lt6109-1 Linear Technology Corporation, lt6109-1 Datasheet - Page 20

• Manufacturer Part Number: lt6109-1
• Description: High Side Current Sense Amplifer With Reference And Comparators
• Manufacturer: Linear Technology Corporation

LT6109-1/LT6109-2
APPLICATIONS INFORMATION
R1 should be chosen such that R1 >> R7 so that V
does not change significantly when the comparator trips.
R3 should be chosen to allow sufficient V
tor output rise time due to capacitive loading.
R2 can be calculated:
Note that the hysteresis being added, V
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:
For example, to achieve I
hysteresis, R7 = 442Ω. Choosing R1 = 4.42k, R3 = 10k
and V
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
by this can be calculated as:
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.
When using the comparators with their inputs decoupled
from the output of the amplifier they may be driven directly
by a voltage source. 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:
20
R2 = R1•
V
∆V
V
HYS
TH(R)
+
OUTA
= 5V results in R2 = 513k.
= 10mV +R1•
= 400mV •R1•
= V



V
V
+
HYS(EXTRA)
+
•
– 390mV

 
R1+R2 +R7



V
OVER
R7

 
+
R1



1
– 390mV
R2
= 900µA with 50mV of total
+
R2

 
1

 



OUTA
OL
HYS(EXTRA)
and compara-
error caused
, is in
OUTA
The input falling edge threshold which causes the output
to transition from low to high is:
Comparator Outputs
The comparator outputs can maintain a logic low level
of 150mV while sinking 500µA. The outputs can supply
higher currents at elevated V
Typical Performance Characteristics. Load currents are
conducted to the V
range between 0V and 60V with respect to V
of the supply voltage used. As with any open-drain device,
the outputs may be tied together to implement wire-OR
logic functions. The LT6109-1 can be used as a single-
output window comparator in this way.
EN/RST Pin
The EN/RST pin performs the two functions of resetting
the latch on the comparators as well as shutting down
the LT6109.
Applying a pulse to the EN/RST pin will reset the compara-
tors from their tripped state as long as the input on the
comparator is below the threshold and hysteresis for an
inverting comparator or above the threshold and hysteresis
for a noninverting comparator. For example, if V
pulled higher than 400mV and latches the comparator, a
reset pulse will not reset that comparator unless its input
is held below the threshold by a voltage greater than the
10mV typical hysteresis.
Figure 15 shows the reset functionality of the EN/RST
pin. The width of the pulse applied to reset the compara-
tors must be greater than t
t
40µs typically (or tying the pin low) will cause the part
to enter shutdown. Once the part has entered shutdown,
the supply current will be reduced to 6µA typically and the
amplifier, comparators and reference will cease to function
until the EN/RST pin is transitioned high. When the part
is disabled, both the amplifier and comparator outputs
are high impedance.
RPW(MAX)
V
TH(F)
= 390mV •R1•
(15µs). Applying a pulse that is longer than
–
pin. The output off-state voltage may

 
R1
1
RPW(MIN)
+
OL
R2
1
levels as shown in the

 
–
(2µs) but less than



V
+
R2
•R1
–
, regardless



INC1
610912f
is