive input
of the comparator reaches a higher potential than the negative,
and the output of the AD8214 switches from low to high. The
input-to-output response of the AD8214 is less than 100 ns. The
output resistor in this case is selected so that the logic level high
signal is 3.3 V.
The output changes states from low to high in the case of an
overcurrent condition. However, the input offset voltage is
typically 1 mV; therefore, this must be taken into consideration
when choosing the threshold voltage. When the current in the
system drops back down to normal levels, the AD8214 changes
states from high to low. However, due to the built-in 10 mV
hysteresis, the voltage at (–IN) must be 10 mV higher than the
threshold for the output to change states from high to low. This
built-in hysteresis is intended to prevent input chatter as well as
any false states.
Table 4 shows typical resistors combinations that can be used to set
an input threshold voltage. Numbers are based on a 2.43 V V
REG
.
Table 4.
Threshold (mV) R1 (kΩ) R2 (kΩ)
30 1.5 120
50 1.6 75
60 2 80
75 2.4 75
110 8.06 169
BATTERY
R
SHUNT
(0.005Ω)
C1
0.01µF
R2
193.75kΩ
R1
6.25kΩ
I
OUT
V
OUT
R
OUT
= 3.3kΩ
2.4V
REGULATOR
1
3
6
5
8
2
06193-006
I
LOAD
I
Figure 30. Typical Application
