LT6118
19
6118f
For more information www.linear.com/LT6118
Note that the hysteresis being added, V
HYS(EXTRA)
, is in
addition to the typical 10mV of built-in hysteresis. For very
large values of R2 PCB related leakage may become an
issue. A tee network can be implemented to reduce the
required resistor values.
The approximate total hysteresis is:
V
HYS
=10mV +R1•
DD
R2
For example, to achieve I
OVER
= 900µA with 50mV of total
hysteresis, R5 = 442Ω. Choosing R1 = 4.42k, R3 = 10k
and V
DD
= 5V results in R2 = 513k.
The analog output voltage will also be affected when the
comparator trips due to the current injected into R5 by
the positive feedback. Because of this, it is desirable to
have (R1 + R2) >> R5. The maximum V
OUTA
error caused
by this can be calculated as:
∆V
OUTA
= V
DD
•
R5
R1+R2+R5
In the previous example, this is an error of 4.3mV at the
output of the amplifier or 43µV at the input of the amplifier
assuming a gain of 100.
Since the comparator can be used independently of the
current sense amplifier, it is useful to know the threshold
voltage equations with additional hysteresis. The input
rising edge threshold which causes the output to transi
-
tion from high to low is:
V
TH R
( )
= 400mV • 1+
R2
applicaTions inForMaTion
The input falling edge threshold which causes the output
to transition from low to high is:
V
TH F
( )
= 390mV 1+
R2
– V
DD
R2
Comparator Output
The comparator output can maintain a logic-low level of
150mV while sinking 500µA. The output can sink higher
currents at elevated V
OL
levels as shown in the Typical
Performance Characteristics. Load currents are conducted
to the V
–
pin. The output off-state voltage may range
between 0V and 60V with respect to V
–
, regardless of the
supply voltage used.
Reverse-Supply Protection
The LT6118 is not protected internally from external rever
-
sal of supply polarity. To prevent damage that may occur
during this condition, a Schottky diode should be added
in series with V
–
(Figure 17). This will limit the reverse
current through the LT6118. Note that this diode will limit
the low voltage operation of the LT6118 by effectively
reducing the supply voltage to the part by V
D
.
Also note that the comparator reference, comparator out-
put and LE input are referenced to the V
–
pin. In order to
preserve the precision of the reference and to avoid driving
the comparator inputs below V
–
, R2 must connect to the
V
–
pin. This will shift the amplifier output voltage up by
V
D
. V
OUTA
can be accurately measured differentially across
R1 and R2. The comparator output low voltage will also
be shifted up by V
D
. The LE pin threshold is referenced
to the V
–
pin. In order to provide valid input levels to the
LT6118 and avoid driving LE below V
–
the negative supply
of the driving circuit should be tied to V
–
of the LT6118.
