LT6119-1/LT6119-2
23
611912f
For more information www.linear.com/LT6119-1
applicaTions inForMaTion
R7 can be calculated from the amplifier output current which
is required to cause the comparator output to trip, I
OVER
.
R7 =
I
OVER
, Assuming R1+ R2
( )
>> R7
To ensure (R1 + R2) >> R7, R1 should be chosen such
that R1 >> R7 so that V
OUTA
does not change significantly
when the comparator trips.
R3 should be chosen to allow sufficient V
OL
and compara-
tor output rise time due to capacitive loading.
R2 can be calculated:
R2 = R1•
V
DD
– 390mV
V
HYS(EXTRA)
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•
V
DD
– 390mV
R2
For example, to achieve I
OVER
= 900µA with 50mV of total
hysteresis, R7 = 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 R7 by
the positive feedback. Because of this, it is desirable to
have (R1 + R2) >> R7. The maximum V
OUTA
error caused
by this can be calculated as:
∆V
OUTA
= V
DD
•
R1+R2+R7
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 comparators 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
The input falling edge threshold which causes the output
to transition from low to high is:
V
TH(F)
= 390mV • 1+
R1
R2
–V
DD
R1
R2
Comparator Outputs
The comparator outputs can maintain a logic low level of
150mV while sinking 500µA. The outputs 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. As with any open-drain device, the
outputs may be tied together to implement wire-OR logic
functions. The LT6119-1 can be used as a single-output
window comparator in this way.
